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Why are 128kbit/s MP3s usually 44,100 Hz?
eamon123
post Oct 14 2012, 01:44
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Since 128kbit/s MP3s usually has a low-pass filter at 16,000 Hz, and the Nyquist-Shannon theorem states that all frequencies under f/2 Hz can be totally described at an f Hz sampling rate, why not encode at 32,000 Hz? Wouldn't it save more space?
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saratoga
post Oct 14 2012, 04:03
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QUOTE (eamon123 @ Oct 13 2012, 20:44) *
Since 128kbit/s MP3s usually has a low-pass filter at 16,000 Hz, and the Nyquist-Shannon theorem states that all frequencies under f/2 Hz can be totally described at an f Hz sampling rate, why not encode at 32,000 Hz? Wouldn't it save more space?


Since encoding happens in the frequency domain anyway, there isn't much savings. It'll just not encode those frequencies, which is pretty close to downsampling, but much easier to implement.

That said, at very low bitrates LAME does downsample.
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eamon123
post Oct 15 2012, 05:02
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QUOTE (saratoga @ Oct 14 2012, 04:03) *
Since encoding happens in the frequency domain anyway, there isn't much savings.


Listen to these test files and tell me what you think (24 vs 48 kHz, both with 11.5 kHz lpf):


https://rapidshare.com/files/666351554/Baba...y24.11.5lpf.mp3
https://rapidshare.com/files/2623776446/Bab...y48.11.5lpf.mp3

QUOTE (halb27)
pre-echo issues get worse


If you ask me the attack sounds as good in the 24 kHz file as the 48 kHz, and everything else sounds better. What do you make of it?


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Dynamic
post Oct 16 2012, 18:07
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QUOTE (eamon123 @ Oct 15 2012, 05:02) *
Listen to these test files and tell me what you think (24 vs 48 kHz, both with 11.5 kHz lpf):
QUOTE (halb27)
pre-echo issues get worse


If you ask me the attack sounds as good in the 24 kHz file as the 48 kHz, and everything else sounds better. What do you make of it?


Counterintuitively, actually, that's a different situation to comparing 32 kHz and either 44.1 kHz or 48 kHz and the attack should be expected to sound as good!

MPEG-1 layer 3 uses 1152 sample frames at either 32, 44.1 or 48 kHz sampling rates
MPEG-2 layer 3 uses 576 sample frames at either 16, 22.05 or 24 kHz sampling rates

Thus, the short-block duration (in milliseconds) to handle pre-echo and transients is the same for both 24 kHz and 48 kHz.

The full frame durations are:
36 ms for 16 or 32 kHz sampling rates
26 ms for 22.05 or 44.1 kHz sampling rates
24 ms for 24 or 48 kHz sampling rates

and this duration can be divided into three short blocks of 192 samples each (and a third of the duration) to handle transients with greater time resolution for the same bitrate at the expense of worse frequency resolution for the same bitrate (very high bitrate overcomes this, but you're talking about CBR)

So, to get the maximum 50% difference in short-block length, compare 32 kHz against 48 kHz or 16 kHz against 24 kHz (or indeed 32 kHz (poorer short block) against 24 kHz (better short blocks)). Converesely, as halb27 says, tonal problem samples are helped by the longer frame durations.
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eamon123
post Oct 17 2012, 19:31
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QUOTE (Dynamic @ Oct 16 2012, 18:07) *
tonal problems


I'm not really sure what those are. Could you explain that for me please?
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halb27
post Oct 17 2012, 22:19
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QUOTE (eamon123 @ Oct 17 2012, 20:31) *
I'm not really sure what those are. ...

As an example for a real ugly tonal problem encode lead-voice using any VBR level you like and listen to the first 2 seconds.
(BTW there's hope for the future: robert gave me a pre-3.100 version for testing which greatly improves upon samples like this.)

This post has been edited by halb27: Oct 17 2012, 22:23


--------------------
lame3100m -V1 --insane-factor 0.75
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