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RTSP/SDP question

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RTSP/SDP question   

Postby thetoolwiz » Mon May 19, 2014 9:37 pm

I'm not sure this is the right place for this, but I thought I'd give it a try.

I'm writing a little RTSP client that's streaming data from an H.264 stream using TCP mux on a unicast configuration.

I'm able to split the two mux streams into audio and video.

I've also got the video playing, thanks to several examples I found that show how to deal with video codecs.

However, what I'm now looking for is an example of how to decode the audio RTP data packets (those following the '$' in the data stream) and how to set up and stream the audio to an audio codec.

I think the audio part of an H.264 / MPEG4 stream is MP3, but I'm just not finding anything online where anybody has discussed this.

If you understand what I'm asking about and can offer suggestions, I'm all ears.

Or if you can suggest a better place to go for questions like this, I'd be most appreciative.

Thanks!
-David
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Re: RTSP/SDP question   

Postby mad_ady » Wed May 21, 2014 12:24 am

Quick question - what are you trying to achieve? Play a RTSP stream on the WDTV or do something else?

If it's the first case, take a look at the IPCamera UMSP plugin. I wrote it and use ffmpeg to receive the RTSP stream and then I'm changing the container so that the WDTV can play it back. Examples are in that thread (and you can also check out the source code): http://forum.wdlxtv.com/viewtopic.php?f=53&t=7500
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Re: RTSP/SDP question   

Postby thetoolwiz » Wed May 21, 2014 8:04 am

Actually, what I'm trying to do is find ANY sort of useful documentation that describes how to interpret the AUDIO RTP payload data that's delivered in an H.264 (muxed) data stream over RTSP. I've found several RFCs that detail the video payload formats, but nothing that says anything useful about audio. They say the data is either "codec specific" or it's "out of band" so isn't discussed there. So where IS it discussed?

I did find somewhere that H.264 apparently uses AAC encoding, not MP3. But I'd imagine the header of the audio payloads should say what format it is.

I've posted on several forums now and people keep chasing me off because it's not specifically relevant to THEIR products. How do developers find out this info if nobody wants to discuss it?

Thanks
-David
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Re: RTSP/SDP question   

Postby mad_ady » Wed May 21, 2014 10:29 pm

Well, I'm afraid I can't help you much there, since this media player that we're playing with here has closed sources (and doesn't support RTSP anyway). However, you could try to look through the implementations of other players that support RTSP - such as ffmpeg or vlc.

As far as I know the audio codec for RTSP streams can be anything. In my particular case I have RTSP streams from IP Cameras, and those usually send out H.264 video + AAC audio streams (or even PCM streams). What the pluging I pointed you to does is remux the RTSP stream to a "file format" that the WDTV can decode, but it tries to keep the video and audio codecs unchanged.

See if this resource is of any help: http://stackoverflow.com/questions/15091246/same-media-format-for-audio-and-video-on-rtsp
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Re: RTSP/SDP question   

Postby thetoolwiz » Fri May 23, 2014 10:00 am

Thanks. That link is informative, but not helpful.

I believe H.264 specifies AAC for the audio. I just can't find any discussion on how to deal with the audio channel.

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Re: RTSP/SDP question   

Postby mad_ady » Mon May 26, 2014 2:33 am

Sorry - I don't have that information :(
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