16 bit vs 24 bit, any samples that work? Options

[AndyH-ha]

There are real differences between 16 bit and 24 bit files, just as there are real differences between uncompressed and mp3. As with the differences between wav and mp3, 16 bit vs 24 bit is often difficult to identify by listening.

Test signals are one thing, but has anyone found any 24 bit music recording that can be successfully identified via ABX testing against a properly resample to 16 bit version of same?

[cabbagerat]

Being a newcomer to this site, I’ve only now read some of the other threads on the 24 vs 16 bit debate. I’ve also now read some of the material on the complex subject of dithering.

Before submitting any sample of 24-bit audio that contains a section that can be demonstrably distinguished from 16 bits (testing with ABX software), I’d just like to check on the ground rules for such an exercise.

Question 1:
Given that it can be demonstrated (using low level test tones) that artefacts in a 24-bit to 16-bit conversion are more evident if dithering is not used, and given that dithering is widely in use and recommended, is the use of some form of dithering compulsory for this exercise? If it is not compulsory, then I might seek to identify a sample where artefacts were in evidence.

Yes, dithering should be compulsory for this kind of test. It is well understood than the quantization process works "better" if there is the "right" amount of noise added to the signal. This has to do with reducing the correlation between the quantization error (or residue) and the signal to be quantized. Where error is not correlated with the signal, the only result of quantization is the addition of noise - not the addition of distortion.

Not applying the optimal type of dither before performing such a test would seriously call into doubt the validity of the results.

Question 2:
If the answer to question 1 is that dithering is compulsory, or at least acceptable, and given that dithering introduces an amount of noise, does this exercise exclude from consideration simpler forms of dithering that introduce much more noise than is necessary? More specifically, are any of the dither methods available with readily available software such as Audacity or foobar acceptable for this exercise? If acceptable dithering method(s) can be indicated, I would seek to identify a sample where the noise of the dithering is detectable when comparing a derived 16-bit version with a 24-bit original.

If your hypothesis is "reduction of 24bit music samples to 16 bits using optimal techniques is not transparent", then you would have to use the theoretically "best" dither available. If your hypothesis is "reduction of 24bit music samples to 16 bits using X technique is not transparent", then you can use any kind of dither available. Note that confirmation of the second hypothesis is still useful.

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Substituting a slightly different (and much simpler!) question, for sine waves with no direct current offset and which are at a frequency not far below half the sampling rate, the maximum [peak] amplitude that can be represented either side of zero is half of 2^16 , and the minimum [peak] amplitude that can be represented either side of zero is 1. Half of 2^16 is 32768. Voltage ratios can be expressed in decibels using the expression 20 x log (voltage ratio). That gives us the figure 90.3 dB as the ratio between the peak level of the loudest sine wave and the softest [highly imperfect] representation of that sine wave without dither. The low level wave doesn’t really have a shape to make the description ‘peak’ meaningful, being some variety of rounded square wave. And the filtering needed might alter the peak value of the low level ‘sine wave’ somewhat.

Without dither, some of the assumptions behind the quantization process break down. In most basic analyses, it is assumed that the quantization error for high bit counts is not correlated with the signal to be quantized - where this does not hold the analysis becomes much, much more complex. I would recommend borrowing or buying a copy of a good DSP book (Oppenheim and Schafer, for example) for answers to these questions better than those that I can offer you.

[MLXXX]

~ Not applying the optimal type of dither before performing such a test would seriously call into doubt the validity of the results.

~ If your hypothesis is "reduction of 24bit music samples to 16 bits using X technique is not transparent", then you can use any kind of dither available. Note that confirmation of the second hypothesis is still useful.

Thanks cabbagerat for your various remarks. I am not sure I am all that attracted to the idea of proving that last hypothesis, 'X technique is not transparent' if better dithering is apparently so readily available. As I have recently discovered, dither using Audacity appears to be much better [certainly quieter] than another method I've used (software I paid for!), and Audacity is a free download.

~ It may be possible, using poor quality dithering, with no noise shaping or poor noise shaping, to make an audible difference because of the added noise. That is not "properly" done. Many people here will remember that in the recently published year long ABX test of DVD-A & SACD vs resampled to 16 bit, the only differences detected by anyone were the "silence" between tracks, turned up to extremely high levels, where the unshaped dither used in their resampling was audible.

Proper resampling means, to me, what CoolEdit/Audition does with resampling, dithering, and noise shaping. I won't go into the evidence that has been presented in various places, but the program's resampling is probably as good as it gets. ~

Thanks, AndyH. Just as well I asked. I had assumed the dither that is built in to some software I've been using for a few years, n-track studio, would do a passable job in terms of added noise. But I find that is not the case, now that I've started comparing the results of different forms of dither.

With some recordings I have recently made at 24-bits, and processed to 16-bits, I have obtained these results:

1. N-track Studio 4 set to 1-bit dither - the added noise is quite noticeable.

2. N-track Studio 4 set to 1-bit dither plus shaping - the added noise not as noticeable as in 1, but nevertheless is detectable at a loud but realistic listening level (detectability verified with ABX software).

3. Audacity: dither left at default (triangular) - the noise is slightly different to that in the 24-bit original but so low in amplitude I think it unlikely to be detectable at a realistic listening level [by me anyway].

4. Audacity: dither set to shaping - same as 3, though a slightly different quality to the noise.

The extracts in which I have detected differences due to the noise level have used the dithering method of 2 above, so I guess that will not be good enough for the current exercise.

If I am to pursue this further, I will probably need to concentrate on other artefacts than noise, though the more I read, the less optimisitic I am of success.

It makes current complaints, in other web forums, about home theatre personal computers not being able to access all 24 bits from some Blu-ray audio formats (partly because of unresolved issued regarding HDMI and content protection protocols) seem unnecessarily alarmist, unless there is no dither used for the reduction to 16 bits. It even has to be queried why 24 bits are being transferred in the first place to some Blu-ray discs, which I guess is what was behind the DVD-A & SACD testing you mention. In relation to surround sound, I guess where 6 or so full range channels are in place, the total noise floor may begin to be an issue, particularly if a movie is created with a very wide dynamic range. It may be a slightly different exercise to produce a two channel CD at a normalized maximum recorded level.

When I first used a 24-bit audio card, a few years ago, for capturing amateur musical performances, I could immediately detect the improvement compared with capturing with a 16-bit sound card. However, it appears that provided dither is used in the mixdown from a 24-bit source, enough of the 24-bit intensity detail should normally find its way into the 16 bits, at least at lower signal frequencies, where we might otherwise detect the lack of intensity resolution.

I haven't given up, but as I say, I am no longer as optimistic.

This post has been edited by MLXXX: Mar 3 2008, 17:17

[Soap]

When I first used a 24-bit audio card, a few years ago, for capturing amateur musical performances, I could immediately detect the improvement compared with capturing with a 16-bit sound card.

Not to be pedantic - but the only objective conclusion you can draw is that you immediately noticed a difference between the quality of the two cards. Unless you performed more testing than you have indicated you have no reason to believe the quality difference you detected had anything to do with one being 16 bit and the other 24 bit.

This post has been edited by Soap: Mar 3 2008, 14:11

[MLXXX]

Thanks, Soap. I should have been more precise. When I started using the new card (an Audigy 2), I compared recordings made with it with its drivers set to 16-bits and with drivers set to 24-bits; and the 24-bit setting gave a noticeably different result (not a dramatic difference of course). As I was not intending to release the recordings in red book CD format, or other uncompressed 16-bit format, I never investigated dithering techniques, though I was aware they were recommended.

I'd also mention that once I had a 24-bit card available I no longer sought to make recordings at close to clipping level, but allowed the recordings much more headroom.

This post has been edited by MLXXX: Mar 3 2008, 15:25