3 Hz high-pass filter is causing severe generational loss

[Mr-Z-2697]

opus/src/opus_encoder.c

Line 1900 in b5dc74f

dc_reject(pcm, 3, &pcm_buf[total_buffer*st->channels], st->hp_mem, frame_size, st->channels, st->Fs);

First introduced in: 0869829

I know this is not a general use case, but maybe worth to take a look?

[jmvalin]

Care to elaborate a bit?

[Mr-Z-2697]

The audio is recursively decoded and encoded without any other alteration, there's an unusual amount of artifacts.
By restoring the code back to the pcm samples copying behavior, the artifacts are gone, or at least imperceptible.
The samples are using 512 kbps VBR, yes, even the highest bitrate won't save it.
sample.7z.gz

Some additional discussion at:
https://forum.doom9.org/showthread.php?t=186465&page=3

[Mr-Z-2697]

May I ask what are the reasons of adding this DC rejection filter, other than to address the problem with DC offset?
Not all audio have DC offset problem, in fact they should and will be actively corrected in mixing stage.
While not all audio being encoded by libopus will have gone through mixing stage, it's still not worth risking all audios just for a few problematic ones, since we now know the damage it can do, rather than being inaudible.

My suggestion is following the guidance of this philosophy:

Does Opus have an echo canceller like Speex does?
Echo cancellation is completely independent from codecs.
You can use any echo canceller (including the one from libspeexdsp) along with Opus.

DC rejection is completely independent from codecs.
You can use any DC rejector (including the one from libopus???) along with Opus.

Or at least make it adaptive.