[SIP Beyond VoIP] Stereo with sylk-webrtc

[Penelope Vennisse]

Hi, we've been trying for a couple of days to get stereo working in Chrome
with the sylk-webrtc browser project. Modifying the SDP and setting
echoCancellation to false have not had any effect. Is there a known issue
with sylk-webrtc and sylkserver, for stereo with OPUS? Appreciate any
feedback.

We've tried what works for other webrtc apps:

1. Force Chrome to disable audio processing:

constraints.audio = {
            mandatory: {
                echoCancellation: false
            }
        };


2. Set the SDP to prefer OPUS, add stereo and high-bitrate. We also tried
removing all other codecs to see if that helped.

v=0
o=- 2951467701722993374 2 IN IP4 127.0.0.1
s=-
t=0 0
a=msid-semantic:  WMS
a=group:BUNDLE audio
m=audio 9 RTP/SAVPF 111
c=IN IP4 0.0.0.0
a=rtpmap:111 opus/48000/2
a=fmtp:111 minptime=10;useinbandfec=1; stereo=1; sprop-stereo=1;
maxplaybackrate=524288; sprop-maxcapturerate=524288;
maxaveragebitrate=524288
a=rtcp:9 IN IP4 0.0.0.0
a=extmap:1 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=setup:active
a=mid:audio
a=recvonly
a=ice-ufrag:Q2dY
a=ice-pwd:ixK6U9znPvXZeQCOzUZSJgJ/
a=fingerprint:sha-256 9D:32:5E:A5:06:6C:01:53:C4:BE:
08:F9:6B:F2:3D:83:CD:F6:51:B9:A4:5B:F1:34:F9:13:B4:C3:60:9F:2C:F6
a=rtcp-mux


The connection works fine and the audio is using the bitrate directive, but
it's always mono. Advice is welcome! We've verified that stereo works in
the browser with other apps, for example the appr.tc reference app using
https://appr.tc/r/955394799?stereo=true&audio=echoCancellation=false


(Note that Chrome 65 and up currently has a stereo regression bug so we're
testing in Chrome 59 using Electron until that's resolved)
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