"Theo" <
[email protected]> wrote in message news:n-q*
[email protected]...
I don't know specifics of DVB-S, but in many video formats they send a
full
frame regularly (every second or few) and between send the differences
from
each succeeding frame. There's more bandwidth reserved for the full
frames.
But if you cut from one scene to another you may still be receiving deltas from the previous scene, and that can cause fuzzy images until the next
slot
for the full frame. Depending on the timing it could cause things to be fuzzy for a short while until the updates catch up.
(This is how basic MPEG/etc works. I don't know if the encoders are a bit more adaptive in that they can adjust the full frame timing to align with scene changes while still keeping within the bandwidth limit. There's possibly less scope for that on live feeds where you can't look ahead)
I did the usual test: I recorded the same film on both DVB-S and DVB-T. I
found a break bumper where there was a single frame glitch on DVB-T and
looked for the corresponding glitch on DVB-S. There wasn't one. So it wasn't the signal that Film 4 were sending out; it was an anomaly in the MPEG encoding. Like an idiot, I forgot to check what type of frame it was - ie
was it a full frame, or was it an incremental difference frame of one form
or another? Until then, I was still wondering whether it was a cue-dot type
of signalling.
Strange that I've only noticed it on Film 4. Other channels have break
bumpers that have static images so would be an equivalent test for coding glitches.
Normally MPEG encoders detect a change of shot and always throw a full frame
at the point, even if it's not at the normal cycle of full frame followed by
n difference frames. I was curious so I single-stepped through a video using the very useful VideoReDo app that I use for removing continuity and adverts from things I want to keep: it can be set to display the type of frame, I,
B, P etc.
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