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

["Robert Schlabbach" <robert_s@gmx.net>]

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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