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Old July 14th 17, 08:36 AM posted to uk.tech.digital-tv
NY
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Posts: 1,062
Default BT/Plusnet TV Recorder (DTR-T2100)

"NY" wrote in message
o.uk...
"thesimpsons" wrote in message
...
When playing back an off air recording, if I select fast forward the
screen displays a perfect speeded up image.
If playing back a recording from an internet channel, fast forward
displays "snap shots" of the speeded up image.
Just wondering why this should be, it's the same on both a ten year
Panasonic and a recent Sony TV.


Different types of compression.

SD TV uses MPEG which requires comparatively little computational effort
to decode - so the decoder can do every single frame even at faster than
normal speed, so you get smooth motion.

Many videos on the internet (eg Youtube) use MP4/H264 compression which
requires a lot more processing and the decoder can't display every frame
at faster than normal, so you get every nth frame and jerky motion. HD
television (including SD channels on a DVB-T2 "HD" multiplex) also uses
H264.

I have seen this difference when I use VideoRedo to edit out commercials
from my recordings on my PC: SD can scrub through at any speed, whereas HD
lags and "gets stuck" if you try to move the programme position too
quickly.

This is one of the downsides of H264 - it produces smaller files but they
are more computational-intensive to decode so most decoders can't manage
faster-than-normal playing.


Incidentally, Sky boxes (well, ours, anyway) can't manage smooth fast play
(even at 2x, never mind faster) with SD - probably the decoder is only just
fast enough to play at normal speed and has no additional capacity for 2x
etc playback.

Software such as VLC on my PC can play SD and HD at fast speed - I've had SD
at 4x (sound is not very intelligible!) and HD at about 3x before I can see
jerkiness. That's for off-air TS files in both cases.