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Removing frequencies for low bitrate transcoding
subinbar
post Oct 14 2012, 06:08
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Would it be effective to remove certain frequencies from an audio file in order to make it more compressible?

I am currently encoding some of my movies to a portable format, and I'm looking to save space where I can. I'd like to transcode the audio tracks at 32-48kbs with QuickTime HEv1 AAC. (Of course I'd like to use Opus, but compatibility is basically non existent.)

I've done some listening tests with QuickTime vs FhG HE, and my conclusion is that I prefer QuickTime. QuickTime seems to include more high frequencies, while FhG seems to eliminate more. However sometimes I believe QuickTime includes too many (or too high) frequencies that are not able to fit into such small bandwidth, resulting in that "underwater" flanging type sound, similar to low bitrate mp3's. It usually happens with sudden high sounds, such as crashing or cymbals in music.

At these low bitrates it looks like QuickTime applies a hard low-pass at about 15khz.

I'm wondering if I can identify the specific frequency range(s) that these artifacts occur in, and remove or lessen this range, if it will help lessen these resulting artifacts and lessen the complexity of the audio. Similar to de-noising a video clip.

Is this a sound theory, or would removing these frequencies do nothing to reduce the complexity of the audio, and therefore the bitrate requirements? Am I better off reducing the bit-depth instead? In my sound tests, this seemed less effective and made the audio sound too dull.

Thanks.
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subinbar
post Oct 14 2012, 08:18
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Well after testing a few scenarios, it seems as though the frequency removal would have be done by the encoder itself. Removing frequencies and setting a lowpass before conversion seems to give QuickTime less information and the resulting transcode sounds worse.

Removing frequencies after conversion was somewhat effective - I was able to carve out some nasty frequencies and clean up the sound a bit. However this doesn't really help unless the encoder can perform the actions itself.

Maybe some day encoders will be advanced enough to allow custom instructions on which frequencies to keep.
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