[linux-dvb] How does bad reception influence quality of macroblocks?

timecop timecop at gmail.com
Thu Feb 15 23:02:10 CET 2007


> > You will lose one or more TS packets (188bytes) if RS decoder (or some
> > other step before that) cannot fully recover the packet. Those packets
> > are usually marked with 'transport error indicator' flag, and mpeg
> > decoder skips them, or perhaps blindly tries to decode data in them.
> > Usually results in ugly crap across the screen in a particular section
> > that was ruined, even if it was only a single TS packet that was
> > affected.
> >
> > -t
>
> Thanks  for  Your  reply.  After  reading some docs about mpeg-TS I still have
> difficulties to realize the effects of loosing TS packets to  the  macroblocks
> (16x16 pixels) of the mpeg-2 video. Is there at least one full macroblock in a
> TS packet?

I never dealt with mpeg2 on the "macroblock" level in TS, but as you
probably read, a single mpeg "frame" will span several TS packets.

> To put my question in other words:
> If my dvb-t device doesn't get the  full  stream  will  this  always  lead  to
> missing  macroblocks  or  can  it  yield  less  obvious  loss of quality (like
> unsharpness)?

Losing a single TS packet, in HD, and especially in SD broadcast will
not lead to "unsharpness", you will receive visible ruined image
artifacts, which usually look like inverted/green/or otherwise random
stream of blocks after the missed packet, as mpeg decoder resyncs to
try to decode.

> Does the type of device influence the qualtity of the recieved stream? (Why do
> devices differ in prices so much?)
The only difference would be a more sensitive tuner.
If your C/N ratio is high, BER is low/or zero, and no continuity
errors in the stream, you're getting everything at full quality.

If the signal 'isn't perfect' you will get ugly dropouts/noise as
described above.



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