[linux-dvb] Bad TS from dvb-t recording?

Måns Rullgård mans at mansr.com
Fri Mar 9 03:06:17 CET 2007

Greg 'groggy' Lehey <grog at lemis.com> writes:

> [Format recovered--see http://www.lemis.com/email/email-format.html]
> Output lines unwrapped.
> On Thursday,  8 March 2007 at 17:03:40 +0100, Torbjrn Lundquist wrote:
>> Hi,
>> I'm trying to make a recording from my dvb-t card (Avermedia). I do like
>> this:
>>> tzap -r -c channels.conf -t 5 -o test.ts SVT24
>> using '/dev/dvb/adapter0/frontend0' and '/dev/dvb/adapter0/demux0'
>> reading channels from file 'channels.conf'
>> tuning to 522000000 Hz
>> video pid 0x04e1, audio pid 0x04e0
>> status 00 | signal 58cf | snr 0000 | ber 00000000 | unc 00000000 | copied 1708168 bytes (333 Kbytes/sec)
>> status 1f | signal f1cf | snr c0c0 | ber 00000000 | unc 00000000 | FE_HAS_LOCK
>> But then I cannot view the file in VLC for instance. If I run dvbsnoop on it
>> I receive some errors, for instance:
>> MPEG-DescriptorTag: 63 (0x3f)  [= ITU-T.Rec.H.222.0|ISO/IEC13818-1 Reserved]
>>            Descriptor_length: 199 (0xc7)
>>              ----> ERROR: unimplemented descriptor (mpeg context), Report!
> This sounds like vlc's way of saying "I don't understand MPEG TS".

No, VLC understands MPEG TS, although it's sometimes a bit fussy about
the timestamps.  That error message is saying that the stream has
(probably in the PMT) a descriptor with tag 63.  This value is
reserved in ISO 13818-1, meaning that it may not be used in extensions
for specific applications, such as DVB.  Refusing to play the stream
only because of this strikes me as somewhat unfriendly, though.

>> I live in Sweden and receive the signal from the Motala-tranciever.
> I don't think that this is related to the transmitter.

It could be, in the sense that different broadcasters use different
encoding parameters.

>> What am I doing wrong? Or can I use some other program for recording (I
>> tried mencoder without success)?
> I do almost exactly this and view the result with mplayer, which

MPlayer will play almost any transport stream, even without a PMT, in
which case it assumes MPEG2 video and MPEG1 audio.

> recognizes it without problems.  Note that the timing information in
> the stream can be crazy (here, for some reason, it wraps around about
> every 26½ hours).  I've done a few patches to mplayer to try to
> display saner times, but they don't work all the time.

The wraparound happens simply because the timestamps are measured at
90kHz and stored in 33 bits.  That happens to last for about 26.5
hours.  DVB streams can contain packets with actual calendar time, but
they might not be present on all multiplexes.

Måns Rullgård
mans at mansr.com

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