[Elphel-support] Some questions about Elphel 353

[Sebastian Pichelhofer]

Hi Jens

On Mon, Jun 14, 2010 at 7:47 AM, Jens Bürger
<jens.buerger at cs.tu-dortmund.de> wrote:
> Hi,
>
> our student group is constantly discovering the various areas of code and structure of the cam, so we  come to additional questions from time to time.
>
> At the moment, we have the following ones:
>
> What is the difference between the elphel_wait_frame_abs and elphel_wait_frame functions?
> As I unterstood it, elphel_wait_frame 'syncs' to completed, compressed images, but to what does the _abs-function 'sync'? I think I read somewhere that it 'syncs' to 'all' images. Does this mean, that uncompressed image-data is also available via the Linux-System? If yes: where are they available and how / when do they bypass compressor.v ?
>
> Where is the RTSP-Streaming-Server implemented?
> This is /apps/astreamer, right?
>
> Do we have to enable the gstreamer-support somewhere? We did try to connect to the cam via gstreamer, but gstreamer did receive no data.
>
> What does /dev/jpeghead do? It looks like it does some sort of finishing the JPEG-Compression?
>
> What sets the timedelay in the RTSP-Stream? Running some tests, we felt that the delay is actual some seconds.

The camera internal delay that the data needs to pass the complete
pipeline from the sensor frontend to the Ethernet stream is extremely
low.

The delay or latency you see in the end is always caused by either
some network issues (some router/switches can cause delays), easiest
way to test if that is the case is to connect the camera directly to a
computer (no crossed out cables required) or - as in most cases - the
video player.

With mplayer I got varying results between 20-100ms latency
With gstreamer latency was in general lower but also varried quite strongly.

BEST results I got with VLC from commandline by setting the
--rtsp-caching=XY parameter correctly:

"vlc --rtsp-caching=20"

This buffer in milliseconds sets how long VLC should "wait" to receive
a full frame from the rtsp stream before displaying it.

I have very good experiences with values between 15-25 (the higher the
datarate from the camera: resolution, fps, compression, format, etc,
the bigger the buffer needs to be, if it is too low there will not be
enough time to receive a full frame and the result will be the
opposite of the desired effect: shuttering video and high latency (up
to several seconds)) with Full HD resolution.

Regards Sebastian

>
> We are interested in the PHP Constants, as listed in
> http://wiki.elphel.com/index.php?title=Elphel_PHP_constants
> Where can we find documentation of the meaning / effect of the various constants?
>
> Thank you in advance,
> Jens Bürger
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