Talk:Transparency (data compression)

I agree about testing being difficult. Personally I am aware that my hearing is not always in the same condition - sometimes I have wax in my ears, and sometimes even signs of tinnitus.

Superficially for MP3 at 192 kbps I can't tell the difference between most CDs and the encoded version. However if I listen to the music while encoding, I can tell that there are some critical parts where there is a perceptual loss even with 192 kbps encoding. These are infrequent perhaps only a few seconds in a long piece of music, but often these are the parts where I feel "wow" in a good recording. The MP3 encoding will often remove the wow factor.

In order to do a fair test the recordings used should be good. In my opinion, many people, even some with quite impaired hearing, can tell the difference between a good recording and a poor one. Recently in a blind test I was asked about the quality of 160, 128 and 64 kbps (I think) MP3 files. I was not given access to the original recording. There was little difference between the 160 and 128 kbps encoded material. There was a harshness in the string sound (orchestral, Tchaikovsky - Serenade for Strings) which very probably did not reflect the sound of the orchestra. The low bit rate recording did not have too many obvious MP3 artefacts, and it was clear that it had a smoother, but also possibly duller frequency response. I chose that as my preferred version - to the amusement of the testers. However if I'd been given a reference recording, or been asked to do an ABX comparison I would probably had been able to do that. The point I'm making here is that the filtering of high frequency sounds in the particular source actually made it pleasanter to listen to. Also I thought that the stereo separation was better in the low bit rate version. I would have expected a really high quality sound source to have sounded smooth, but also significantly less dull than the low bit rate recording.

Where this kind of testing is carried out, the testers really do need to know what they are doing. Significant care needs to be taken when generating the encoded material, and possibly some familiarity with the music used for testing, and the nature of artefacts in the encoding process will help the testers to develop material for a valid test.

In the limit, for higher quality encoding, the differences may be very subtle and small, and doing the evaluation of subjects could be very hard. It is possible that some differences in the sounds perceived may be slight modifications and have little to do with the encoding. These could include level differences and frequency response differences. For me after reading about the MAD MP3 decoder I've been using Winamp 2.95 with the MAD decoder in preference to newer versions of Winamp (see http://en.wikipedia.org/wiki/Talk:Transparency_%28data_compression%29 ). There are articles which tell me that this should sound better than the standard decoder. I am thus pre inclined to prefer this, even if I really cannot tell the difference. I feel however that I can actually detect a difference and recordings do sound pleasanter, more real, and more involving, but this could be due to frequency changes rather than the finer resolution of the decoding due to the dithering used. I have no way of knowing this, and only an independent test would establish this in my perception. I suppose I could evaluate the frequency content of samples using different versions of Winamp to determine whether frequency modifications are a possible explanation of my preference for the MAD decoded data.

David Martland 11:01, 3 May 2007 (UTC)

There's a lot of audiophile "I can hear more than you plebs" style boasting surrounding this topic. The only valid tests are double-blind with at least a 90% confidence threshold (i.e. there's less than a 10% chance that you got it right by guessing). Anything less is just too likely to be imagination (or baseless boasting). The 90% figure is arbitrary. Obviously you can choose a higher one if you want to be more certain but anything less will probably be laughed at. 155.198.65.29 (talk) 18:13, 12 May 2009 (UTC)

Double blind testing and proof

This wiki page correctly states that:

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"There is no way to prove whether a certain lossy compression methodology is transparent. To scientifically prove that a compression method is not transparent, double-blind tests may be useful. The ABX method is normally used."

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User Binksternet has sought to change this pre-existing definition to read as follows:

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If two different versions of a sound are given the same score in a double blind test, the two sounds are proven "transparent"—to have no subjective difference.

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The short answer to this is that it is false, and therefore violates WP:EL. The longer answer is that it belies a correct understanding of proof and exactness, and the role of what hypothesis testing does and can do. The original wiki defn appears correct. The difference is perhaps subtle, but it is important. (talk) 12:58, 5 October 2009 (UTC))

The reference can't possibly violate any of the guidelines about external links! It's a perfectly good reference. Pohlmann is an expert on the subject. Don't remove it. Binksternet (talk) 18:12, 5 October 2009 (UTC)

Also, if you wish to defend the earlier text, you will need to have a reference to back it up. The fact that it was pre-existing here doesn't count for anything in the face of a complete rebuttal in the form of an expert source. The expert: "Ken C. Pohlmann is a professor at the University of Miami in Coral Gables, Florida, and the director of the Music Engineering program..." If you find a source that states otherwise, we can come to a consensus about how to bring both viewpoints to the article. Binksternet (talk) 18:30, 5 October 2009 (UTC)