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

lynx.abraxas at freenet.de lynx.abraxas at freenet.de
Fri Feb 16 00:02:27 CET 2007


On 16/02/07 07:02:10, timecop wrote:
> >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.

Thanks a lot. This makes it clear.

> 
> >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.

This  is  very valuable to know. Sadly I couldn't find any info on this in the
net.
What would You recon is ment with "signal strength" and  "signal  quality"  in
the  M$  windows  dvb-t  programme?  Would  it  be  SIG and SNR as reportet by
dvbsnoop or tzap?  What  do  the  other values discribe? SNR is never changing 
although it is reported. Here is  some  output  of usbsnoop and tzap:

tzap ard
using '/dev/dvb/adapter0/frontend0' and '/dev/dvb/adapter0/demux0'
reading channels from file '/home/lynx/.tzap/channels.conf'
tuning to 191500000 Hz
video pid 0x0201, audio pid 0x0202
status 03 | signal 8675 | snr 0000 | ber 001fffff | unc 00000000 |
status 1f | signal 8435 | snr 0000 | ber 00000000 | unc 00000013 | FE_HAS_LOCK
status 1f | signal 8455 | snr 0000 | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal 844e | snr 0000 | ber 00000000 | unc 00000000 | FE_HAS_LOCK

dvbsnoop -s signal -pd 9 -n 9 -timeout 1000
dvbsnoop V1.4.00 -- http://dvbsnoop.sourceforge.net/
   DEMUX : /dev/dvb/adapter0/demux0
   DVR   : /dev/dvb/adapter0/dvr0
   FRONTEND: /dev/dvb/adapter0/frontend0

---------------------------------------------------------
Transponder/Frequency signal strength statistics...
max cycle count: 9
Capabilities: BER: 1  SNR: 1  SIG: 1  STAT: 1  UBLK: 1
---------------------------------------------------------
cycle: 1  d_time: 0.001 s  Sig: 33912  SNR: 0  BER: 0  UBLK: 0  Stat: 0x1f [SIG CARR VIT SYNC LOCK ]
cycle: 2  d_time: 1.017 s  Sig: 34416  SNR: 0  BER: 0  UBLK: 0  Stat: 0x1f [SIG CARR VIT SYNC LOCK ]
cycle: 3  d_time: 1.020 s  Sig: 33891  SNR: 0  BER: 0  UBLK: 0  Stat: 0x1f [SIG CARR VIT SYNC LOCK ]
cycle: 4  d_time: 1.020 s  Sig: 33933  SNR: 0  BER: 0  UBLK: 0  Stat: 0x1f [SIG CARR VIT SYNC LOCK ]
cycle: 5  d_time: 1.020 s  Sig: 33933  SNR: 0  BER: 0  UBLK: 0  Stat: 0x1f [SIG CARR VIT SYNC LOCK ]
cycle: 6  d_time: 1.016 s  Sig: 33945  SNR: 0  BER: 0  UBLK: 0  Stat: 0x1f [SIG CARR VIT SYNC LOCK ]
cycle: 7  d_time: 1.016 s  Sig: 33959  SNR: 0  BER: 0  UBLK: 0  Stat: 0x1f [SIG CARR VIT SYNC LOCK ]
cycle: 8  d_time: 1.016 s  Sig: 33956  SNR: 0  BER: 0  UBLK: 0  Stat: 0x1f [SIG CARR VIT SYNC LOCK ]
cycle: 9  d_time: 1.016 s  Sig: 33939  SNR: 0  BER: 0  UBLK: 0  Stat: 0x1f [SIG CARR VIT SYNC LOCK ]


Regards,
Lynx





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