> > I'm talking about actual waveform analysis compared to the origional > > sound.
Tom replied:
> Ah, so who cares what it sounds like - we'll tell you what you should be > thinking.
Dan responded:
I'm not getting what you are arguing. Are you saying that fidelity to the sound put out by the folks who made the CD is not an important part of a player?
Fidelity is not always fidelity. There a lot of psychoacoustic tricks that can be
used to create compression that loses a lot of data and therefore looks quite
a bit different from the original sound in a waveform analysis, but does not
affect the perceived sound (or doesn't affect it much). In fact, compression
schemes such as MP3 would be impossible without using some of those
psychoacoustic phenomena. The trick is simply to get rid of sound that
most people don't perceive (or perceive only weakly) anyway.
For example, parts of the sound that would cancel in the air due to phase cancellation can be reduced or removed in some circumstances, as can sound that is in one channel but is masked by sound from the other channel. Codecs that do this kind of compression typically expect that you will be listening through speakers, as this kind of cancellation would not occur if you were wearing headphones (and since most professional audio mixes are done using speakers as opposed to headphones, this kind of lossy compression would actually give headphone listeners a more accurate version of the recording than if they listened to the original uncompressed version through headphones).
Wikipedia also has an entry that offers some additional description of this with links. http://en.wikipedia.org/wiki/Psychoacoustic_model
I haven't spent a lot of time listening to different compression schemes to see which one I think sounds most like the original, but I would guess that to some extent it would vary from person to person. Each person perceives sound a little differently, hearing some frequencies more strongly than others, and I would guess that some compression schemes will remove sound that some people perceive strongly (resulting in them thinking that particular compression scheme sucks) whereas other poeple may not perceive the frequencies being removed as much and so therefore think the compressed audio sounds much like the original, uncompressed version.
I would also guess that much sound removed by various compression schemes (particularly MP3) is very high frequency sound, which many people can't hear anyway. High frequency hearing loss is the most common kind of hearing loss, and can affect perception of music long before it becomes an issue in being able to understand speech.
So I guess the simple answer to your question, Dan, would be that fidelity to the sound of the original does not always equal having the waveforms look the same or similar, and the compression scheme that looks most like the original will not always be the one that best preserves the sonic impression of the original. When it comes to sound, I trust my ears much more than my eyes.
Reggie Bautista
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