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Author: Merlijn Wajer
To: maemo-leste, Pavel Machek
Subject: Re: [maemo-leste] Nokia N900 voice calls

Hi Arno,

On 14/04/2023 18:29, Arno ll wrote:
> Hi Maemo-Leste Community,
> Calls are working both ways on n900 with an acceptable sound quality.
> [snip]
> *Why it was not working in both ways ?*
> Audio recording through cmt_speech is not working with sample rates >
> 4000 Hz
> (8000 Hz is possible but needs some work)
> Forcing pulseaudio to allow 4000 Hz frequency with no resampling solves
> the main issue.

This is a great find and great news, and it's good to know that with
some hacks we can now get voice calls to work on the Nokia N900.

Pavel, I suggested to Arno that we can lower the latency in pulseaudio
and I think he said he got it down to 30ms (from 600ms) or so. It's not
a clear implementation yet (kind of hack), but he said the audio quality
was good. So we might be getting somewhere here.
I tried to force the latency to be low with this commit:
- but Arno reported it doesn't work well and makes things really choppy.
I read this article
and wrote the commit based on that. Maybe it's outdated info.

Looking at the other points in the email (that I snipped out), some
things we can do now:

* Correct permissions for /dev/cmtspeech through udev rules
* Figure out a clean way to have PA allow recording at 4000Hz
* Allow pulseaudio to talk to (read from) ofono
* Work on getting UCM2 for Nokia N900

It looks like even after that there will be some hacks required to get
this working, but maybe the above is a good start.