[opus] Opus RTP/RTSP support

[Timothy B. Terriberry]

Giulio Mazzoleni wrote:
> Is there any program that is currenlty able to play a RTSP/RTP stream
> encoded with opus?

Both (the current beta release of) Firefox and Chrome (as of M24) 
support Opus in (S)RTP as part of WebRTC. Of course, that also requires 
ICE and DTLS-SRTP (for Firefox) or SDES-SRTP (for Chrome... DTLS-SRTP 
support will be coming in M25), so it may not be the most convenient for 
simple testing.

The underlying media engine used by both browsers can be tested 
separately, however. See 
<http://www.webrtc.org/reference/getting-started> to get the code and 
build it. After that, you can run voe_cmd_test to set up an RTP receiver 
listening for Opus (just follow the on-screen prompts).

There's no signaling (e.g., SDP), so you'll have to hard-code the 
payload type you send on (like opusrtp in opus-tools does), or figure 
out how to generate suitable signaling to send to your own application. 
The handling of mono and stereo is being changed slightly in 
<http://review.webrtc.org/933022/> (to run on a single payload type to 
avoid negotiation failure). We recently clarified how this is supposed 
to work in <http://tools.ietf.org/html/draft-spittka-payload-rtp-opus-02>.