"Radium" wrote in message
ups.com...
Hi:
Why is it that MP3s needs some amount of encoded audio in order to
have any audio at all, while WMA can simply make its own audio? [To
better understand this question, read below]
I have Adobe Audition 1.5 in which I do audio experiments.
Below is my first experiment:
1. I make a silent 44.1 KHz-sample-rate, 16-bit-resolution, monaural
wave file that is 4 seconds long. [To do this, go to "generate" and in
the drop-down menu click "silence". A small windows pops up giving the
number of seconds, I put it at '4']
2. I save it as "silent.wav."
3. I then convert this wave file to a 44.1-KHz-sample-rate, monaural,
20kbps WMA file -- "silent.wma."
4. I close silent.wma
5. I open silent.wma and convert it to a 44.1 KHz-sample-rate, 16-bit-
resolution, monaural wave file and save it as "silent.wav" again --
this overwrites the original "silent.wav."
6. I then close silent.wav and then re-open it. Then I convert it back
to 44.1-KHz-sample-rate, monaural, 20kbps "silent.wma" file
[overwriting the original "silent.wma"].
After generating the silent.wav file I repeat steps 2-6 at least 4
times. Now when I play silent.wma I notice audio in the file that
resembles the characteristic artifacts of WMA.
In my second experiment, I do the exact same thing, except I use MP3
instead of WMA:
1. I make a silent 44.1 KHz-sample-rate, 16-bit-resolution, monaural
wave file that is 4 seconds long. [To do this, go to "generate" and in
the drop-down menu click "silence". A small windows pops up giving the
number of seconds, I put it at '4']
2. I save it as "silent2.wav."
3. I then convert this wave file to a 44.1-KHz-sample-rate, monaural,
32kbps MP3 file -- "silent2.mp3."
4. I close silent2.mp3
5. I open silent2.mp3 and convert it to a 44.1 KHz-sample-rate, 16-bit-
resolution, monaural wave file and save it as "silent2.wav" again --
this overwrites the original "silent2.wav."
6. I then close silent2.wav and then re-open it. Then I convert it
back to 44.1-KHz-sample-rate, monaural, 32kbps "silent2.mp3" file
[overwriting the original "silent2.mp3"].
After generating the silent2.wav file I repeat steps 2-6 more than 4
times. No matter how many times I repeat 2-6, silent2.mp3 still
remains completely silent. Why is this?
Thanks,
Radium
P.S. In my post I am describing lossy WMA compression. The standard
WMA.
MP3 is a perceptual encoder, that means that within the many narrow filter
bands, sounds that are masked by other sounds are not encoded.
I may be wrong on this, but I don't think WMA is a perceptual encoder. If
so, multiple encode/decode cycles will add low-level artefacts which with
repeated code/decode cycles will become very audible. MP3, being a
perceptual encoder, will encode silence "perfectly", and decode it equally
perfectly, so artefacts won't be created. Try the same experiment with some
audio, and you will see (hear?) MP3 artefacts adding. If you take some
audio, encode MP3, decode, then encode MP2 then decode, then encode MP3 (or
alternatively encode/decode MP3 with different bit rates) you will hear the
artefacts easily. In other words, MP3 creates artefacts on audio, but only
when there is some audio to create them on. It is the reason why in the UK
radio stations with a digital output (DAB, DSAT or whatever) do not allow
the use of any data-compressed programme source as multiple encode/decode
cycles are very audible.
S.
--
http://audiopages.googlepages.com