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Old 03-01-2012, 14:21   #1
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Default audiophiles are crazy... (notes on compression)

....or so i read today...again:

so heres a lil snippet from a nice page on compression and audio quality for those who struggle to hear differences, brought to you by a crazy person:


What To Listen For An excellent way to hear compressor/limiters in action is an outside broadcast on TV. While the presenter is speaking, the level is constant, with very little variation - even when they are off-axis from the microphone. When there is a break in the commentary, the background noise can be heard to increase at a (relatively) fixed rate, until it is as loud as the presenter's voice, or someone starts speaking again. It is then instantly reduced to where it was before.
As you get used to what to listen for, you will hear many CDs where the level of the backing track falls when the singer (or someone else) starts making their noises - this (to many audio professionals) is quite normal, but it is not - it is a typical case of over application of compression in the final mix. When an additional sound is added to those already present, it is supposed to get louder - this is called dynamics (or even "micro-dynamics" - a reduced scale version of the real thing).
These effects are especially noticeable with commercials on radio or TV - listen for them so the sound can be identified. Should you purchase a CD that does the same, complain to the record company - they have ruined your music!
What About Lossy Data Compression? MP3 - love it or hate it, it is here (probably) to stay. As can be determined from a multitude of sources, MPEG Layer 3 (or MP3 for short) discards information that theory (and a lot of experimental testing) indicated would be inaudible. It uses a well known characteristic of our hearing called "masking", where it is known (and can be proven) that certain frequencies and levels are completely inaudible when accompanied by another signal at a higher level. The points where masking take effect are beyond the scope of this article, but differ according to relative levels and frequency, and the frequency band itself. An MP3 encoder breaks the signals into sub-bands using filters, and each is treated differently according to a set of rules built into the encoder signal processor.
While it is generally considered that a high bit rate (128kb/s or above) MP3 track is of "near CD quality", many people will dispute this vehemently. My own experiments and listening tests indicate that imaging is poor, and the precise placement of instruments and vocals is missing. Some instruments - especially the harpsichord - sound completely different when encoded, almost regardless of the bit rate. A good test is to "rip" some pink noise (preferably generated by an analogue source), and compare the difference.
There should be no difference - or at least it should be inaudible, but this is not the case! At 320kb/s the difference is barely audible - one has to listen carefully to hear it, or figure out just what to listen for ... but there is a difference, and it also shows up very clearly on the analyser of Winamp. The peaks are flattened, so the dynamic range (or peak to average ratio) is degraded, and the sound of the noise lacks "life" compared to the original recording.
If we can hear a difference with noise, why would music be any different? It isn't! Ok, noise has a relatively constant bandwidth (DC to daylight in the extreme), and excites all frequencies more or less simultaneously. Well, so does a lot of music, albeit for short periods at a time.
Will my comments here make MP3 go away? Of course not, and nor should it go away, because it is a useful way to archive recordings, or provide people (who insist on not hearing approaching traffic while they run or cycle) a convenient medium for portable sound.

my only issue with the above --- who can hear traffic while bunnyhopping up n down kerbs at 30mph with the wind in your ears, thats what eyes are for
"Space Travels in my blood, there ain't nothing I can do about it" - The Only Ones

Last edited by Enzyme; 03-01-2012 at 14:30..
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Old 03-01-2012, 15:35   #2
el maco
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I'd say listening to mp3 artifacts using noise is not relevant (unless you're a fan of Venetian Snares etc.) Other than that, the rest doesn't seem too crazy to me. Especially the part about clipping which follows your quoted paragraph. I bet majority of my record collection is recorded so loud that the signal clips frequently
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Old 03-01-2012, 18:11   #3
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o i do have Venetian snares in my breakbeat\4step type collection ><

and you dont have to tell me about the clipping, sooo many badly recorded files out there :\
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