[Speex-dev] Notch Filter in AEC

[Jean-Marc Valin]

Quoting Steve Underwood <steveu at coppice.org>:
> You make that sound like its just a matter of meeting some arbitrary
> spec. Let's be more specific.....

Well, the corresponding spec is G.712, though the reason for doing that has more
do do with quality (as you point out) than just meeting the spec.

> If you use narrow band voice down to deep bass frequencies:
>
>      - 16 bit linear audio sounds good

Even there I tend to disagree because when you chop all the high frequencies,
keeping the deep bass makes the sound even more "muffled" IMO.

>      - alaw or ulaw sounds muddy
>      - low bit rate codecs, like speex or G.729, sound awful.

Definitely agree here. It's amazing to see how many systems don't do this
filtering before encoding and end up with poor quality speech.

   Jean-Marc