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Topic: Why do some encoders use really specific lowpass values? (Read 1780 times) previous topic - next topic
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Why do some encoders use really specific lowpass values?

Some encoders seem to use really specific lowpass values. For example, the EZ CD converter's version of FDK AAC uses a lowpass value of 15.7khz for 128kbps CBR, and 17.2khz for 160kbps. Why are these specific values used, instead of just 16khz and 17khz? You also see this in LAME too. They use 15.1khz for 96kbps instead of just 15khz.

https://www.poikosoft.com/ezcd-fraunhofer-fdk-aac-encoder

Were these values picked algorithmically? Is the lowpass filter not 100% accurate? or is there another reason?

Re: Why do some encoders use really specific lowpass values?

Reply #1
https://wiki.hydrogenaud.io/index.php?title=Fraunhofer_FDK_AAC#Bandwidth - not sure about EZ CD converter having its own customizations

as for why, maybe a good place to start is the source code, then maybe there is git blame (or equivalent) and maybe more context in the commits, also maybe asking the authors of those commits nicely can reveal some more context
a fan of AutoEq + Meier Crossfeed

 

Re: Why do some encoders use really specific lowpass values?

Reply #2
I think cutoff points are calculated from a reference value and there is no reason to use "rounded" values. Also, I think there's a possibility that some encoders specify cutoff points as FFT / MDCT bands and not kHz.

Also, by "calculated from a reference value", I don't mean just multiplying it. Higher bitrates should have lower lowpass values than what a pure multiplication gives, because otherwise increasing the bitrate would only increase bandwidth and not increase quality. For example (for FhG MP3Enc), the cutoff point of 11025Hz 16kbps mono MP3 is slightly above 4kHz, but the one of 22050Hz 32kbps mono MP3 is almost 8kHz, and the one of 44100Hz 64kbps mono MP3 is almost 13kHz.