[linux-dvb] Re: Channel change delay with DVB-T

[Johannes Stezenbach <js@convergence.de>]

Robert Schlabbach wrote:
> 
> From my observations, broadcasters typically send about 2 I-Frames per
> second. This means that even after only switching the video stream _on the
> same_ channel or transponder, you will have a delay of 0 to about 500ms
> waiting for the first I-Frame on the new stream. For comparison, analog PAL
> TV has a field rate of 50Hz, i.e. you would have a delay of only 0 to 20ms.
> 
> When you also change the channel/transponder, you will have to wait for the
> tuner PLL to settle on the new frequency and for the demodulator to lock
> the signal. The time required for this varies with the DVB flavor: My
> experience is that DVB-S demodulators can do this in less than 10ms, DVB-C
> in less than 50ms, and DVB-T demodulators take 500-750ms.
> 
> So there are your technical limits: DVB-S and DVB-C roughly half a second,
> DVB-T one to 1.5 seconds. That's the best it can get. So your expensive
> Philips TV seems to be 1-2 seconds slower than it could theoretically be.

My experience is that switching between services on the same multiplex
is faster with DVB-T than with DVB-C or DVB-S.

I think that DVB-T here in Berlin uses statistical multiplexing (i.e.
the channel bandwidth is distributed dynamically between all services
on the channel, which means real-time MPEG encoding), and the encoder
makes shorter GOPs when bandwidth is available.

I haven't confirmed this hypothesis, though. Does anyone know
a software which can display the GOP structure of a captured stream?

Also IIRC the Philips TU1216, and the frontnend in one of those ugly
Nokia boxes tune significantly faster than 500ms.


Johannes


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