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"Re-rendering" 44.1 to 48 kHz helps compression?
HUNDOLOS
post Oct 20 2011, 12:24
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Hello everyone! This is my first post so I hope it's a meaningful question I got. So I found a "remastered" version of the Tomb Raider 3 soundtrack, which is originally in ADPCM. The person dealt with the noise and made some minor changes and, as he said, "re-rendered" the music to 48000 kHz (instead of the original 44.1) so it loses less during compression (to 320 kbps mp3 in this case).

The question is, is this true, does it work at all? Do you end up with losing less by doing "compressed 44.1 -> compressing again on 48" than letting it stay on 44.1, or is it just something that puts one's conscience to sleep? tongue.gif I'm asking in general, the ADPCM was just additional info (since it's different in structure from the more advanced compressions of today). Thanks everyone smile.gif
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dhromed
post Oct 20 2011, 13:07
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The guy probably just resampled to 48KHz to give him some more headroom for editing. There's no problem with this practice, unless the resampler is very bad at its job.

Keeping it at 48KHz for the final version is pointless, though, especially given the lowpass of mp3 and the fact that ADPCM -> MP3 is essentially a lossy-lossy transcode.
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HUNDOLOS
post Oct 20 2011, 13:20
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Do all mp3 encoders have that lowpass level though? I remember seeing a soundtrack done in 48 kHz (pointless, I know) and 320 kbps, it said it's ripped with iTunes, and it didn't show the usual symptoms tongue.gif
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probedb
post Oct 20 2011, 13:30
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If it was a soundtrack from a DVD-V it would have been 48KHz. I seem to remember the default sample rate for DVD-V is 48KHz rather than 44.1KHz.
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HUNDOLOS
post Oct 20 2011, 13:33
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QUOTE (probedb @ Oct 20 2011, 14:30) *
If it was a soundtrack from a DVD-V it would have been 48KHz. I seem to remember the default sample rate for DVD-V is 48KHz rather than 44.1KHz.


That's true, but the reason I said pointless is that I know for a fact it was just a CD soundtrack rip. Anyway, this is a bit off topic in a sense, doesn't matter as much.
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[JAZ]
post Oct 20 2011, 19:23
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The answer cannot be a strict yes neither a strict no.

If we take strictly the sentence "I use 48Khz instead of 44Khz so that encoding it to mp3 keeps more audio data", the general answer would be not really. Yet, there's a tiny detail which is that the pre-echo effect will be reduced by the consequence of having a higher sampling rate.
This does not mean that it keeps more audio data.

And it would be a plain "NO" if he really means that the 48Khz file is bigger that the 44Khz even after being decompressed from the MP3. Having more samples does not equal to having more audio information. One can resample the 44Khz file to 48Khz and that is not "decompressing" anything.

In general, it is wise to use the sampling rates at which the codec has been tuned (For example, CELT/OPUS codec is tuned at 48Khz sampling rate).
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HUNDOLOS
post Oct 20 2011, 19:27
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Excellent answer [JAZ], thanks smile.gif (I would assume btw that mp3 was tuned for 44.1.)
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DVDdoug
post Oct 20 2011, 19:32
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QUOTE
...which is originally in ADPCM. The person dealt with the noise... I know for a fact it was just a CD soundtrack rip.
CDs are "regular" linear PCM. I don't know anything about that particular soundtrack, but there's there's no reason for a modern CD to have audible noise.

QUOTE
Do all mp3 encoders have that lowpass level though?
Yes, but you can disable it.

It's generally best to allow the MP3 encoder to low-pass. It usually works best when you allow it work as it's designed. It's lossy compression and it's gong to throw some information. If you force it to keep the high frequencies, you are forcing it to throw-away something else that might be more noticable! You are making the psychoacoustic model "less smart".

Resampling to 48kHz is unrelated to the low-pass issue, since resampling doesn't add high frequencies to the audio.
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HUNDOLOS
post Oct 20 2011, 19:36
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QUOTE (DVDdoug @ Oct 20 2011, 20:32) *
QUOTE
...which is originally in ADPCM. The person dealt with the noise... I know for a fact it was just a CD soundtrack rip.
CDs are "regular" linear PCM. I don't know anything about that particular soundtrack, but there's there's no reason for a modern CD to have audible noise.


I said "I remember seeing a soundtrack," not the one I was talking about in the first place. TR3 is from in-game ADPCM source, the other one I mentioned is just any odd music released on CD.
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