OK, I don’t want to diverge too much from the practical to the theoretical, so
I’m going to run down what is usual, not what is possible, because it narrows
the field of discussion.
Most people I know are using recording systems that bussing audio at 32-bit
float, minimum, and use 64-bit float calculations in plug-ins and significant
processing. They may still be using 24-bit audio tracks on disk, but for the
most part they are recorded and are dithered one way or another (primarily
gaussian noise in the recording process). They may bounce things to tracks to
free processor cycles. I think in large majority of cases, these are
self-dithered, but even if it doesn’t happen for some, I don’t think it will
impact the audio. And if people are worried about it, I don’t understand why
they aren’t using 32-bit float files, as I think most people have that choices
these days.
Some of the more hard core will send audio out to a convertor (therefore
truncated at 24-bit), and back in. Again, I think the vast majority of cases,
these will self dither, but then there’s the fact error is at a very low level,
will get buried in the thermal noise of the electronics, etc.
Maybe I left out some other good ones, but to cut it short, yes, I’m mainly
talking about final mixes. At 24-bit, that often goes to someone else to
master. The funny thing is that some mastering engineers say “only dither
once!”, and they want to be the one doing it. Others point out that they may
want to mess with the dynamic range and boost frequencies, and any error from
not dithering 24-bit will show up in…you know, the stereo imaging, depth, etc.
I think it would be exceptional to actually have truncation distortion of
significant duration, except for potential situations with unusual fades, so
I’m not worried about saying don’t dither 24-bit, even heading to a mastering
engineer (but again, do it if you want, it’s just no big deal for final
outputs–in contrast to the pain in the rear it is to do it at every point for
the items I mentioned in previous paragraphs).
Down the more theoretical paths, I’ve had people argue that this is a big deal
because things like ProTools 56k plug-ins need to be dithered internally…but
why argue legacy stuff that “is what it is”, and secondly, these people usually
don’t think through how many 24-bit truncations occur in a 56k algorithm, and
you only have so many cycles. The other thing I sometimes get is the specter of
the cumulative effect (but what if you have so many tracks, and feedback,
and…)—but it seems to me that the more of this you get going on, to approach a
meaningful error magnitude, the more it’s jumbled up in chaos and the less easy
it is for your ear to recognize it as “bad”.
> On Feb 9, 2015, at 7:54 AM, Vicki Melchior <vmelch...@earthlink.net> wrote:
>
> Nigel, I looked at your video again and it seems to me it's confusing as to
> whether you mean 'don't dither the 24b final output' or 'don't ever dither at
> 24b'. You make statements several times that imply the former, but in your
> discussion about 24b on all digital interfaces, sends and receives etc, you
> clearly say to never dither at 24b. Several people in this thread have
> pointed out the difference between intermediate stage truncation and final
> stage truncation, and the fact that if truncation is done repeatedly, any
> distortion spectra will continue to build. It is not noise-like, the peaks
> are coherent peaks and correlated to the signal.
>
> You don't say in the video what the processing history is for the files you
> are using. If they are simple captures with no processing, they probably
> reflect the additive gaussian noise present at the 20th bit in the A/D,
> based on Andy's post, and are properly dithered for 24b truncation. My
> point is that at the digital capture stage you have (S+N) and the amplitude
> distribution of the S+N signal might be fine for 24b truncation if N is
> dither-like. After various stages of digital processing including non-linear
> steps, the (S+N) intermediate signal may no longer have an adequate amplitude
> distribution to be truncated without 24b dither.
>
> I think the whole subject of self dither might be better approached through
> FFT measurement than by listening. Bob Katz shows an FFT of truncation
> spectra at 24b in his book on 'Itunes Music, Mastering for High Resolution
> Audio Delivery' but he uses a generated, dithered pure tone that doesn't
> start with added gaussian noise. Haven't thought about it but I can imagine
> extending his approach into a research effort.
>
> Offhand I don't know anything that would go wrong in your difference file ("
> ...if the error doesn't sound wrong). It's a common method for looking at
> residuals.
>
> Vicki
>
>
> On Feb 8, 2015, at 6:11 PM, Nigel Redmon wrote:
>
>>> Beyond that, Nigel raises this issue in the context of "self-dither”...
>>
>> First, remember that I’m the guy who recommended “always” dithering 16-bit
>> (no “always” as in “alway necessary”, but as in “do it always, unless you
>> know that it gives no improvement”), and to not bother dithering 24-bit. So,
>> I’m only interested in this discussion for 24-bit. That said:
>>
>>> ...In situations where there is a clear external noise source present,
>>> whether the situation is analog to digital conversion or digital to digital
>>> bit depth change, the external noise may, or may not, be satisfactory as
>>> dither but at least it's properties can be measured.
>>
>> For 24-bit audio, could you give an example of when it’s likely to not be
>> satisfactory (maybe you’ve already given a reference to determining
>> “satisfactory")? Offhand, I’d say one case might be with extremely low
>> noise, then digitally faded such that you fade the noise level below the
>> dithering threshold while you still have enough signal to exhibit truncation
>> distortion, and the fade characteristics allow it to last long enough to
>> matter to your ears—if we weren’t talking about this distortion being down
>> near -140 dB in the first place. I’d think that, typically, you’d have
>> gaussian noise at a much higher level that is needed to dither 24-bit; that
>> could change with digital processing, but I think that in the usual
>> recording chain, it seems pretty hard to avoid for your "analog to digital
>> conversion” case.
>>
>> I’m still interested in what you have to say about my post yesterday (“...if
>> the error doesn’t sound wrong to the ear, can it still sound wrong added to
>> the music?”). Care to comment?
>>
>>
>>> On Feb 8, 2015, at 8:09 AM, Vicki Melchior <vmelch...@earthlink.net> wrote:
>>>
>>> I have no argument at all with the cheap high-pass TPDF dither; whenever it
>>> was published the original authors undoubtedly verified that the moment
>>> decoupling occurred, as you say. And that's what is needed for dither
>>> effectiveness. If you're creating noise for dither, you have the option
>>> to verify its properties. But in the situation of an analog signal with
>>> added, independent instrument noise, you do need to verify that the
>>> composite noise source actually satisfies the criteria for dither. 1/f
>>> noise in particular has been questioned, which is why I raised the spectrum
>>> issue.
>>>
>>> Beyond that, Nigel raises this issue in the context of "self-dither". In
>>> situations where there is a clear external noise source present, whether
>>> the situation is analog to digital conversion or digital to digital bit
>>> depth change, the external noise may, or may not, be satisfactory as dither
>>> but at least it's properties can be measured. If the 'self-dithering'
>>> instead refers to analog noise captured into the digitized signal with the
>>> idea that this noise is going to be preserved and available at later
>>> truncation steps to 'self dither' it is a very very hazy argument. I'm
>>> aware of the various caveats that are often postulated, i.e. signal is
>>> captured at double precision, no truncation, very selected processing. But
>>> even in minimalist recording such as live to two track, it's not clear to
>>> me that the signal can get through the digital stages of the A/D and still
>>> retain an unaltered noise distribution. It certainly won't do so after
>>> considerable processing. So the short
>>> answer is, dither! At the 24th bit or at the 16th bit, whatever your
>>> output is. If you (Nigel or RBJ) have references to the contrary, please
>>> say so.
>>>
>>> Vicki
>>>
>>> On Feb 8, 2015, at 10:11 AM, robert bristow-johnson wrote:
>>>
>>>> On 2/7/15 8:54 AM, Vicki Melchior wrote:
>>>>> Well, the point of dither is to reduce correlation between the signal and
>>>>> quantization noise. Its effectiveness requires that the error signal has
>>>>> given properties; the mean error should be zero and the RMS error should
>>>>> be independent of the signal. The best known examples satisfying those
>>>>> conditions are white Gaussian noise at ~ 6dB above the RMS quantization
>>>>> level and white TPDF noise at ~3dB above the same, with Gaussian noise
>>>>> eliminating correlation entirely and TPDF dither eliminating correlation
>>>>> with the first two moments of the error distribution. That's all
>>>>> textbook stuff. There are certainly noise shaping algorithms that shape
>>>>> either the sum of white dither and quantization noise or the white dither
>>>>> and quantization noise independently, and even (to my knowledge) a few
>>>>> completely non-white dithers that are known to work, but determining the
>>>>> effectiveness of noise at dithering still requires examining the
>>>>> statistical properties of the error signal and showing
>>>
>>>> th
>>>>> at the mean is 0 and the second moment is signal independent. (I think
>>>>> Stanley Lipschitz showed that the higher moments don't matter to
>>>>> audibility.)
>>>>
>>>> but my question was not about the p.d.f. of the dither (to decouple both
>>>> the mean and the variance of the quantization error, you need triangular
>>>> p.d.f. dither of 2 LSBs width that is independent of the *signal*) but
>>>> about the spectrum of the dither. and Nigel mentioned this already, but
>>>> you can cheaply make high-pass TPDF dither with a single (decent) uniform
>>>> p.d.f. random number per sample and running that through a simple
>>>> 1st-order FIR which has +1 an -1 coefficients (i.e. subtract the previous
>>>> UPDF from the current UPDF to get the high-pass TPDF). also, i think Bart
>>>> Locanthi (is he still on this planet?) and someone else did a simple paper
>>>> back in the 90s about the possible benefits of high-pass dither. wasn't a
>>>> great paper or anything, but it was about the same point.
>>>>
>>>> i remember mentioning this at an AES in the 90's, and Stanley *did*
>>>> address it. for straight dither it works okay, but for noise-shaping with
>>>> feedback, to be perfectly legitimate, you want white TPDF dither (which
>>>> requires adding or subtracting two independent UPDF random numbers). and
>>>> i agree with that. it's just that if someone wanted to make a
>>>> quick-and-clean high-pass dither with the necessary p.d.f., you can do
>>>> that with the simple subtraction trick. and the dither is not white but
>>>> perfectly decouples the first two moments of the total quantization error.
>>>> it's just a simple trick that not good for too much.
>>>>
>>>> --
>>>>
>>>> r b-j r...@audioimagination.com
>>>>
>>>> "Imagination is more important than knowledge."
>>>>
>>>>
>>>>
>>>>
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