Topic: Would someone like to build a little passive mixer for me?

[faninor]

I have a pair of Core Sound Cardioids and a pair of Core Sound Binaural microphones, which I'd like to be able to mix together and record onto a single M-Audio Microtrack when I'm doing stealthy taping. So it needs to have two stereo 1/8" inputs and either 1/8" or dual 1/4" outputs for the Microtrack. Someone gave me a schematic of a passive mono mixer that just makes a 50/50 mix of the two inputs, it looks very simple. If it would be easy to add a control to allow for different mixes than just 50/50 that would be preferred.

Does anybody think they might be able to put something together for me?

[boojum]

Looks like it is time to guy that soldering iron.   

[faninor]

Looks like it is time to guy that soldering iron.   

Looks that way, haha.

[Church-Audio]

Looks like it is time to guy that soldering iron.   

Looks that way, haha.

No offence but you should not really do that with a passive mixer.. You need an active mixer so you dont load down the mics and so you can provide power to the mics separately.. Although you can power two sets of electret capsules from a single source its not recommended for many reasons including the fact that you will degrade the performance of the microphones when you load them down like that. Not to mention why would you want to mix a set of cardioid mics with a set of omni mics? You have to realize that the mics outputs will not be the same so you will not get a 50/50 mix you will get more like a 30/70 the sensitivity of these two sets of mics will not match. And you will also end up with some wild phasing issues.

Chris

[faninor]

No offence but you should not really do that with a passive mixer.. You need an active mixer so you dont load down the mics and so you can provide power to the mics separately.. Although you can power two sets of electret capsules from a single source its not recommended for many reasons including the fact that you will degrade the performance of the microphones when you load them down like that. Not to mention why would you want to mix a set of cardioid mics with a set of omni mics? You have to realize that the mics outputs will not be the same so you will not get a 50/50 mix you will get more like a 30/70 the sensitivity of these two sets of mics will not match. And you will also end up with some wild phasing issues.

Chris

I will be powering both of the microphones with individual battery boxes before going into the mixer, so powering them won't be an issue -- in the past when I've recorded with either of these microphones and a minidisc I've just plugged them right into the line input rather than mic input so it should be okay.

I want to mix them because I'm not particularly fond of how either of the microphones sound on their own (my CSB's usually get plenty of bass even when using the battery box's roll-off filter, but higher frequencies and especially acoustic guitar and drums don't turn out so well if I'm not able to record from a sweet spot -- the CSC's are just the opposite, they pick up drums and acoustic guitar great but are dreadfully lacking for bass). I probably wouldn't get a 50/50 mix of course. If it had potentiometers for the inputs that wouldn't be a problem. Otherwise... well, I might still prefer the sound of the mix over either microphone on its own.

I've played around with running two minidisc decks, one with each microphone, and mixing the two recordings later, and my ears have preferred how that sounds over either of the sources on their own. I just have to deal with fixing problems with the recordings drifting apart (which has its own phasing issues or artifacts from time stretching or condensing one source to match the other -- I wouldn't mind trying out a passive mixer and seeing if what it can do will turn out better, worse, or approximately the same with considerably less editing time involved than what I've been able to do in the past running two recorders and mixing after.

As for the wild phasing issues -- this would be due to the different distances from sound source to mic 1 vs. sound source to mic 2?

They're very small microphones -- I should be able to position one CSC directly above a CSB and about 1 cm away. So at the very worst case, a sound source directly above or below the microphones, which are spaced exactly 1 cm apart from each other, and in conditions where sound is travelling at 345 meters per second, the difference between the time it takes to get from one mic to the other would be 1/34500 of a second... about .00002899 seconds. This would mean deconstructive interference, especially around 17.25kHz which would be bad...

However, the sound source I'm trying to record wouldn't ever be directly above or below me -- sounds coming from the front, or sides, or from an angle above or below that is not too steep, would have practically no difference between the time it takes for the path the sound travels to one mic vs. the other -- I think there wouldn't be too much phasing to ruin anything in the audible range. Even if the difference in length from source to mics were only reduced to .6 cm by pointing them in more or less the right direction instead of the worst case scenario, that would be 6/345000, or about .00001739 seconds different. If I'm recording at 48kHz, each sample would be about .00002083 seconds long -- so the sources would be out of sync by less than 1 sample -- and even the worst case above or below is less than 1 1/2 samples different. I may be able to get recordings that accurately synced at a few specific places in time, but I don't think I'd be able to get an entire show constantly synced within 1 1/2 samples for the entire recording.

So the short version is, unless there's additional reasons for phasing issues besides distance from sound sources to microphones that I'm not aware of, I should still be better off in this case with a mixer than I am mixing two recordings after the fact.

[Church-Audio]

No offence but you should not really do that with a passive mixer.. You need an active mixer so you dont load down the mics and so you can provide power to the mics separately.. Although you can power two sets of electret capsules from a single source its not recommended for many reasons including the fact that you will degrade the performance of the microphones when you load them down like that. Not to mention why would you want to mix a set of cardioid mics with a set of omni mics? You have to realize that the mics outputs will not be the same so you will not get a 50/50 mix you will get more like a 30/70 the sensitivity of these two sets of mics will not match. And you will also end up with some wild phasing issues.

Chris

I will be powering both of the microphones with individual battery boxes before going into the mixer, so powering them won't be an issue -- in the past when I've recorded with either of these microphones and a minidisc I've just plugged them right into the line input rather than mic input so it should be okay.

I want to mix them because I'm not particularly fond of how either of the microphones sound on their own (my CSB's usually get plenty of bass even when using the battery box's roll-off filter, but higher frequencies and especially acoustic guitar and drums don't turn out so well if I'm not able to record from a sweet spot -- the CSC's are just the opposite, they pick up drums and acoustic guitar great but are dreadfully lacking for bass). I probably wouldn't get a 50/50 mix of course. If it had potentiometers for the inputs that wouldn't be a problem. Otherwise... well, I might still prefer the sound of the mix over either microphone on its own.

I've played around with running two minidisc decks, one with each microphone, and mixing the two recordings later, and my ears have preferred how that sounds over either of the sources on their own. I just have to deal with fixing problems with the recordings drifting apart (which has its own phasing issues or artifacts from time stretching or condensing one source to match the other -- I wouldn't mind trying out a passive mixer and seeing if what it can do will turn out better, worse, or approximately the same with considerably less editing time involved than what I've been able to do in the past running two recorders and mixing after.

As for the wild phasing issues -- this would be due to the different distances from sound source to mic 1 vs. sound source to mic 2?

They're very small microphones -- I should be able to position one CSC directly above a CSB and about 1 cm away. So at the very worst case, a sound source directly above or below the microphones, which are spaced exactly 1 cm apart from each other, and in conditions where sound is travelling at 345 meters per second, the difference between the time it takes to get from one mic to the other would be 1/34500 of a second... about .00002899 seconds. This would mean deconstructive interference, especially around 17.25kHz which would be bad...

However, the sound source I'm trying to record wouldn't ever be directly above or below me -- sounds coming from the front, or sides, or from an angle above or below that is not too steep, would have practically no difference between the time it takes for the path the sound travels to one mic vs. the other -- I think there wouldn't be too much phasing to ruin anything in the audible range. Even if the difference in length from source to mics were only reduced to .6 cm by pointing them in more or less the right direction instead of the worst case scenario, that would be 6/345000, or about .00001739 seconds different. If I'm recording at 48kHz, each sample would be about .00002083 seconds long -- so the sources would be out of sync by less than 1 sample -- and even the worst case above or below is less than 1 1/2 samples different. I may be able to get recordings that accurately synced at a few specific places in time, but I don't think I'd be able to get an entire show constantly synced within 1 1/2 samples for the entire recording.

So the short version is, unless there's additional reasons for phasing issues besides distance from sound sources to microphones that I'm not aware of, I should still be better off in this case with a mixer than I am mixing two recordings after the fact.

Most phasing problems are not just a matter of reversing the  polarity..  180 degrees and calling it a day they are a matter of calculating by what degree your out of phase and inserting the proper delay onto the signal that is out.. That's the only way to fix phase. You cant fix phase if your combining the signal because you have no separation to insert the delay on the signal that has the phasing issue. You would need both sets of mics to have separate tracks. It sounds to me like your trying to hit a square peg through a very small round hole. Why not sell both mics if your not happy with them and buy something else. There are plenty of used mics around in the yard sale section here. Or look at getting new mics plenty of people are selling new mics. But what you want to do will cause more trouble then its worth. I know this from experience. But sometimes we just have to figure this stuff out on our own. Remember the two mics will not be the same levels... So you need to be able to adjust the levels independently in order to do what you want to do. Good luck.

Chris

[Roving Sign]

No offence but you should not really do that with a passive mixer.. You need an active mixer so you dont load down the mics and so you can provide power to the mics separately.. Although you can power two sets of electret capsules from a single source its not recommended for many reasons including the fact that you will degrade the performance of the microphones when you load them down like that. Not to mention why would you want to mix a set of cardioid mics with a set of omni mics? You have to realize that the mics outputs will not be the same so you will not get a 50/50 mix you will get more like a 30/70 the sensitivity of these two sets of mics will not match. And you will also end up with some wild phasing issues.

Chris

I will be powering both of the microphones with individual battery boxes...unless there's additional reasons for phasing issues besides distance from sound sources to microphones that I'm not aware of, I should still be better off in this case with a mixer than I am mixing two recordings after the fact.

Wouldnt your time and money be better spent by finding some mics that perform to your liking...?

[Krispy D]

^^uh oh.  That sounds like logic!

[faninor]

Wouldnt your time and money be better spent by finding some mics that perform to your liking...?

My time, well... compared to recording and then mixing, absolutely yes. Compared to mixing while recording, the time invested to find a mixer to use probably wouldn't be any greater than the time I would take choosing a new microphone. My money, not so much -- I started off with the CSCs only, because at that time from listening to samples of different microphones and comparing the prices of microphones that is what I could afford. After using them for a while (and getting a better speaker system for listening at home) I decided that they worked okay but I was a bit unsatisfied with how the bass sounded. Later I picked up some used CSBs for a fairly good price to try them out instead. So I already have all the equipment I would be using except for the mixer, and if the cost of the mixer is not too great, for the time being I'd rather try to get what I want out of what I already have until I do decide what mics I would rather have and save up enough extra money to purchase them.

[Roving Sign]

BOSS made a little mixer that ran of a single 9V - BX4



Should be able to find one for less than 20 bucks...

My time, well... compared to recording and then mixing, absolutely yes. Compared to mixing while recording,

Finding a mixer...and building one are two different things...timewise.

[faninor]

Most phasing problems are not just a matter of reversing the  polarity..  180 degrees and calling it a day they are a matter of calculating by what degree your out of phase and inserting the proper delay onto the signal that is out.. That's the only way to fix phase. You cant fix phase if your combining the signal because you have no separation to insert the delay on the signal that has the phasing issue. You would need both sets of mics to have separate tracks. It sounds to me like your trying to hit a square peg through a very small round hole. Why not sell both mics if your not happy with them and buy something else. There are plenty of used mics around in the yard sale section here. Or look at getting new mics plenty of people are selling new mics. But what you want to do will cause more trouble then its worth. I know this from experience. But sometimes we just have to figure this stuff out on our own. Remember the two mics will not be the same levels... So you need to be able to adjust the levels independently in order to do what you want to do. Good luck.

Chris

I'm well aware that phasing is an issue for whatever phase angle the two signals are off by... 180 degrees is the absolute worst case but there are many shades of grey between there and 0 degrees.

However, I'm also well aware that, because the mics are placed at a single absolute distance from each other, if you were to take the two signals from the mic and perform a Fourier analysis on them, then match up each term of the two resulting Fourier series and compare the phase angle for each term (each frequency), then the phase angle is related inversely to the period of the simple sine wave for all terms with a period at least two times greater than the difference in time it takes for a sound source to get to one microphone compared to the other... If I'm able to keep the mics pretty close together and nearly equidistant from sound sources, then the phase angle of the two signals should be pretty small for most or all of the audible spectrum -- at least enough so to do better than I can mixing the two recordings afterwards (as I said above, at the worst case if they're placed 1 cm apart from each other, the two microphones would be within 1 1/2 samples of each other for sound sources directly above or below the microphones when recording at 48kHz, which is probably better than I can do when mixing after recording if the recordings have any drift at all, because I'd either be chopping the recording up and matching it at certain places, and it would drift to be several samples out of sync before the next place I correct it, or else I would be stretching or compressing one recording to match the other, which is another process that I have a feeling might be a little bit devastating in its own way to higher frequencies which may not be preserved well in the interpolation process of stretching or compressing the recording.

BOSS made a little mixer that ran of a single 9V - BX4



Should be able to find one for less than 20 bucks...

My time, well... compared to recording and then mixing, absolutely yes. Compared to mixing while recording,

Finding a mixer...and building one are two different things...timewise.

Ooh, thank you! I was thinking about building one since I hadn't been able to find one yet that seemed like what I was looking for.

edit - oh haha, I completely missed your point there. Building one might be fun though! Certainly more fun than I have getting two recordings to line up while they're constantly trying to drift apart.

I hadn't found this one yet, but at a glance it looks like it might be closer to what I'm looking for than anything I've found yet -- just the right number of inputs... it needs power, but since it runs on a 9v at least I won't have to plug in anywhere (which is the bigger issue). I'll have to see if I can find one to buy -- it looks like they might not make it anymore based off of the google results I'm getting (hmm, that's probably why I hadn't found it). 

[Krispy D]

not to chime in out of turn, (I really only skimmed the thread so I may be way off) but why not just pick up a mixpad 4 or something similar.  they are easy to find for under a hundred bucks, have four inputs, would negate your phase issues, and run on 9v batteries.  I think I saw one in the yard sale recenty as well.

keep in mind this is just my non-asked for two cents

[faninor]

not to chime in out of turn, (I really only skimmed the thread so I may be way off) but why not just pick up a mixpad 4 or something similar.  they are easy to find for under a hundred bucks, have four inputs, would negate your phase issues, and run on 9v batteries.  I think I saw one in the yard sale recenty as well.

keep in mind this is just my non-asked for two cents

Thank you too! I hadn't seen this model of mixer yet either. I'm looking at a user manual for it online now, so I'll read though it and see if this would do the trick for me -- if so it'd probably be easier to find than that old BOSS mixer.

I think the phase issue is something that just has to be lived with and would still be as much of an issue (but I think in my situation it shouldn't really be a problem).

edit - it looks like that one would work fine... with its inputs I would need one kind of adapter to plug one mic set into the mono 1/4" inputs (channel 1 and 2), and a different adapter for the single unbalanced stereo 1/4" input (channel 3 and 4)... with the BOSS I would need two of that first type of adapter. I know where I can get the first type of adapter, hopefully it's not too hard to find the 2nd. There's also a lot more controls, which could either be good or bad.

[Church-Audio]

I'm well aware that phasing is an issue for whatever phase angle the two signals are off by... 180 degrees is the absolute worst case but there are many shades of grey between there and 0 degrees.

However, I'm also well aware that, because the mics are placed at a single absolute distance from each other, if you were to take the two signals from the mic and perform a Fourier analysis on them, then match up each term of the two resulting Fourier series and compare the phase angle for each term (each frequency), then the phase angle is related inversely to the period of the simple sine wave for all terms with a period at least two times greater than the difference in time it takes for a sound source to get to one microphone compared to the other... If I'm able to keep the mics pretty close together and nearly equidistant from sound sources, then the phase angle of the two signals should be pretty small for most or all of the audible spectrum -- at least enough so to do better than I can mixing the two recordings afterwards (as I said above, at the worst case if they're placed 1 cm apart from each other, the two microphones would be within 1 1/2 samples of each other for sound sources directly above or below the microphones when recording at 48kHz, which is probably better than I can do when mixing after recording if the recordings have any drift at all, because I'd either be chopping the recording up and matching it at certain places, and it would drift to be several samples out of sync before the next place I correct it, or else I would be stretching or compressing one recording to match the other, which is another process that I have a feeling might be a little bit devastating in its own way to higher frequencies which may not be preserved well in the interpolation process of stretching or compressing the recording.

Hummmm well you cant really tell with fft unless you have a reference. You "seem" to know what your talking about but abit of your theory is off. I suggest not placing the mics so close together as the closer the mics are to one another the more "likely" phase cancellation will occur.. I would suggest doing some more studying on this subject, I would invest in some mics that you dont have to "fix" to get them to sound good. I would hardly call wearing two sets of mics stealth taping... I would look at getting a pair of AT 853 mics I am not going to plug my mics because well.. I dont have too. But I think you should look around and listen to some of the other veterans here that are trying to say the same thing I am.. If you dont like the sound of your mics. There is one company that did this AKG used two capsules in there 3000 one for low end one for high end. They used a crossover to divide the mics so they did not overlap each others frequence response. You then end up with a time coherent signal with "less" phasing issues. Also you will not be able to "fix" the phase issues unless you use a 4 channel recorder and then sent the individual tracks to a computer so you can then change the time arrival. But even then you will still have comb filtering issues. Sounds to me like just buying a better set of mics might be simpler

[faninor]

Hummmm well you cant really tell with fft unless you have a reference. You "seem" to know what your talking about but abit of your theory is off. I suggest not placing the mics so close together as the closer the mics are to one another the more "likely" phase cancellation will occur.. I would suggest doing some more studying on this subject, I would invest in some mics that you dont have to "fix" to get them to sound good. I would hardly call wearing two sets of mics stealth taping... I would look at getting a pair of AT 853 mics I am not going to plug my mics because well.. I dont have too. But I think you should look around and listen to some of the other veterans here that are trying to say the same thing I am.. If you dont like the sound of your mics. There is one company that did this AKG used two capsules in there 3000 one for low end one for high end. They used a crossover to divide the mics so they did not overlap each others frequence response. You then end up with a time coherent signal with "less" phasing issues. Also you will not be able to "fix" the phase issues unless you use a 4 channel recorder and then sent the individual tracks to a computer so you can then change the time arrival. But even then you will still have comb filtering issues. Sounds to me like just buying a better set of mics might be simpler

I will probably be replacing these mics eventually, I'm just looking for something to temporarily use maybe for the next year or so. I don't have very many recordings with me at the moment, but if memory serves correctly I've enjoyed a lot of recordings made with DPA 4061's -- but at the moment I don't want to spend $700-ish (I think that's approximately what they cost)... I'm moving into my first apartment and there are lots of other ways that I could more responsibly be spending that much money.

Two sets of mics isn't extremely stealthy, but I haven't been caught yet. Whenever I'm worried about being able to pull it off with 2 mics, battery boxes, minidisc recorders, and two times the blank discs and batteries in the past I've just guessed at which mic I would prefer on its own for that particular show and leave half the gear at home.

My degree is in mathematics and I've only taken one class about the physics involved with sound (and that class was sort of geared towards the non-physics, non-math crowd so a lot of stuff was glossed right over). So I'm definitely no expert and might have some misconceptions. If you could help me understand where my theory is off that would be wonderful, as I've been thinking about this for a while and can't figure out where my thinking has gone sour.

You say that the closer two microphones are to each other the more likely phase cancellation will occur, and you have a lot of practical experience with this sort of thing so you're probably saying this from experience which is hard to argue with. But I just don't see how the physics and mathematics behind it explain this.

In a simple model where there's one sound source, and one mic placed 1 meter away, and a second mic placed right between them then there's no problems when there are simultaneously high pressures at both mics or low at both mics, which would be whenever the wavelength of the signal the source nicely divides the meter distance from the source to the 1st mic and the half meter distance from the source to the 2nd mic. When the wavelength is something that almost evenly divides those distances they won't be perfectly in phase, but would be only a few degrees out of phase. This wouldn't be a huge problem as long as it's only a few degrees out of phase, would it? However, with this setup than sounds coming from the source with a wavelength of 1 meter, or a wavelength of 1/3 meter, would be 180 degrees out of phase at the two microphone positions. If the 2nd microphone is moved closer and closer to the 1 meter point where the 1st microphone is, then when it is very close to 1 meter from the source, then the sound with a 1/2 meter wavelength will be only a few degrees out of phase, as will the sound with the 1 meter wavelength and the sound with the 1/3 meter wavelength. To find anything that is severely out of phase you have to look at the shorter wavelength, higher frequency sounds. Keep moving the two mics closer together and everything within the audible spectrum should be within a few degrees of being perfectly in phase (there would just be tons of ultrasonic phasing that we can't hear) -- theoretically this could be continued infinitely by pushing the microphones closer and closer so that more and more low frequencies are even closer to being perfectly in phase and the only frequencies that are dreadfully out of phase are even higher and higher. Taking this to the limit you'll end up when every finite frequency is perfectly in phase and the distance between the microphones is zero so that they're essentially the same microphone (the better but likely more expensive solution that you've suggested).

In practice it is surely more complicated than this, but I don't see how, if two microphones are sufficiently close to each other, there would be be a great deal of audible deconstructive interference or phase cancellation within the audible hearing range of hearing -- all of the significant phase cancellation would be happening at ultrasonic frequencies. Everything would be slightly out of phase at all audible frequencies, but not by too many degrees if the mics are sufficiently close enough. If the mics are farther apart, then some audible frequencies would suffer absolutely no phase cancellation (such as the 1/2 meter wavelength in my example). And so for that particular wavelength a greater distance is better than when the two microphones are very close together and they are out of phase by a few degrees. But when they're farther apart like that there would also be audible frequencies that are severely out of phase such as the 1/3 meter and 1 meter wavelengths that are 180 degrees out of phase.

I'd really appreciate it if you'd help me understand where my theory is off at!

[SparkE!]

Hey faninor,  I think it's you that's correct on this.

Chris is worried about signal inversion between the two mic sources.  A rise in instantaneous pressure at the mic capsule should provide the same polarity of voltage swing at the output of the mic.  If they produce the opposite polarity signal from each other, then they are 180 degrees out of phase with each other and contrary to what Chris says, you can't cure this problem with a simple time shift of one of the signals relative to the other.  You actually have to invert one of the signals and that cannot be done with a passive mixer unless that mixer uses a transformer for a mixing element.  The most commonly used convention is that a rise in instantaneous pressure on the mic capsule should produce a rise in instantaneous voltage at the output of the mic. So, if you send a puff of air at the mic capsule by saying a word that starts with a "P", then the resulting waveform should have a rising edge at the first of the recording of the "P".

So, as long as your mics have the same polarity of output, you should not have to worry about phasing issues if the mics are located at approximately the same distance from the sound source.

As far as placement of the mics go, sound travels about 1137 feet per second.  If you are using a sample rate of 44.1 kHz, your Nyquist frequency is 22.05 kHz and you'll get 180 degrees of shift at that frequency if you are just one sample off.  Of course the phase shift at lower frequencies for that same time shift is smaller. At 220.5 Hz, your phase shift would only be 1.8 degrees.  In one sample period, sound travels just over 3/10ths of an inch, so it does not take much mic separation to give significant relative phase shift between two mic source at the higher frequencies.  Then again, your own ears are about 17 cm apart in terms of the acoustic delay from one side of your head to the other.  Our brains use that phase shift at the higher frequencies to help us locate the sources of the sounds around us.

[faninor]

Ah okay, I hadn't thought about signal inversion -- that makes a lot of sense. I think my mics have the same polarity though so it shouldn't be a problem. I just checked and the microphones have a diameter of just about .25". If I allow just a very small gap between the two microphones then the discrepancy between the two should be between 0 and 1 samples, and by then placing them wisely with respect to the sound source I should be able to keep directions with a 1 sample discrepancy to places where it shouldn't have as important an effect.

[Lil Kim Jong-Il]

I'll add another vote for just getting new mics.  There are some people here who are looking to get in and you could probably sell both those pair and take a nice step up to another mic with the proceeds plus what you would put toward the mixer.   If you are trying to be low profile, the mixer isn't something you want to add.  Even for normal situations adding the mixer and second pair of mics is just adding potential points of failure on your chain.  IMO you will be lucky to mix the CSB and CSC mics and get a result that is not a lesser quality result than either mic alone.  All the guys I know who get good results from a four mic mix are doing the mix in post and appling filters.  I think you will be happier in the long run with a single pair of mics. 

[faninor]

I think I want to hang onto my Core Sound microphones even after upgrading, at least for a while... in the past I've loaned them out to friends, but the more expensive my microphones are, the less willing I'd be to take risks with them. I think I might try to build a mixer myself based off of the schematic I have and just try it out. In the past I've mixed recordings from these mics in post with good results. But sync in post has its own problems with drift and any way I go about fixing that has its own problems quality-wise. And even if there's absolutely no drift, because of the finite samples in digital storage, it is impossible to guarantee the accuracy of synchronization by more than 1/2 a sample -- so recording at 48kHz, the Nyquist frequency is 24kHz, but if the two sources you're mixing were recorded offset by half a sample from each other (or x.5 samples off from each other, your pick of x) then whichever way you line them up you'll still be half a sample out of sync, your 24kHz frequency will be 180 degrees out of phase no matter what. This is a real benefit of recording at 96kHz I think -- if you're just recording with a single microphone set at 96kHz that's great, but if you're recording from multiple mics and then mixing it together 96kHz should give enough precision to guarantee that all your audible frequencies don't get hurt in the mix. Do the guys you know record at a high sample rate? And do they use a multitracker or something that should take care of drift problems?

Given the physics of the situation and intelligent mic placement I think I'll be able to do just as well and most likely considerably better mixing it on the fly as I can in post recording at 44.1 or 48kHz. In the long run I'll be getting a different pair of mics though.

[Lil Kim Jong-Il]

Do the guys you know record at a high sample rate? And do they use a multitracker or something that should take care of drift problems?

Now they use 96K and timecode.  But it hasn't always been that way, it used to be multiple DAT sources.


I still think a single pair of better quality mics will give the best result but it is fun to build things so you might as well try it. 

[Church-Audio]

I think I want to hang onto my Core Sound microphones even after upgrading, at least for a while... in the past I've loaned them out to friends, but the more expensive my microphones are, the less willing I'd be to take risks with them. I think I might try to build a mixer myself based off of the schematic I have and just try it out. In the past I've mixed recordings from these mics in post with good results. But sync in post has its own problems with drift and any way I go about fixing that has its own problems quality-wise. And even if there's absolutely no drift, because of the finite samples in digital storage, it is impossible to guarantee the accuracy of synchronization by more than 1/2 a sample -- so recording at 48kHz, the Nyquist frequency is 24kHz, but if the two sources you're mixing were recorded offset by half a sample from each other (or x.5 samples off from each other, your pick of x) then whichever way you line them up you'll still be half a sample out of sync, your 24kHz frequency will be 180 degrees out of phase no matter what. This is a real benefit of recording at 96kHz I think -- if you're just recording with a single microphone set at 96kHz that's great, but if you're recording from multiple mics and then mixing it together 96kHz should give enough precision to guarantee that all your audible frequencies don't get hurt in the mix. Do the guys you know record at a high sample rate? And do they use a multitracker or something that should take care of drift problems?

Given the physics of the situation and intelligent mic placement I think I'll be able to do just as well and most likely considerably better mixing it on the fly as I can in post recording at 44.1 or 48kHz. In the long run I'll be getting a different pair of mics though.

Theory is nice... Experience tells me your not going to get good sound by placing an omni capsule right beside a cardioid capsule and mixing the signal together with out being able to have separate tracks to address the phasing issues, you have to remember the omni mic in "theory" pics up sound equally from all directions. The cardioid has more pickup on axis. The cardioid also has ports in the back of the capsule that are 180 degrees out from your omni mic.. The two patterns from the back lobe of the cardioid and the back lobe of the omni will mix and cause phase cancellation IN YOUR AUDIO not to mention that the arrival time WILL NEVER BE THE SAME of because your using two different types of microphones "strapped" together. I will be more then willing to bet your not "better off mixing it live" then you are with a multitrack and fixing it in post.

Chris
 

[SparkE!]

Mixing it live, he can align the sources to within one sample of each other.  Mixing in post, he'd be lucky to keep it within about 200 samples.  So, alignment of the signals won't be the issue.  However, getting the proportion of one signal's contribution to the other's could be an issue.  When you are mixing in post, you have the luxury of being able to experiment with different ratios of signal amplitude and choose the ratio that sounds the best.  Doing that mix live, you've got to get it right the first time.

[Church-Audio]

Mixing it live, he can align the sources to within one sample of each other.  Mixing in post, he'd be lucky to keep it within about 200 samples.  So, alignment of the signals won't be the issue.  However, getting the proportion of one signal's contribution to the other's could be an issue.  When you are mixing in post, you have the luxury of being able to experiment with different ratios of signal amplitude and choose the ratio that sounds the best.  Doing that mix live, you've got to get it right the first time.

Brad if he has separate tracks he can fix it. If not he is stuck with things the way they are. I would never do an experiment like this with out having the ability to treat the tracks separately. I dont know what your referring to when you say ( align the sources to with in one sample of each other..) I am not sure I follow you.

Chris

[SparkE!]

Mixing it live, he can align the sources to within one sample of each other.  Mixing in post, he'd be lucky to keep it within about 200 samples.  So, alignment of the signals won't be the issue.  However, getting the proportion of one signal's contribution to the other's could be an issue.  When you are mixing in post, you have the luxury of being able to experiment with different ratios of signal amplitude and choose the ratio that sounds the best.  Doing that mix live, you've got to get it right the first time.

Brad if he has separate tracks he can fix it. If not he is stuck with things the way they are. I would never do an experiment like this with out having the ability to treat the tracks separately. I dont know what your referring to when you say ( align the sources to with in one sample of each other..) I am not sure I follow you.

Chris

200 samples at 44.1 kHz is a little less than 5 ms.   5 ms is 1/2 of a cycle (180 degrees) at 100 Hz and if both sources were perfectly matched in amplitude for the on-axis signals, you'd get comb filtering with nulls at 100, 300, 500, 700... Hz and so on with nulls every odd multiple of 100 Hz.  This is one reason why matrix mixes often are done 70/30 or 60/40 and not at 50/50.  5 ms of skew across a recording is common when you use two separate sources.

If you place the two sets of mics so the both left channel mics are within about 1/3 of an inch of the same distance from their intended sound source, then you'll get less than 1 sample of acoustic delay between those two sources.  Yes, this is only true if both mics are pointed at the intended sound source and you will get phasing issues when you take into account the back pattern of the cardioid mic, but remember, that's precisely what makes a null in the pattern to the rear of a cardioid mic.  By mixing cardiod and omni, you are going to end up with something like a subcardiod pattern if the mics are located very close to one another.  In fact, I think you'll find that this is how some multipattern mics actually make their subcardioid pattern.

[Church-Audio]

Mixing it live, he can align the sources to within one sample of each other.  Mixing in post, he'd be lucky to keep it within about 200 samples.  So, alignment of the signals won't be the issue.  However, getting the proportion of one signal's contribution to the other's could be an issue.  When you are mixing in post, you have the luxury of being able to experiment with different ratios of signal amplitude and choose the ratio that sounds the best.  Doing that mix live, you've got to get it right the first time.

Brad if he has separate tracks he can fix it. If not he is stuck with things the way they are. I would never do an experiment like this with out having the ability to treat the tracks separately. I dont know what your referring to when you say ( align the sources to with in one sample of each other..) I am not sure I follow you.

Chris

200 samples at 44.1 kHz is a little less than 5 ms.   5 ms is 1/2 of a cycle (180 degrees) at 100 Hz and if both sources were perfectly matched in amplitude for the on-axis signals, you'd get comb filtering with nulls at 100, 300, 500, 700... Hz and so on with nulls every odd multiple of 100 Hz.  This is one reason why matrix mixes often are done 70/30 or 60/40 and not at 50/50.  5 ms of skew across a recording is common when you use two separate sources.

If you place the two sets of mics so the both left channel mics are within about 1/3 of an inch of the same distance from their intended sound source, then you'll get less than 1 sample of acoustic delay between those two sources.  Yes, this is only true if both mics are pointed at the intended sound source and you will get phasing issues when you take into account the back pattern of the cardioid mic, but remember, that's precisely what makes a null in the pattern to the rear of a cardioid mic.  By mixing cardiod and omni, you are going to end up with something like a subcardiod pattern if the mics are located very close to one another.  In fact, I think you'll find that this is how some multipattern mics actually make their subcardioid pattern.

Yes your talking about the difference between left and right.. What he wants to do is take a single stereo recorder and plug 4 mics into a mixer and blend them down the sample rate does not factor into the phasing issues.


The phasing issues will be there because although the back of the cardioid mics are 180 degrees out of phase from the front the omni mics are not. So when you combine them together side by side you will have phasing issues.  But dont take my word for it why not build this your self and see if I am right or wrong.. Its pretty simple thing to do. I have done it my self and I have noted the effects. The effects were not pleasant. The best way to make a sub cardioid is simply to block some of the vents on the back of the capsule. There are a few companies that use an omni and a cardioid to make a subcardioid BUT they do not put both mics side by side on the same axis.. If they did they would have a phasing nightmare of a sound.
When they do try and make a cardioid into a sub cardioid using two capsules one method is to place the omni capsule directly behind the cardioid capsule but the vents of the capsule are not facing backwards towards the omni they are facing sideways. Please forgive my very bad drawing Its pretty simple rear lobe of the cardioid 180 degrees out of phase give or take because its dependent on the fundamental + omni rear lobe in phase = phase cancellation and not at a predictable group of frequencies. Because the directionality of a frequency in free space changes as the fundamental gets lower.

Chris

[faninor]

If you place the two sets of mics so the both left channel mics are within about 1/3 of an inch of the same distance from their intended sound source, then you'll get less than 1 sample of acoustic delay between those two sources.  Yes, this is only true if both mics are pointed at the intended sound source and you will get phasing issues when you take into account the back pattern of the cardioid mic, but remember, that's precisely what makes a null in the pattern to the rear of a cardioid mic.  By mixing cardiod and omni, you are going to end up with something like a subcardiod pattern if the mics are located very close to one another.  In fact, I think you'll find that this is how some multipattern mics actually make their subcardioid pattern.

Yes your talking about the difference between left and right.. What he wants to do is take a single stereo recorder and plug 4 mics into a mixer and blend them down the sample rate does not factor into the phasing issues.

The phasing issues will be there because although the back of the cardioid mics are 180 degrees out of phase from the front the omni mics are not. So when you combine them together side by side you will have phasing issues.  But dont take my word for it why not build this your self and see if I am right or wrong.. Its pretty simple thing to do. I have done it my self and I have noted the effects. The effects were not pleasant. The best way to make a sub cardioid is simply to block some of the vents on the back of the capsule. There are a few companies that use an omni and a cardioid to make a subcardioid BUT they do not put both mics side by side on the same axis.. If they did they would have a phasing nightmare of a sound.
When they do try and make a cardioid into a sub cardioid using two capsules one method is to place the omni capsule directly behind the cardioid capsule but the vents of the capsule are not facing backwards towards the omni they are facing sideways. Please forgive my very bad drawing Its pretty simple rear lobe of the cardioid 180 degrees out of phase give or take because its dependent on the fundamental + omni rear lobe in phase = phase cancellation and not at a predictable group of frequencies. Because the directionality of a frequency in free space changes as the fundamental gets lower.

Chris

I'm not certain that the phasing issue of the rear lobe should be that great a concern. Isn't this just what gives the mic the pickup pattern that it has? And whether or not you're mixing live or mixing in post it is impossible to put the rear lobe of the cardioid in phase without taking the front lobe out of phase. I'm not sure what could be gained by mixing in post over mixing live (aside from a greater ability to select the volume of each source) if sound traveling from the source you'd like to record to the front lobe of either microphone is already in phase. In a couple weeks when I have my regular computer with me I will try to do a few tests to see what the actual results of what I'd like to try would be at a variety of frequencies.

[Lil Kim Jong-Il]

Why not filter the mic signals to eliminate the phase issues and then sum them? 

A basic HPF or LPF feeding a buffer is pretty simple.  With complementary pass filters and a summer you could isolate the desired ranges of each mic.   You could run your CSC tracks through a HPF and your CSBs through a LPF. 

[Church-Audio]

Why not filter the mic signals to eliminate the phase issues and then sum them? 

A basic HPF or LPF feeding a buffer is pretty simple.  With complementary pass filters and a summer you could isolate the desired ranges of each mic.   You could run your CSC tracks through a HPF and your CSBs through a LPF. 

Good idea.

[Church-Audio]

If you place the two sets of mics so the both left channel mics are within about 1/3 of an inch of the same distance from their intended sound source, then you'll get less than 1 sample of acoustic delay between those two sources.  Yes, this is only true if both mics are pointed at the intended sound source and you will get phasing issues when you take into account the back pattern of the cardioid mic, but remember, that's precisely what makes a null in the pattern to the rear of a cardioid mic.  By mixing cardiod and omni, you are going to end up with something like a subcardiod pattern if the mics are located very close to one another.  In fact, I think you'll find that this is how some multipattern mics actually make their subcardioid pattern.

Yes your talking about the difference between left and right.. What he wants to do is take a single stereo recorder and plug 4 mics into a mixer and blend them down the sample rate does not factor into the phasing issues.

The phasing issues will be there because although the back of the cardioid mics are 180 degrees out of phase from the front the omni mics are not. So when you combine them together side by side you will have phasing issues.  But dont take my word for it why not build this your self and see if I am right or wrong.. Its pretty simple thing to do. I have done it my self and I have noted the effects. The effects were not pleasant. The best way to make a sub cardioid is simply to block some of the vents on the back of the capsule. There are a few companies that use an omni and a cardioid to make a subcardioid BUT they do not put both mics side by side on the same axis.. If they did they would have a phasing nightmare of a sound.
When they do try and make a cardioid into a sub cardioid using two capsules one method is to place the omni capsule directly behind the cardioid capsule but the vents of the capsule are not facing backwards towards the omni they are facing sideways. Please forgive my very bad drawing Its pretty simple rear lobe of the cardioid 180 degrees out of phase give or take because its dependent on the fundamental + omni rear lobe in phase = phase cancellation and not at a predictable group of frequencies. Because the directionality of a frequency in free space changes as the fundamental gets lower.

Chris

I'm not certain that the phasing issue of the rear lobe should be that great a concern. Isn't this just what gives the mic the pickup pattern that it has? And whether or not you're mixing live or mixing in post it is impossible to put the rear lobe of the cardioid in phase without taking the front lobe out of phase. I'm not sure what could be gained by mixing in post over mixing live (aside from a greater ability to select the volume of each source) if sound traveling from the source you'd like to record to the front lobe of either microphone is already in phase. In a couple weeks when I have my regular computer with me I will try to do a few tests to see what the actual results of what I'd like to try would be at a variety of frequencies.

Again theory is a wonderful thing.. But again when you record with a cardioid mic you are absolutely capturing sound from behind the microphone that back vents are picking up sound that absolutely contributes to the over all sound of the microphone. Want to try an experiment block some of the vents and listen to the difference in sound quality. What is gain by doing things in post is you can rotate the cardioid microphone in time/space by knocking it out of phase say 20degrees or 40 degrees in relation to the omni signal. That's the benefit of separate tracks for all mics. You can manipulate the phase of each mic. With mixing on the fly you can adjust the phase of the stereo signal but that will not fix the phase relationship between two microphones place side by side and mixed to the far right or left, as you would have to with a stereo recording. Control of individual tracks means control of individual phase this means you can change the time/frequency relationship between the capsules.

Another good idea that has been suggested would be the use of hi pass and lo pass filters but filters also introduce phase issues of there own. These could work if you can get the slop past 6 db per octave, if not you will have too much overlap of the frequency bands and still have phasing issues. If you use an active 18db per octave or greater filter you can achieve what you want. You would simply do a fft on each mic find out where the response starts to get back and make that your cross point between the two mics. In effect your creating a two way transducer but your not using speakers your using microphones.

This has been done by AKG with a dynamic microphone and a condenser microphone called the c3000 where they used a cardioid and omni capsule one on top of the other and crossed it over very high around 8-10k and used the omni capsule for sibilance enhancement, because its very hard to get a large diaphragm microphone to have extended frequency response above 10k they used this little trick to make there 3000 microphone a much better mic in the high frequency range with out having to spend a lot of money making a better 1inch diaphragm.

Chris Church

[kuuan]

I had built various small pramps, had plans to build a mixer.
The best preamp in the end is the one of which I had taken out the level control potentiometer and substituted it with a switch for 2 levels. Why? Because all the pots I had bought were too bad and noisy. Panning pots are hardly available, good pots generally cost a fortune, this is why in the end, I never built the mixer.