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?

[MLXXX]

AndyH, perhaps you might be kind enough to answer my two questions below.

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.

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.

************************************************************

What is the theoretical signal to noise level available for 16-bit format of a conventional audio compact disc?

It is often suggested that 16-bits provide around 96.3 dB of signal to noise ratio, based on there being 2^16 instantaneous levels that can be represented with 16 bits. However, it’s an extremely complicated question. The S/N ratio can be argued to be infinite (if a series of zeroes is used between tracks of the CD), or extremely high (if a series of zeroes is sent interspersed with an occasional positive bit, or occasional negative bit [which could apply is using high frequency dithering to make a low frequency low level tone audible]).

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.

Recording at 24-bits and keeping the noise below that which applies for the 16-bit format

Without attempting to pin down a definitive answer to the S/N ratio question, I have found in practice that the required signal to noise ratio of recording equipment needed to compete with that available in the 16-bit format is quite high. What I have found is that is stretches my equipment to the limit. A Behringer analogue mixer struggles to provide a signal that has noise much less than 90dB down when set up so as not to clip in loud sections of a recording that approach 0 dB. A Creative Audigy 4 soundcard with external hub struggles to provide better than about 94 dB with some audio drivers. By locating the Audigy 4 card in the pc in a PCI slot away from interference, and by using an ASIO ALL driver (instead of the normal a creative driver), and by setting the electronic fader in the creative recording mixer to less than 100%, I was able to get over 100 dB available for use for 24-bit recording.

Then there is a question of room noise where the recording is made. A close microphone technique is needed.

When all of this is done, I find that a dithered 16-bit version of a 24-bit recording can be audibly noisier at a realistic (though rather high) listening level, during quieter passages of a recording.

However, a lot may depend of the type of dither used. I would prefer not to submit any sound recording extract until I have a better understanding of what forms of dither are considered acceptable.

This post has been edited by MLXXX: Mar 3 2008, 16:09