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Offline Walstib62

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Re: The myth of unity gain
« Reply #15 on: December 26, 2011, 11:20:53 AM »
DSatz,

Thanks for the post. It is sometimes diificult to find a way to explain complex technical terms in "layman's" terms. I think the whole point of the OP was a caution that using the "unity gain" setting of a given recorder does not prevent, in all cases, input stage overload, or clipping. (Often called brickwalling here)

First of all, the term unity gain, as DSatz mentioned, means voltage in=voltage out for a given amplifier. Normally, this term is applied to a SINGLE amplifier stage. The term has been adopted here to mean votage in to recorder=voltage out of recorder. The recorder can (will) have numerous amplifier stages in use. You can still input too high of a voltage to the input stage of a recorder and have a reasonable reading on the record level meters because they are measuring an amplifier stage which is not the input stage. (e.g A/D stage)
Here's a good real world exampe:
You are using mics (brand doesn't matter) into a V3. the line out of the V3 is then connected to the XLR input of a PMD 661. Instead of selecting "line in" the "mic in" is selected and input attenuation is applied, say -12 dB. You set the recorder at 4. You then adjust the levels on the V3 to just below 0dB on the recorder's meter. You have more than likely just overloaded the input stage of the recorder with never seeing a meter indication over 0. This is because the input stage clipped, and the record level control set that clipped signal to a level just below 0, and the meter was happy.

It would be ideal to have 2 sets of meters. One to monitor the input stage, and another to monitor the D/A stage. Then you would know you have the correct settings where they are most critical. You can actually do this if you record to a computer, because most software will allow selection of input signal monitoring as well as output levels. Plus, you can see the waveform as it's recording. I once saw a clipped soundboard waveform that was recording at -10 dB . I knew I had to attenuate the input signal to get it under control. Otherwise, I would not have known until playback. (Music was too loud to monitor correctly with headphones, at least with MY ears)   
 

Offline Gutbucket

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Re: The myth of unity gain
« Reply #16 on: December 26, 2011, 11:48:14 AM »
Good thread!

I think an important issue here is simply one of terminology.

We need a good clear term that everyone can use to describe "the lowest safe setting of the record level control without fear of overload".  That phrase just doesn't roll off the tounge and is too much to type, and the term 'unity gain' has become miss-appropriated around here.  We need a good short term for it.
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Offline Walstib62

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Re: The myth of unity gain
« Reply #17 on: December 26, 2011, 12:42:13 PM »
Good thread!

I think an important issue here is simply one of terminology.

We need a good clear term that everyone can use to describe "the lowest safe setting of the record level control without fear of overload".  That phrase just doesn't roll off the tounge and is too much to type, and the term 'unity gain' has become miss-appropriated around here.  We need a good short term for it.

As someone already said, it might be better to think of it as the "highest safe setting." A lot of folks are using external preamps to "condition" the signal so that the recorder does not add unwanted noise. It seems in some cases that these ext. pre's might be driven a little too high in the hopes that a hotter preamp signal is better. In most cases, with even a resonalbly decent recorder, this is just not the case. Usually the weak link in the noise chain is the mics anyway. Driving even a stellar preamp does not reduce the amount of noise in the signal. It just amplifies that noise cleanly.
While we're there, another often used term is "headroom" This  means how much additional, useable function is available in a given audio circuit,  in terms of gain. For example, the headroom for a preamp is how much additional gain is available for a given signal before clipping occurs. So, if your preamp has 70 dB of clean gain, and the signal going into that preamp is 40 dB, then you have 30 dB of headroom. I have heard some folks say that they listen to a recording and it has a lot of great "headroom" on playback. Or that a given preamp has more headorom than another based on how it sounds on playback. What does that mean???? It is not something that can be heard, it is something that can be measured.

stevetoney

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Re: The myth of unity gain
« Reply #18 on: December 26, 2011, 12:54:56 PM »
Maybe not, because below that point you might just have 0dB gain in the input stage and attenuation after the input stage which doesn't help input stage clipping and may or may not help with ADC clipping.  If in testing you discover that there is some point below which the input is still clipped and just being reduced below 0dBFS (such that you have a signal that is clipped with its flat-top peaks at say -12dBFS), then there is no reason to use those settings--they are giving you a false sense of security.

Jon, please forgive me...I just don't understand your response or the point about false sense of security. 

In the context of most conversations where the 'unity gain' term was used, it was in reference to a setting on a handheld...R-09, R-09HR, M10, PMD-620, and a maybe a couple others in which people (mostly noobs) were trying to gauge where they should set their recorders.  As you know, handheld recorders have one setting only...a control with a correponding numeric range that goes from 0 to X. 

Is it OK or is it not OK to state that the 'highest recommended setting' (or whatever term will be used in place of the formerly incorrecly applied 'unity gain' term) on the numeric scale of the recorder should be Y to avoid distortion, with Y being a value that has been determined to be roughly where a noob should set his recorder so that there is a nearly 100% chance that the recorder won't contribute any distortion? 

Isn't the false sense of security you're talking about removed, for example, by setting an m10 at 3 instead of 4 (because maybe 4 was felt to be the optimal setting because that's where someone incorrectly thought that voltage in = voltage out).
« Last Edit: December 26, 2011, 01:11:20 PM by tonedeaf »

Offline Walstib62

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Re: The myth of unity gain
« Reply #19 on: December 26, 2011, 01:08:20 PM »
Maybe not, because below that point you might just have 0dB gain in the input stage and attenuation after the input stage which doesn't help input stage clipping and may or may not help with ADC clipping.  If in testing you discover that there is some point below which the input is still clipped and just being reduced below 0dBFS (such that you have a signal that is clipped with its flat-top peaks at say -12dBFS), then there is no reason to use those settings--they are giving you a false sense of security.

Jon, please forgive me...I just don't understand your response or the point about false sense of security. 

In the context of most conversations where the 'unity gain' term was used, it was in reference to a setting on a handheld...R-09, R-09HR, M10, PMD-620, and a maybe a couple others in which people (mostly noobs) were trying to gauge where they should set their recorders.  As you know, handheld recorders have one setting only...a control with a correponding numeric range that goes from 0 to X. 

Is it OK or is it not OK not to state that the 'highest recommended setting' (or whatever term will be used in place of the formerly incorrecly applied 'unity gain' term) on the numeric scale of the recorder should be Y to avoid distortion, with Y being a value that has been determined to be roughly where a noob should set his recorder so that there is a nearly 100% chance that the recorder won't contribute any distortion? 

Isn't the false sense of security you're talking about removed, for example, by setting an m10 at 3 instead of 4 (because maybe 4 was felt to be the optimal setting because that's where someone incorrectly thought that voltage in = voltage out).
Gnerally speaking, yes, but not in all cases. For example, say you have a screaming hot input to the deck. You could set the record level at at 1 to just get below 0 dB, with no clip indicators flashing. There is almost a 100% chance that the signal is clipped all to hell at the input stage. The real take away here is that if you are pushing the recorder input down and pushing the external preamp signal up then you are increasing the chances of overloading the recorder's input.
Buddha said it best, "take the middle path"
« Last Edit: December 26, 2011, 01:11:38 PM by Walstib62 »

Offline ScoobieKW

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Re: The myth of unity gain
« Reply #20 on: December 26, 2011, 01:11:43 PM »
Next time I'm making a bootleg, I'll make sue the knob thingy is set below unity gain.

;) vs.

Next time I record a show, I'll set my input levels lower to avoid clipping. If that doesn't work, I'll attenuate the input to avoid overloading the input stage.

While inaccurate language works, accurate terminology increases the chance of accurate communication, and makes learning easier
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stevetoney

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Re: The myth of unity gain
« Reply #21 on: December 26, 2011, 01:26:47 PM »
While inaccurate language works, accurate terminology increases the chance of accurate communication, and makes learning easier

I don't disagree with the first portion of your statement, but the part in bold is your humble opinion.  Analogies and non-techical language are very helpful and needed in many situations to help communicate with people that don't have a similar background or technical understanding.

While I'm finally getting an understanding of what several people are trying to communicate, I personally have found nothing 'easy' about the accurate terminology that's been used in this thread.

Just sayin...
« Last Edit: December 26, 2011, 01:31:17 PM by tonedeaf »

Offline Will_S

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Re: The myth of unity gain
« Reply #22 on: December 26, 2011, 01:38:10 PM »

Is it OK or is it not OK to state that the 'highest recommended setting' (or whatever term will be used in place of the formerly incorrecly applied 'unity gain' term) on the numeric scale of the recorder should be Y to avoid distortion, with Y being a value that has been determined to be roughly where a noob should set his recorder so that there is a nearly 100% chance that the recorder won't contribute any distortion? 


I think you're still getting it backward, as are a few other folks in this thread.  From the point of avoiding distortion (at least, the kind that wouldn't be given away by the level meters indicating clipping), there is no "highest safe" setting.  There may be a higher than optimal from an added noise standpoint, but the problem is when folks turn the recording level down to accommodate an overly hot input.  There are many recorders that allow you to reduce the recording level so that a too-hot input signal (i.e., one the recording deck simply cannot accommodate - one that needs to be attenuated BEFORE it even hits the recorders input) gives levels on the dB meters that look ok, but in fact is simply clipping below full scale because it is overloading something in the deck upstream of the level meters, and that overdriven stage is simply passing on a clipped wave form that later stages are boosting or attenuating, but cannot unclip.

So what you need to look for is the LOWEST setting.  If you need to set your recorder BELOW that to avoid clipping on the meters, your input signal is too hot, and turning down your recording levels just gives you a clipped waveform that has been shrunk down.
« Last Edit: December 26, 2011, 07:06:28 PM by Will_S »

Offline Walstib62

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Re: The myth of unity gain
« Reply #23 on: December 26, 2011, 02:19:59 PM »
Jon,

Thanks for the very detailed post. I was just trying to make a point of how the term is misunderstood and putting it in very (over) simplified terms. I'm sure your values are correct. I was just throwing numbers out there for example, to sort of illustrate the concept. I should not have thrown random numbers out without checking the accurcy. I don't have that stuff on my brain anymore. I have to look it up. Anyway thanks for the correction.

Offline Todd R

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Re: The myth of unity gain
« Reply #24 on: December 27, 2011, 12:19:16 PM »
Lots of good info in this thread, thanks to DSatz for a well-reasoned and informative post to get it started.

I would like to amplify on something Will brought up:  DSatz has explained how "unity gain" is not the same thing as what is being called minimum level setting (or more clearly to me, maximum input level), but Will also noted that many here are assuming unity gain is great because it means like dood, you just have a straight wire then, man.

That is also a huge myth, perhaps not as dangerous as the maximum input level myth that DSatz brings up, but I'd guess more widespread here.  The idea is that if you can find unity gain on your recorder, then you're just recording exactly what you preamp put out, and you haven't changed the signal at all -- no different than having just a straight wire.

Except that it isn't -- you don't have effectively a straight wire just because you are at unity gain.  The recorder's signal path doesn't change at all, so if you're going through a series of passive attenuators and active gain stages, you're still going through all those same signal path elements regardless of where you have your gain set.  Whether or not it will sound better being at unity gain probably depends on a lot of design factors and parts used, but unity gain is not some magical place where you have effectively replaced the entire signal path with a straight wire.
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Recorders:  Sound Devices MixPre-6, Sony PCM-M10, Zoom H4nPro

Offline Todd R

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Re: The myth of unity gain
« Reply #25 on: December 27, 2011, 12:34:12 PM »
Also, particularly to Steve and perhaps others: I think you've got some confusion on the issue that DSatz and Will have been talking about, since like Will said, it seems you have it backwards.

You've got examples saying that if you have distortion at level setting 4, then the solution is to keep no higher than setting 3.  That isn't the issue here, and it won't solve the problem, it will only mask it since you won't be seeing digital clipping by trying to go above 0dbFS.

You could have 3 kinds of distortion:  signal input is too high for the max input of the recorder, output signal from the active gain stage is higher than the gain stage can provide (output distortion from the analog gain stage), or distortion from going above 0dbFS when converting to a digital signal.

In a well-designed recorder, you wouldn't ever get the second kind of distortion:  it would not be possible to get into the output distortion realm without already going over 0dbFS after your A>D conversion stage.  This kind of distortion though would be an area where you are getting distortion at level setting 4, but it is resolved if you dial it back to level setting 3.  So that would be a case of needing a max level setting, but again this shouldn't happen if the recorder is designed well.

The third type of distortion is one we all are familiar with and try to avoid:  don't go into the realm of digital clipping by trying to exceed 0dbFS.

The discussion mainly in this thread is about input level distortion:  you are feeding a signal that is too hot for the recorder to handle without distortion.  And really it isn't so much about the minimum gain setting, in the sense that you can avoid this distortion by changing your gain setting.  It is that if you need to go below a certain gain setting to avoid going over 0dbFS, then you know you are going to have input distortion.  Thus, it isn't really a minimum gain setting that is the issue, it is that there is a maximum input level the recorder can take.  The minimum input level setting is just an indication that you will be exceeding that maximum input level.

The gain setting you use at that point will not solve your problem, it will only indicate that you have a problem.  The only solution as Will notes is to use an external attenuator to bring that signal down before it gets to the inputs of your recorder.

Hopefully that clears it up a bit more.
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Re: The myth of unity gain
« Reply #26 on: December 27, 2011, 01:03:09 PM »
Lots of good info in this thread, thanks to DSatz for a well-reasoned and informative post to get it started.

I would like to amplify on something Will brought up:  DSatz has explained how "unity gain" is not the same thing as what is being called minimum level setting (or more clearly to me, maximum input level), but Will also noted that many here are assuming unity gain is great because it means like dood, you just have a straight wire then, man.

That is also a huge myth, perhaps not as dangerous as the maximum input level myth that DSatz brings up, but I'd guess more widespread here.  The idea is that if you can find unity gain on your recorder, then you're just recording exactly what you preamp put out, and you haven't changed the signal at all -- no different than having just a straight wire.

Except that it isn't -- you don't have effectively a straight wire just because you are at unity gain.  The recorder's signal path doesn't change at all, so if you're going through a series of passive attenuators and active gain stages, you're still going through all those same signal path elements regardless of where you have your gain set.  Whether or not it will sound better being at unity gain probably depends on a lot of design factors and parts used, but unity gain is not some magical place where you have effectively replaced the entire signal path with a straight wire.

Great summary of the myth...I can see where there have been some wrong conclusions along the way.

For me - some of my misconceptions come from using standalone A/D converters with no knobs...

Im can remember the term being used here in the jb3 heyday - as people were trying to interface various rigs with it.

The whole "lowest safe setting" and "straightwire" business seem to be subsequent conclusions.

Im as guilty as anyone in using the term - but I surely never meant to imply any level of "safety" - just a good, relatively safe, starting point for someone trying to get a feel for new gear.

I've read all the replies in this thread - and still - I dont see why it isnt a good piece of general advice for a guy who just got some new gear and is trying to put it through the paces?

Offline Gutbucket

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Re: The myth of unity gain
« Reply #27 on: December 27, 2011, 01:03:49 PM »
I've often wondered why manufacturers of inexpensive recorders don't set the input-stage gain controls up as described in this thread.  Why should the user have to use test tones to find the lowest input gain setting before overloading the input-stage?  Why don't manufacturers simply use that point as the starting point of the input gain adjustment so that the input stage will not clip before the ADC?

The only reason I can think of for allowing the input setting range to go lower would be to allow the user to fade-in or fade-out using the input gain control.  Do users actually do that with these recorders, even in the 'larger real world market' outside of TS?
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runonce

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Re: The myth of unity gain
« Reply #28 on: December 27, 2011, 01:16:56 PM »
I've often wondered why manufacturers of inexpensive recorders don't set the input-stage gain controls up as described in this thread.  Why should the user have to use test tones to find the lowest input gain setting before overloading the input-stage?  Why don't manufacturers simply use that point as the starting point of the input gain adjustment so that the input stage will not clip before the ADC?

The only reason I can think of for allowing the input setting range to go lower would be to allow the user to fade-in or fade-out using the input gain control.  Do users actually do that with these recorders, even in the 'larger real world market' outside of TS?

"Why should the user have to use test tones to find the lowest input gain setting before overloading the input-stage?"

Ok - now im confused again - that sounds totally counter-intuitive...lowest gain...before overloading - how does low input gain cause overloading?

Offline Todd R

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Re: The myth of unity gain
« Reply #29 on: December 27, 2011, 01:18:13 PM »

I've read all the replies in this thread - and still - I dont see why it isnt a good piece of general advice for a guy who just got some new gear and is trying to put it through the paces?

I don't think it is necessarily a bad thing or bad advice.  And it does seem logical to assume that a recorder will behave "normally" (eg, not distort) if you have levels set somewhere near unity gain.

But it is important to understand what you're getting and not getting.  Not getting as DSatz points out some guarantee that you won't have distortion, for example.  Also, all the talk about unity gain does then seem to lead some to freak out that they have their recorder set at 7 when unity is 4, especially when there is every likelihood (or at least possibility) that it will sound exactly the same when set at 7 (and the external preamp adjusted accordingly), than it would when set to 4 (with a slightly different gain setting on the external preamp).
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