Re: [Phys-l] Digital TV signal delay

["John Clement" <clement@hal-pc.org>]

It is all in the design of the cable boxes.  I pointed out that encoding is
known to be much more time consuming than decoding.  This is not inherent in
the digital process, but is dependent on the choice of compression
algorithms.  In the audio field there are a number of compression algorithms
and they vary as to the time to encode or decode.  It should be evident that
slow encode with fast decode is desirable from an economic point of view.
Having fast processors in every consumer box can increase the cost so much
that consumer acceptance would be low, so fast decode is the norm.

The other reason for slow encode is that encoding is best done as a 2 or
more pass process where the initial pass gathers the data necessary to make
the encoding decisions.  So the encoder needs to gather a segment of the
video before encoding.  It is in the best interest of the station to
transmit the very best signal, so slow encode is the norm.

As to delay between one cable box and another, the wires and splitters can
not introduce a perceptible delay as long as they maintain signal integrity.
If you read the reviews of converter boxes you will find that some boxes are
praised for having very quick channel switching times, and others are damned
for being excessively slow to switch channels.  Nobody seems to review the
issue of delay time in the signal.  If you really want to know the source of
the delay anyone who has 2 boxes can readily perform a simple experiment to
see where the delay is located.  After all we are physicists!  All analog
TVs should have negligible delay, but a digital TV might also be a source of
delay.

There are a number of articles on the web that one can readily find that
discuss things like encode/decode speed and CPU demands.

The slow encode time has nothing to do with the different speeds of
decoding.  I don't know if the encode delay time has been standardized yet,
but eventually it will have to be so that the TV stations can depend on it
for their programming schedules.  After all the technician has to start the
tape, of DVD earlier than the time it is supposed to air.  As to the exact
source of the different delays in decoding, it is all in the design.  While
all of the decoding processors have to be fast enough to keep up with the
transmitted signal, the more expensive boxes might have faster processors.
The slower processors might have less delay if they use compromises rather
than full decoding.  I suspect that there are only a few chips that do the
decoding and the specific chip used in the box is probably the determining
factor.  There is also the time to generate the specific type of output,
composite, component, or HDMI, but that should be negligible.  In the case
of HD radio everyone uses the same single chip for decoding.  One might be
able to look up the chips used in the boxes, as this information may be on
the web.

John M. Clement
Houston, TX


> I agree with John Denker that encoding is (or ought to be) more time
> consuming than decoding.  But that is inconsistent with what people are
> observing when different cable boxes and TVs in the same house are
> putting the same channel on the screen at times differing by a couple
> seconds.  That data would suggest one of the cable boxes is the slow
> link, or one of the TVs is the slow link.
>
> I also notice that after I converted to HD-digital (after having had
> analog), the time to change channels increased from essentially nothing
> to a couple seconds.  Of course either the cable box or TV (depending on
> how they're set up) has to decide if the broadcast is 480p, 780i, 780p,
> or 1080i and make that switch.  How long does that take?
>
> In my case, my Sony XBR4 TV can do 1080p and supposedly does a good job
> of dealing with different resolutions, so I have my cable box set to
> give the TV the same format as the broadcast, so any screen formatting
> takes place in the TV rather than the cable box.  Anyway, when I switch
> from one channel to the next, even if the resolution does not change,
> there is approximately a 2-second delay with a black screen before the
> new channel comes up.
>
> What's causing this channel switching delay if the decoding process is
> the fast process (and encoding is the slow process)? And, what's causing
> two TVs in the same house to be out of synchronization if they are both
> HD-digital viewing the same station, but with different cable boxes
> and/or different TVs?