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Short samples: 24 bit vs rendered to 16 bits dithered
post Mar 10 2008, 15:37
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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]: Attached File  SprightlyHandelclip.flac ( 1.26MB ) Number of downloads: 646

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?:

QUOTE (AndyH-ha @ Nov 3 2006, 12:19) *
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
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post Mar 11 2008, 07:06
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I downloaded the clips this morning but did not have time to do anything with them until this night. I’m writing this without reference to whatever may have been added since the thread was opened.

Listening only to the 0.4 seconds before the music starts, with the amplifier volume at maximum, I can easily hear a difference in the two files. I have enough experience to not be so foolish as to attempt to listen to the music at anywhere near that setting.

Comparing the 16 bit file I created from the original (in CoolEdit), on that opening 0.4 seconds, and the same volume setting, I hear no difference.

Still looking only to that opening bit before the music starts.
In all three files, CoolEdit’s measurements (Analyze/Statistics) are quite near to each other. This is not the case if one generates silence at 32 bit, then dithers when converting to 16 bit. There the dither noise is very high compared with the original (just about the same measurements as after dithering this file’s noisy opening). I guess, in this file, the background noise being as high as it is accounts for most of the signal level; the dither noise adds relatively little.

The largest Statistic difference is in my 16 bit conversion: the peak amplitude is 2dB higher than the 24 bit file while the download 16 bit version is only 1dB higher. The noise shaping I used accounts for the difference in sound. It puts much more of the dither noise at frequencies higher than I can hear, while the downloaded 16 bit sample has more dither noise at lower frequencies.

I played with listening to the entire clip, as well as shorter sections of the music, comparing my 16 bit conversion to the 24 bit download. I could detect nothing upon which to make a guess about identity. Perhaps later I will have more energy to give to another attempt, but it has been a long day.

Again, I have no wish to read my posts in the thread which started this. I know that I have sometimes gone into more detail than saying “properly resampled and dithered” or whatever I wrote in the first post, but I don’t know if I was as specific somewhere in that thread as I have been in other posts on this topic.

Unless the file has no quiet passages, fade outs, or something of that aspect, it is not hard to make sure the dither is audible. I can dither so that the difference at the beginning of the downloaded clip is audible at a somewhat lower volume setting, but I don’t consider that relevant to the question. Can any difference in the music be distinguished? Can the between track sections even be heard at reasonable listening levels, let alone heard to be different?

I don’t see that between tracks is relevant anyway. While it is something that one hears in some recordings, if that, and only that, is somewhat different in sound from the original master, who is ever going to know it without access to that original master? One isn’t going to be able to say that the character of the between track sections somehow reduces the quality of the recording (unless it was so in the original recording, of course).

Fade in sometimes starts below the level of audibility and fade out sometimes goes down below audibility. That is not a part of the performance for which one is expected to crank the volume to maximum. The fade in and fade out, as such, are part of the performance. If one could hear a difference, while there was actually music signal present, with the volume setting remaining unchanged from the level that made the main part of the performance enjoyable, then there would be a legitimate difference.
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post Mar 11 2008, 07:16
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Thanks for doing this, MLXXX

I can't download the files (due to a restrictive firewall), but I would appreciate it if somebody would try apply Sebastian's noise shaped dither from the end of this thread, and try to ABX again. Be careful turning it up all the way, though - it could damage tweeters.

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post Mar 11 2008, 09:07
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QUOTE (cabbagerat @ Mar 11 2008, 07:16) *
[...] but I would appreciate it if somebody would try apply Sebastian's noise shaped dither from the end of this thread [...]

>java -jar requant.jar SprightlyHandelclip.wav SprightlyHandelclip_b16_d17_slameath48.wav 16 -d1.7 -s@lameath48.sos

Attached File(s)
Attached File  sprightlyhandelclip_b16_d17_slameath48.flac ( 579.65K ) Number of downloads: 230
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