Short samples: 24 bit vs rendered to 16 bits dithered Options

[MLXXX]

Source audio: Upright piano, amateur performance, Handel, Gigue No. 1, first two bars, 9 sec aprox, recorded with a matched pair of Rode NT1-A condensor microphones, connected to a Behring MX 802A mixer, driving a Creative Audigy 4 hub for the analogue to digital conversion, connected to a PC running Windows XP and Asio4All sound drivers, with n-Track studio 4 as the recording software.

The noise of the Behring mixer was a bit high for 24-bit recording using the normal main outputs. Instead, I took the audio from the Auxiliary 1 and Auxliary 2 mixes.

The top lid of the upright piano was open. The microphones were located a few centimetres above the top of the piano, and displaced horizontally about a quarter of the length of the piano on either side of the middle of the keyboard. They pointed downwards, with a slight tilt towards the centre of the keyboard.

With the Behring mixer turned off, the recording software stereo VU meters registered around -95dB per channel. With the mixer turned on, but phantom power for the mics off, the VU meters showed around -85dB per channel. With phantom power on, the VU meters showed around -75dB per channel but this varied with extraneous noises near the recording room, such as heavy traffic. (The mixer was set to roll off frequences under 75Hz.)

Here is an unmodified extract from the 24-bit stereo 48KHz recording [now in flac format]:  SprightlyHandelclip.flac ( 1.26MB ) Number of downloads: 574

Here is the same extract after bit reduction to 16 bits using triangular dither: [attachment (4313) now deleted to free up my upload quota - this file was not optimally dithered and was unnecessarily noisy]
The format of the modified extract is 24 bits even though the last 8 bits should contain no varying data.

The peak recorded level is -9.5dB. It was played forte but not fortissimo. Another piece recorded in the same session peaked at -4dB. [As a comparison, a lossless format audio file on an HD-DVD disk Superman Returns on my Home Theatre PC came in at -3.3dB at the start of chapter 4 (storm at sea) and -9.7dB in chapter 20 (Superman and Lois go flying from the top of the Daily Planet). This was for Front Left and Front Right. The chapter 20 music of the movie is very dramatic, and worth listening to in its own right. It is not background music. ]


The uploads above are made in response to a request made in November 2006 by AndyH in the Listening Tests thread 16 bit vs 24 bit, any samples that work?:

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?

The bit reduction and triangular dither were carried out using the freeware audio editor, Audacity.

It is a trivial exercise to ABX compare the two versions if a high listening gain is employed and listening restricted to the first 0.3 seconds. High frequency dither noise then becomes quite noticeable. [If this is not the case, your equipment is probably not playing back at 24-bits, but at only 16-bits.]

It is a much more difficult (impossible?) exercise to ABX at a moderate listening level. Before doing any comprehensive listening tests at moderate levels, I would like to get the feedback of AndyH or others as to whether other dithering techniques are to be preferred.

If anyone is thinking of uploading an alternative dithered version, a 16-bit format would do. I put my "dithered to 16 bits" version into a 24-bit format simply to reduce the likelihood of some software players processing it differently to the 24-bit original file when comparing playback.

At very high listening levels I find that the version that has been reduced to 16bits sounds a little duller to my ears. However, my hearing becomes affected very rapidly at such volume levels, so that after a few repetitions I cannot hear any differences.

This post has been edited by MLXXX: May 26 2008, 14:09

[Ron Jones]

Do you happen to have ABX logs available?

I can submit samples dithered with POW-r, UV22, Waves L1, L2, L3 and perhaps others if desirable. The two former algorithms retain 19 to 20 bits of resolution according to Bob Katz.

EDIT: Removed one portion after reading a few posts in the original thread.

This post has been edited by Ron Jones: Mar 10 2008, 17:29

[MLXXX]

Do you happen to have ABX logs available?

I did not bother posting an ABX log as the exercise is dead easy when listening at high gain to the first 0.3 seconds. I was able to get 10/10 (a 0.1% probability of guessing), without any trouble, and in a short time-frame. But listening at a medium level would be much more challenging.

I can submit samples dithered with POW-r, UV22, Waves L1, L2, L3 and perhaps others if desirable. The two former algorithms retain 19 to 20 bits of resolution according to Bob Katz.

I have for a long time guessed that about 20 full actual bits (no dither) would be sufficient to satisfy the human ear, and in any event I understand that nominally 24-bit audio ADCs do not perform reliably for their least significant digits, or even if they did, real life microphones would generate too much noise to make the least significant digits meaningful.

I guess what would be desirable would be a dither that:

(a) generates less audible noise (fairly easy to establish by checking out the first 0.3 seconds of the 24-bit original at high gain); and/or,
(b) a dither that garners more of the missing detail, making the result sound closer to the original (probably quite difficult to test - perhaps more easily established by reducing the level of the original before further processing, i.e working with a lower bit depth version for preliminary testing purposes).

I was not seeking the absolute best dither, merely either of the following:

(1) an opinion that the dither I used (with the Audacity software) is close enough to optimal anyway and good enough to proceed with listening tests at lower volume (i.e tests not simply based on audible dither noise),
(2) to be provided with a dithered version that is of noticeably better quality and "up to scratch" to be used for further listening tests.

Cheers.