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again about compression

Subject: again about compression
From: Gianni Pavan <>
Date: Mon, 01 Apr 2002 19:25:13 +0200
I think the effect of compression on sound quality it is still a debatable 
question. It is very important to know which sound features may be affected 
by compression and how the acoustic context may affect the compression 
algorithm's behaviour. I made some tests with MP3 encoding/decoding and 
several effects can be easily detected by hearing or by spectrographic 
analysis. Of course, with MP3 the compression is scalable and alterations 
increase with increasing the compression factor (which corresponds to 
reducing the bit rate).
The MP3 works both in frequency and in time; by using algorithms based on 
both frequency and time masking, it can cut a short segment of a signal if 
it is just after a strong sound. This happens with sperm whale clicks: if 
they are very loud, after each click the spectrogram shows a short segment 
of silence (it is not noticeable by hearing, but it may cut some important 
features we measure in the 10 ms after a click!).

I don't know how ATRAC behaves. As I can't afford the buy of an MD recorder 
only for this testing, I would like to know if there is some software able 
to perform ATRAC coding/decoding I could use for doing some tests.

Any information will be appreciated,
Gianni Pavan
Centro Interdisciplinare di Bioacustica e Ricerche Ambientali
Universita' degli Studi di Pavia
Via Taramelli 24, 27100 PAVIA, ITALIA
Tel/Fax   +39-0382-525234 Laboratori
Tel/Fax   +39-0382-526208 Segreteria


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