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Gear / Technical Help => Ask The Tapers => Topic started by: moonchilddave on June 11, 2013, 12:22:55 PM
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Ok, I just recently taped a loud rock concert and am seeing something odd I've never seen before...
AT933 (w/sub-cardioid elements) - concealed in the liner of a hat with one above each ear
Sound Professionals Battery Box (dual level controls with bass roll-off) - bass roll-off turned off
Edirol R-09HR (LIMITER: off, PLUG IN POWER: off, LOW CUT: off, MIC GAIN: L) -used LINE IN and had level set to 39
The audio turned out sub-par. Forgive me if I'm not using proper terms here - but when I pull the audio in to my editing software (Sound Studio 4 for Mac), usually I see the waveforms displayed on the center line for each track with equal amounts above and below the center line. For some reason, this time the main audio still seems to be along the center line with a fair amount dipping farther below it than above it (in the negative? direction). Someone suggested fiddling with the DC offset, but that shits the ENTIRE waveform up or down, moving the center above or below where it should be. Any clue what's going on here? See the attached image.
Also, here is a sample from the beginning of the show: https://soundcloud.com/dave-downin/ttw-nothing (https://soundcloud.com/dave-downin/ttw-nothing)
Thanks for any advice / help!
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My first instinct is a dying battery if it sounds bad and you weren't recording brass horns.
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Yeah, dying battery would be my first guess. Only time(s) I saw an asymmetric waveform were from a dying (battery box) battery.
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Hmmm... I was using a Maha PowerEx 9.6V 230mAh IMEDION "Ready When You Are!" Rechargeable battery. I had charged it the night before and put it in the battery box. I wouldn't think that it would have drained that quick, but I guess it's possible (I pulled the battery and charged it again before I even started to mess with the audio).
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Here's one zoomed in... I notice there seems to be some noise that appears to be riding the waveform... I'm going to have another shot this Thursday as I'm headed out to see Queensryche on their 25th Anniversary Operation Mindcrime tour. Really hoping that turns out a little better than this did.
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Ah, ok... No, that probably was not the loudest point. Here's one of the loudest points, zoomed in as far as possible:
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It sounds bad. See the first post for a sample.
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yep, listening to that sample it sounds like the classic "mics being overloaded by too high an spl" situation.
how close were you to the stacks? i expect queensryche will also be pretty loud, and you've probably not got enough time before then to upgrade your mics, so perhaps just standing a little further back may help. just a thought.
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These mics will absolutely overloud under loud shows with out being wired 3 wire or in some cases with my mod. Running these 2 wire in most cases under a very loud show will get your an overloaded recording. I have found that the mic diaphragm on these mics hits the back plate and causes distortion due to the tiny gap between the two. There is not quite enough compression between the two structures you could change this but blocking one of the ports on the back of the diaphragm it self. But I doubt that it would not have an adverse effect on frequency response of the mics.
So long and the short of it is these mics will overload regardless of battery. But in some cases the 3 wire or my mod might be enough to prevent the oversaturation of the fet and reduce distortion accordingly. In a stack tapping situation where there is a lot of air being moved they may bottom out as described above.
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These mics will absolutely overloud under loud shows with out being wired 3 wire or in some cases with my mod. Running these 2 wire in most cases under a very loud show will get your an overloaded recording.
Bingo!
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Thanks all for the input. It was in a Hard Times Cafe and they had it cranked. I was about 12ft directly back from the stack, and you could FEEL it. Generally, I don't like being that close when taping - but was with friends. At least now I know. Never had that happen to me before. I've taped some loud shows before, and gotten good results, but never been that close. Again, thanks to all for letting me know what went wrong.
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OK, looking at the download that is textbook FET clipping--FETs soft clip asymmetrically, which is what I see in the picture below. The mics were badly overloaded, but probably not because of a low battery--these mics would only draw about 1mA, so a fresh battery should last a week. I mean a week of continuous use, not a week on the shelf fully charged. The mics in that configuration probably aren't capable of that SPL handling.
I'm not quite sure I see what you are looking at... I understand what "clipping" is, where part of the waveform is basically chopped off at the peak. I've never heard of "soft clipping", and I'm not sure I see it either. Could you explain a little of what exactly it is and how to recognize it visually?
BTW, Queensryche was wicked loud, almost seemed louder than this show as I could really feel it. I thought my recording was pretty much going to be a waste, but while I can see a little bit of asymmetry it's not nearly as bad. I did have the levels on the battery box down a bit, and I was a fair bit farther back from the stacks this time, but the whole place was shaking. Are omni & sub-cards more susceptible to this than a card would be? I know I've taped loud shows in the past, but it was usually always with card elements. The sub-cards are a fairly new addition for me.
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OK, looking at the download that is textbook FET clipping--FETs soft clip asymmetrically, which is what I see in the picture below. The mics were badly overloaded, but probably not because of a low battery--these mics would only draw about 1mA, so a fresh battery should last a week. I mean a week of continuous use, not a week on the shelf fully charged. The mics in that configuration probably aren't capable of that SPL handling.
I'm not quite sure I see what you are looking at... I understand what "clipping" is, where part of the waveform is basically chopped off at the peak. I've never heard of "soft clipping", and I'm not sure I see it either. Could you explain a little of what exactly it is and how to recognize it visually?
BTW, Queensryche was wicked loud, almost seemed louder than this show as I could really feel it. I thought my recording was pretty much going to be a waste, but while I can see a little bit of asymmetry it's not nearly as bad. I did have the levels on the battery box down a bit, and I was a fair bit farther back from the stacks this time, but the whole place was shaking. Are omni & sub-cards more susceptible to this than a card would be? I know I've taped loud shows in the past, but it was usually always with card elements. The sub-cards are a fairly new addition for me.
Soft clipping does exist and its just what it says. Soft clipping is a slight bottoming out or topping out of a sine wave. Not total distortion but a slight distortion is called soft clipping. Hard clipping is gross distortion of a waveform.
Also "i've taped loud shows in the past" is a relative term. Loud shows relative to what? With out having a way to measure sound pressure its impossible to know if one show or another was louder. So for example your mics might overload at 114 db and be relatively ok at 110 db and I can tell you its impossible to be able to determine without being able to measure it. So a show you think is loud one day may not be as loud as the show that distorted your mics in the first place. Sound loudness also depends on how its mixed. I can mix things so they appear quite loud and mix things so they don't. Measure both and get the same db levels.
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Are omni & sub-cards more susceptible to this than a card would be? I know I've taped loud shows in the past, but it was usually always with card elements. The sub-cards are a fairly new addition for me.
Yes.
The bass makes up the lion's share of the SPL energy of what you are recording. Since most cardioids have a rolled off response at low frequencies, especially miniature cardioids, they act as a high pass filter and transduce less of that low bass energy into electrical signal. As the pickup pattern is shifted towards omni, the low frequency response of a microphone generally becomes flatter and that means more of that low frequecy energy is converted by the microphone into electrical signal, potentially overloading the microphone itself, or something downstream in the signal chain.
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Sorry to bump an old thread.
Referring to the waveforms shown in the first post, some people attributed it to a dying battery and others to soft clipping. My question is, can a dying battery cause the soft clipping? For example, will a certain mic not clip at all at a given spl when given the right amount of power, but soft clip at the same spl when underpowered (e.g., from a dying battery)? Or can the waveforms shown above always be attributed to soft clipping caused by the spl exceeding what the mic can handle?
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My question is, can a dying battery cause the soft clipping? For example, will a certain mic not clip at all at a given spl when given the right amount of power, but soft clip at the same spl when underpowered (e.g., from a dying battery)?
Yes. It's very common for the levels at which the max SPL clipping limit occurs, and the somewhat lower levels at which any particular measured distortion threshold is encountered, to decrease as the microphone is starved for power.
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My question is, can a dying battery cause the soft clipping? For example, will a certain mic not clip at all at a given spl when given the right amount of power, but soft clip at the same spl when underpowered (e.g., from a dying battery)?
Yes. It's very common for the levels at which the max SPL clipping limit occurs, and the somewhat lower levels at which any particular measured distortion threshold is encountered, to decrease as the microphone is starved for power.
Any insight into the technical reasons for this?
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My question is, can a dying battery cause the soft clipping? For example, will a certain mic not clip at all at a given spl when given the right amount of power, but soft clip at the same spl when underpowered (e.g., from a dying battery)?
Yes. It's very common for the levels at which the max SPL clipping limit occurs, and the somewhat lower levels at which any particular measured distortion threshold is encountered, to decrease as the microphone is starved for power.
I assume that one can't tell whether the cause is a dying battery or a mic that's overloaded, at least in terms of soft clipping, purely from a picture of a waveform like the one in the first post. Is that correct?
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I can't, but educated guesses can be made.
Any insight into the technical reasons for this?
Microphones are like people. If you aren't well fed you can't do hard work well. If you aren't fed at all you'll stop working all together.
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Sorry to bump an old thread.
Referring to the waveforms shown in the first post, some people attributed it to a dying battery and others to soft clipping. My question is, can a dying battery cause the soft clipping? For example, will a certain mic not clip at all at a given spl when given the right amount of power, but soft clip at the same spl when underpowered (e.g., from a dying battery)? Or can the waveforms shown above always be attributed to soft clipping caused by the spl exceeding what the mic can handle?
In the case of a battery box the voltage would have to be pretty low like 3 volts and I have never seen a 9v go that low :) The 933 will overload one of two ways electronically or physically Electronically is a matter of overloading the fet of the mic capsule or the input of the recorder. Both can look the same.... The other physical distortion comes from when the diaphragm hits the backplate that tends to be rather harsh looking. You can also starve the fet of voltage this can have one of two effects lowering amplitude and distortion on the other end of the capsule where its connected. Or distortion in extreme cases by reducing headroom due to the lack of voltage. Based on the waveform I would say this was not a lack of voltage because the whole thing would be distorted or more then a few parts provided that the SPL was constant. I would say its possible this could have been backplate distortion on a transient low end spike. But these are all just guesses.