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I am encoding some FLAC files into AAC and I came across the -cutoff option.

I do not see this option in the documentation, however I did find it in the help

$ ffmpeg -v warning -h full | grep cutoff
-cutoff                    E...A. set cutoff bandwidth (from INT_MIN to
                                  INT_MAX)

I am looking for more info as to what this actually does.

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1 Answer 1

up vote 3 down vote accepted

The cutoff option sets the bandwidth of the encoder – bandwidth meaning the highest frequency that will be encoded.

While typically you wouldn't restrict the bandwidth when encoding audio for home theater, music listening, etc., there are other use cases such as telephony or video conferencing, where it is not necessary to transmit the whole frequency spectrum.

For example, the primary frequencies of speech are situated well below 8 kHz, so you can cut off the higher frequencies and save required transmission bandwidth. This was also done in the early days of telephony to be able to share a line among multiple users.

Various encoders support this option, such as:

Especially AAC and Opus are interesting because they were created with low bit rate or narrow band transmissions in mind.

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I think for AAC they're advocating setting it to 15,000 – which should still be enough for most music. The encoder may save bandwidth and you might lower the risk of encoding hissy frequencies above that, which could cause artefacts. There's also some psycho-acoustic trickery going on, but I'm not sure which encoder uses that. –  slhck Apr 1 '13 at 9:37
    
@StevenPenny Also see Quality Assessment of FFmpeg AAC for a graph showing how various -cutoff values may affect perceived quality for the native FFmpeg AAC encoder (-codec:a aac -strict experimental). –  LordNeckbeard Apr 1 '13 at 18:09
    
@LordNeckbeard It's quite interesting how the MOS drops for higher cutoffs – not what you'd really expect. –  slhck Apr 1 '13 at 18:58
    
@slhck it makes sense to me. As you explained a higher cutoff increases the likelihood of artifacts from high freq tones. –  Steven Penny Apr 2 '13 at 3:28

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