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Author Topic: Digital Resoloution and bit depth  (Read 7156 times)

Nika Aldrich

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Re: Digital Resoloution and bit depth
« Reply #15 on: February 11, 2005, 09:14:33 pm »

Whew!  That explanation worked!  Let us know if you have any more questions.

Nika
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howlback

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Re: Digital Resoloution and bit depth
« Reply #16 on: February 12, 2005, 01:45:55 am »

Nika Aldrich wrote on Fri, 11 February 2005 11:12

 - if I have a signal whereby the noise added because of sampling at 16 bits was so low that it could not be detected, what would be the benefit of adding more quantization steps?  Again, remember, the adding of quantization steps only lowers the error-signal (noise) in the system.  If I can't detect that noise anyway, what is the benefit of reducing it?



Nika, I like your explanation.

But as you know, music isn't sine tones.  The point of my previous post, was that you can hear the error-signal (noise) you are talking about with some kinds of music in a 16 bit system.  Sometimes low amplitude signals fade below the noise-floor of the system, then reappear again.  Things like cymbals, or the reverb of a room ringing out.  This is what is called quantization distortion.  

Nika, I know you know this, but I am just writing it here for clarity's sake.

The audibility of this quantization distortion is eliminated by using dither.  This dither decorrelates the quantization error from the low amplitude music signal.  It gives us a better representation of the signal to our ear, by randomizing the distortion, making it noise again.  Effectively, dither gives us a perceived dynamic range of 115 dB.  Probably close enough to our ear's theoretical 120 dB dynamic range.

However, we could also avoid quantization distortion by recording in 24 bits.  This would give us 144 dB of dynamic range.  There might be other reasons to record in 20 or 24 bit too.  I think this is what Nika was getting at when he wanted Peter to think about signal to noise ratio.

Other books I've read state that 20 and 24 bit systems  do not dither the input signal.  I wonder if your more recent investigation have found this to be true Nika?

Best Wishes,

Kent
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Yannick Willox

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Re: Digital Resoloution and bit depth
« Reply #17 on: February 12, 2005, 08:16:58 am »

If I'm correct 20 bit convertors and above are self-dithering, due to the inherent input noise (20 bit = 120 dB dynamic range, and I've never seen any AD convertor with less noise).
So there is no necessity to add (digital) noise.

If I'm correct, quantizing distortion was mainly an issue with 16 bit convertors that were not really 16 bit convertors ?
I thought a correctly dithered (true) 16 bit convertor has dynamic range beyond 96 dB, so we'd never be able to hear quantisation distortion ?
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jfrigo

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Re: Digital Resoloution and bit depth
« Reply #18 on: February 12, 2005, 01:55:00 pm »

Yannick Willox wrote on Sat, 12 February 2005 05:16

If I'm correct 20 bit convertors and above are self-dithering, due to the inherent input noise (20 bit = 120 dB dynamic range, and I've never seen any AD convertor with less noise).
So there is no necessity to add (digital) noise.


There are a few converters with less noise than -120. The original Lavry AD122 was so named because of its 122dB range, and the newest gold AD has noise at -127dB.

As for self dithering, while the end user wouldn't add "digital noise" (dither) while originally converting at 20bits (unless using a 24 bit converter), you can't always count on ambient or electrical noise to provide self-dithering as you can't be sure it has enough high frequency component to be completely effective.

Kent wrote:
Quote:

But as you know, music isn't sine tones.

But remember, it can be broken down into component sine waves - fundamentals and overtones/harmonics in very complex and constantly varying combinations. Learning how a digital system behaves with individual sine waves is essentially learning how it behaves with complex waveforms like music.

Quote:

However, we could also avoid quantization distortion by recording in 24 bits. This would give us 144 dB of dynamic range.


We don't avoid it, rather it exists below our ability to perceive it. With strings of processors running at 24 bits in DAWs, truncation distortion can conceivably become an audible problem again. Ideally, I prefer even to dither to 24 bits from 48 bit processors (but not all processors & plugs offer the option unfortunately).
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howlback

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Re: Digital Resoloution and bit depth
« Reply #19 on: February 12, 2005, 03:16:44 pm »

Quote:



"Quote:

However, we could also avoid quantization distortion by recording in 24 bits. This would give us 144 dB of dynamic range."


We don't avoid it, rather it exists below our ability to perceive it. With strings of processors running at 24 bits in DAWs, truncation distortion can conceivably become an audible problem again. Ideally, I prefer even to dither to 24 bits from 48 bit processors (but not all processors & plugs offer the option unfortunately).


Thanks for correcting me Jay.  I wrote something false.  You wrote what I meant to say.  

-peace

Kent
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jfrigo

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Re: Digital Resoloution and bit depth
« Reply #20 on: February 12, 2005, 03:55:24 pm »

kent walker wrote on Sat, 12 February 2005 12:16


Thanks for correcting me Jay.  I wrote something false.  You wrote what I meant to say.  


I knew what you meant. I just wanted to make sure any lurkers also knew.  Smile
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Nika Aldrich

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Re: Digital Resoloution and bit depth
« Reply #21 on: February 12, 2005, 06:14:02 pm »

kent walker wrote on Sat, 12 February 2005 01:45

But as you know, music isn't sine tones.  The point of my previous post, was that you can hear the error-signal (noise) you are talking about with some kinds of music in a 16 bit system.


That's dangerous territory and needs to be qualified.

Quote:

Sometimes low amplitude signals fade below the noise-floor of the system, then reappear again.  Things like cymbals, or the reverb of a room ringing out.  This is what is called quantization distortion.  


Not necessarily.  It is quantization distortion if there is not enough noise present to decorrelate the quantization error from the valid signal.  I think you touch into some of this in the post below.  Of course, as I said in my post, we were just going to "accept" for the time being that the quantization error is noise.  This is not inherently so.  It is indeed distortion unless noise at an adequate amplitude is present prior to the conversion process.  This delves into the area of "stochastics," an area that we aren't quite ready to go into with respect to the poster who framed the question, but certainly open for dialogue beyond his initial question.

Quote:

Effectively, dither gives us a perceived dynamic range of 115 dB.  Probably close enough to our ear's theoretical 120 dB dynamic range.


Uh?  On a 16 bit signal during recording?  Not actually - the presence of dither only gives us perceived dynamic range of 16 bits, unless colored dither or noise-shaping is used - which it should never be in a recording environment unless the material is going straight to disk.  Those types of dither (or noiseshaping) that are designed specifically to add to the perceived dynamic range cause problems if used in the recording process.

Quote:

Other books I've read state that 20 and 24 bit systems  do not dither the input signal.  I wonder if your more recent investigation have found this to be true Nika?



I'm not sure what you mean?  Are you talking about the converters?  Most 16 bit A/D converters are actually 24 bit converters that dither down to 16 bits internally.

Nika
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howlback

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Re: Digital Resoloution and bit depth
« Reply #22 on: February 12, 2005, 09:52:07 pm »

Quote:

 
Kent wrote previously:

Effectively, dither gives us a perceived dynamic range of 115 dB. Probably close enough to our ear's theoretical 120 dB dynamic range.


Uh? On a 16 bit signal during recording? Not actually - the presence of dither only gives us perceived dynamic range of 16 bits, unless colored dither or noise-shaping is used - which it should never be in a recording environment unless the material is going straight to disk. Those types of dither (or noiseshaping) that are designed specifically to add to the perceived dynamic range cause problems if used in the recording process.

Well, this is what Bob Katz writes in his mastering book anyway.  I've never read the claim that a 16 bit system can give a perceived dynamic range of 115 dB anywhere else, but I bet he has a source or a reason for saying that, maybe it is the noise shaping thing.  Thought I would check it against your findings.

Quote:


Kent wrote previously:

Other books I've read state that 20 and 24 bit systems do not dither the input signal. I wonder if your more recent investigation have found this to be true Nika?


I'm not sure what you mean? Are you talking about the converters? Most 16 bit A/D converters are actually 24 bit converters that dither down to 16 bits internally.


I am seeking your opinion here as to whether or not you think there is any possible value in adding dither to 24 bit audio, even when not changing the bit depth.  This is an option with some DAWs and digital consoles.  Sometimes the implementation is confusing.  E.G. the Sony DMX-R100 console has an option on its 2 track output to dither 24 bits out of the console with either triangular or rectangular dither, but you can only get 24 bits out anyway (not greater bit depth).  I don't know what the internal processing is...  

Are any 24 bit converters using dither?  I know many don't.  I am familliar with the limitations of dynamic range being 120 -something dB.  Just curious if any designers have found it worthwhile to add a little dither anyway.

-Kent

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Bob Olhsson

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Re: Digital Resoloution and bit depth
« Reply #23 on: February 12, 2005, 10:45:55 pm »

Kent, many processes create more than 24 bits and those could benefit from being dithered to 24 bits because it will slow the buildup of distortion from successive processes.

I look at this as being a matter of "it couldn't possibly hurt" because the dither will be less audible than the distortion from not dithering. I'll grant that in many cases neither one is likely to be audible but I think it's silly to not use 24 bit dither when it's available.

It's also important to understand that dither does not hide distortion, it eliminates it.

howlback

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Re: Digital Resoloution and bit depth
« Reply #24 on: February 12, 2005, 11:10:57 pm »

Bob Olhsson wrote on Sat, 12 February 2005 22:45

Kent, many processes create more than 24 bits and those could benefit from being dithered to 24 bits because it will slow the buildup of distortion from successive processes.

I look at this as being a matter of "it couldn't possibly hurt" because the dither will be less audible than the distortion from not dithering. I'll grant that in many cases neither one is likely to be audible but I think it's silly to not use 24 bit dither when it's available.

It's also important to understand that dither does not hide distortion, it eliminates it.

Thanks Bob,

I understand the importance of dithering when changing bit depths and get that we are not hiding distortion, but what is the purpose of eliminating distortion that we can't hear?  Is that still distortion?  The sony implementation is really confusing because they don't offer any choice to dither or not when taking 24 bits from other digital outs, only the program bus.  Does this mean that they are dithering those outputs?  I honestly don't know for certain.  Why would they give a choice for one output but not another?

I wonder what kinds of dither guys working with DVD-A are using.   Jay might know, you too Bob, I know you are a sonic king.

-Best wishes,

KEnt
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Nika Aldrich

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Re: Digital Resoloution and bit depth
« Reply #25 on: February 12, 2005, 11:58:46 pm »

kent walker wrote on Sat, 12 February 2005 21:52

Well, this is what Bob Katz writes in his mastering book anyway. I've never read the claim that a 16 bit system can give a perceived dynamic range of 115 dB anywhere else, but I bet he has a source or a reason for saying that, maybe it is the noise shaping thing. Thought I would check it against your findings.


OK, that's the noise-shaping issue, and it should never be done in the A/D conversion process unless you are going straight to disk.  If it is going to be processed at all the noise-shaped material will bleed around and potentially cause more damage than good.  A 16 bit system can indeed yield as much as even 150dB dynamic range (in certain frequency bands) but this should only be done on the final pass to 16 bits.

Quote:

I am seeking your opinion here as to whether or not you think there is any possible value in adding dither to 24 bit audio, even when not changing the bit depth.


If the bit depth is being reduced then you dither - period.  So if you're going internally from a 32 bit numerical nomenclature to 24 bits on output you dither - period.  If you are going from 24 bits to 16 bits you dither - period.  If you are going from 48 bit to 24 bit you dither - period.

Quote:

Are any 24 bit converters using dither?


Absolutely - it is inherent by means of the thermal noise from resistors, etc.  They don't have to "add" dither - it is there by default.

Having said this, one thing they do to decouple predictable error is they actually use multiple converters simultaneously and then randomly vary, sample by sample, as to which is in use and what weight it is given.  This removes any predictable error from any given converter by randomly toggling between the various options.  It is dither, in a sense - the addition of random noise (because we randomly toggle between various components) in order to eliminate the distortion caused by a single device's error.  It really is dither, though a bit removed from our normal means of discussing it.  This, by the way, was the solution to the 1 bit problem we run across with DSD - the industry abandoned DSD for traditional PCM converters in 2000 or so because it was better to randomly vary the various components in the converter's design.  This can't be done in the way I describe with DSD in the way that Sony has codified the format, so this method of removing the predictable error in the system is reserved for exclusively "traditional PCM" systems.  

Nika
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Nika Aldrich

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Re: Digital Resoloution and bit depth
« Reply #26 on: February 13, 2005, 12:04:02 am »

kent walker wrote on Sat, 12 February 2005 23:10

I understand the importance of dithering when changing bit depths and get that we are not hiding distortion, but what is the purpose of eliminating distortion that we can't hear?


Further processing or other variables in the listening system may expose it.

Quote:

Is that still distortion?


Absolutely!  Just because it falls below the noise floor does not mean it isn't distortion!

Quote:

I wonder what kinds of dither guys working with DVD-A are using.


All I can tell you is that if they are producing 24 bit results then the choice of dither is much less significant.  They're probably using 2 LSB TPDF dither.  Either way, a very wide variety of options can be used here without audibly affecting the signal.  One could argue that in this final pass they could probably even get away with no dither because in typical listening environments the distortion will never get loud enough to hear.  It really makes a difference in intermediate processing where the buildup of repeated truncation can cause problems, and in dithering to 16 bits where we need all of the dynamic range we can get.  Yes, it SHOULD be done everywhere bit reduction is done, but if the pass from 48 to 24 is the final pass - no further processing to be done - then I can imagine they audibility would be moot.

Nika
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cerberus

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Re: Digital Resoloution and bit depth
« Reply #27 on: February 13, 2005, 12:08:24 am »

Could we be clearer about the difference between noise and distortion here ?  
I understand quantization error as "truncation distortion"; which can be mitigated by adding "dither noise". So I have failed to absorb the part of Nika's lesson regarding quantization errors = [random or uncorrelated] noise; which does not make sense to me when it is examined outside of the context it was presented in, where it makes perfect sense to me.  There was a caveat about whether quantization error is really correlated to the signal or not. And the rest of the explanation seemed to hinge on the proof that it's "noise". I guess I am hung up on that.

Nika wrote

Of course, as I said in my post, we were just going to "accept" for the time being that the quantization error is noise. This is not inherently so. It is indeed distortion unless noise at an adequate amplitude is present prior to the conversion process. This delves into the area of "stochastics," an area that we aren't quite ready to go into with respect to the poster who framed the question, but certainly open for dialogue beyond his initial question.


Well now that I've read further down this page; I guess I am not the only one who has a slight problem "accepting".... If stochastics can be explained to a lay person, please try.

Also I have found that dithering 32 bit float signals to 24 bits and then mixing them together gives a very audible effect from the dither. Imagine transferring 100 32 bit tracks to a 24 bit system.. Do you want to hear 100 dithers ?  My answer after trying the strategy is: no. (...or I used the wrong dither)

cerberus

Nika Aldrich

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Re: Digital Resoloution and bit depth
« Reply #28 on: February 13, 2005, 12:49:16 am »

cerberus wrote on Sun, 13 February 2005 00:08

Could we be clearer about the difference between noise and distortion here ?


If you add a signal to another signal and it is related, or correlated to the original signal it is distortion.  This means that as the original signal changes in some way the added signal changes with it in some way.  If the behavior of the new signal is completely unrelated to the original signal then it is noise.  It is said to be "uncorrelated" error.
 
Quote:

I understand quantization error as "truncation distortion"; which can be mitigated by adding "dither noise".


If there is no random behavior in the original waveform and we quantize it then the amplitude of the quantization error indeed is directly related to the amplitude of the signal.  This is distortion.  The error signal is correlated to the original signal.

If, however, there is noise added to the original waveform prior to quantizing (and at significant enough amplitude - let's say a couple of quantization steps in amplitude) then the quantization error after sampling will no longer be related to the signal itself and will instead be related to the random noise that we added first.  Therefore, this quantization error is no longer correlated - it is no longer distortion.  Instead it is completely random - determined by the (also) random noise that was added in the first place.  This turns the quantization distortion into quantization noise.  

Of course, if the signal itself has enough noise present from natural sources then it doesn't have to be added first.  So if we just take a signal and record it in a noisy room with a noisy mic and a noisy pre, through some cables to a noise front-end of a converter then the natural noise is enough to decouple the quantization error from the signal itself.  

Therefore my lesson all holds, but I simply did not want to venture into having to explain that the quantization error is only random if sufficient noise is present in the signal.   This is why I asked him to just "accept" that the quantization error was random - if the recording is done properly it is.  For the sake of the original poster, however, we just wanted to go one step at a time.

Quote:

If stochastics can be explained to a lay person, please try.


That's it.  Stochastics is, in layman's terms, the study of random behavior.  That's what we touched on above.  We have to ensure random behavior is present (and at sufficient amplitude) before we can ensure that quantization error is random.

Nika
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jfrigo

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Re: Digital Resoloution and bit depth
« Reply #29 on: February 13, 2005, 01:16:47 am »

kent walker wrote on Sat, 12 February 2005 20:10


I wonder what kinds of dither guys working with DVD-A are using


There are many who feel that flat dither (TDPF) is the most neutral, and given the low level of it in 24 bit systems, additional noise shaping seems unnecessary. When noise shaping is called for or preferred by a mastering engineer, there are several that are used. POW-r is very popular. There are actually three choices in the POW-r scheme. In other instances a curve from a particular converter or SRC box will be used. Lavry and Weiss both have SRC boxes that are common in mastering studios and the curves in those are sure to be used (Weiss offers POW-r as well). Near-Nyquist implimentations seem to be out of vogue at the moment, though some still use them. These include Apogee UV22 and POW-r type 1. Plenty of other dithers are used as well, but these are some popular ones.
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