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

[Michal Dobrzynski <michal_dobrzynski@mac.com>]

Thanks for that. It was quite informative. Now if there was just a 
utility that could benchmark the time between requesting a certain 
transport stream (including tuning to the frequency it's at) and the 
time that the first I-Frame was received (ideally with a profile of how 
long in step took) we could get some really meaningful numbers.

I guess things like signal strength, bit error rate, etc could all be 
taken into account. Given that I have two identical cards the 
information could be made more meaningful by monitoring signal on one 
card while benchmarking on the other.

Does any of what I have said make any sense? I wouldn't know as I'm 
quite far off from being able to code something like this and I'm 
struggling to  keep up with my university subjects as it is.

Regards,
Michal

On 23/04/2004, at 5:48 PM, Robert Schlabbach wrote:

> From: "Adrian P Challinor" <adrian.challinor@osiris.co.uk>
> > I understand the concerns here, but this delay also happens on
> > commercial TV's. I have a fairly expensive Phillips TV with built
> > in DVB-T receiver, and it takes at least three seconds to switch
> > between digital channels. Analogue is instant.
> >
> > Is this something to do with DVB-T encoding?
> It has something to do with _progressive_ video encoding technologies.
> MPEG-2 utilizes I-, P- and B-Frames, of which only I-Frames (aka "key
> frames") can be decoded by themselves, the others require preceding and
> possibly following frames to be decoded.
>
> Analog TV has no progressive decoding, each frame can be decoded all by
> itself. You could achieve the same with MPEG-2 by utilizing only 
> I-Frames,
> but compression would be much less.
>
> 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.
>
> Regards,
> --
> Robert Schlabbach
> e-mail: robert_s@gmx.net
> Berlin, Germany
>
>
>
> --
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