[Live-devel] Presentation time when streaming video recording from surveillance cameras

[Ross Finlayson]

> We use the RTSPClient from Live555 to access live video and video recordings from surveillance cameras, utilizing the cameras built-in RTSP server and recording storage (An SD card).
> We have a problem regarding the presentation time when streaming video recordings.
> Let’s say we have a video recording from “last week” which we would like to stream. We start the stream; we can even start the stream from a specific time in the video recording, so that’s swell.
> BUT, the presentation time we receive is “now”, not “last week”.

You're misunderstanding the purpose of "presentation time".  The "presentation time" is a timestamp - generated by the server and conveyed to the client for each RTP packet - that tells the client the relative spacing of successive frames of the same media type, and also - if both audio and video substreams are present - allows the client to synchronize the audio and video (i.e., 'lip sync').  The absolute value of the presentation times is undefined and irrelevant.  (It is often the 'current' time, but doesn't have to be.  E.g., if the server's clock happens to be set to 1990, then the presentation times will be from then.)

What you want is something like the "normal play time" (NPT), but (as you discovered) we don't compute that when the client is seeking by absolute time, which I think is what you're doing.

But I think some sort of solution should be possible.  To be sure, please give us an example of a RTSP protocol exchange between your client and the server, when you're asking to stream from some time in the past - i.e., the "DESCRIBE", "SETUP", and "PLAY" commands, and their responses.

Ross Finlayson
Live Networks, Inc.
http://www.live555.com/

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