Mailing List Archive: MythTV: Users

Problem recording AC3 from DVB-T

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dooze at optushome

Jun 22, 2004, 6:50 PM

Post #1 of 3 (571 views)
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Problem recording AC3 from DVB-T

I'm in Melbourne, Australia, running MythTV with a DVB-T card. I've been
trying to record SD broadcasts using the AC3 PID (set up in mythtv-setup in
the channel editor). I'm using the "scan" tool to find the correct PIDs,
here's the dump for one channel in particular:

-------------------------------------------------------------
scan -c
using '/dev/dvb/adapter0/frontend0' and '/dev/dvb/adapter0/demux0'
Frontend can not do INVERSION_AUTO, trying INVERSION_OFF instead
0x0000 0x0430: pmt_pid 0x0100 Nine Melbourne -- Nine Digital (running)
0x0000 0x0431: pmt_pid 0x0101 Nine Melbourne -- Nine Digital HD (running)
0x0000 0x0432: pmt_pid 0x0102 Nine Melbourne -- Nine Guide (running)
dumping lists (3 services)
Nine Digital (0x0430) 01: PCR 0x0087 V 0x0207 A 0x02d0 (eng) TT
0x0247 AC3 0x02d1
Nine Digital HD (0x0431) 01: PCR 0x0080 V 0x0200 TT 0x0240 AC3
0x028a
Nine Guide (0x0432) 01: PCR 0x0085 V 0x0205 A 0x02bc (eng)
-------------------------------------------------------------

I can record from the standard audio PIDs fine (eg. 0x02d0) and they play
back great either via the offical front-end or the XBox Media Center
frontend I'm running, but if I specify the AC3 PID (0x02d1) the mythbackend
doesn't appear to correctly identify the audio as an AC3 stream. When a
recording starts using the AC3 PID the backends logs this:

-------------------------------------------------------------
2004-06-23 11:10:09 DVB#0 Recorder: Card opened successfully.
Videostream: ASPECT: 16:9 Size = 720x576 FRate: 25 fps BRate: 10.00
Mbit/s
Audiostream: Layer: 1 BRate: 288 kb/s Freq: 48.0 kHz

-------------------------------------------------------------

As you can see, it thinks the AC3 stream is still a Layer 1 stream, just
with a higher bit rate than usual. I'm running the released 0.50.1 build,
I've also tried earlier CVS snapshots over the past with the same result.

Any ideas?

Stuart

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hamish at cloud

Jun 23, 2004, 7:49 AM

Post #2 of 3 (561 views)
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Re: Problem recording AC3 from DVB-T [In reply to]

On Wed, Jun 23, 2004 at 11:50:52AM +1000, Stuart Cameron wrote:
> I'm in Melbourne, Australia, running MythTV with a DVB-T card. I've been
> trying to record SD broadcasts using the AC3 PID (set up in mythtv-setup in
> the channel editor). I'm using the "scan" tool to find the correct PIDs,
> here's the dump for one channel in particular:
[..]
>
> I can record from the standard audio PIDs fine (eg. 0x02d0) and they play
> back great either via the offical front-end or the XBox Media Center
> frontend I'm running, but if I specify the AC3 PID (0x02d1) the mythbackend
> doesn't appear to correctly identify the audio as an AC3 stream. When a
> recording starts using the AC3 PID the backends logs this:
>
[..]
> As you can see, it thinks the AC3 stream is still a Layer 1 stream, just
> with a higher bit rate than usual. I'm running the released 0.50.1 build,
> I've also tried earlier CVS snapshots over the past with the same result.

Hi Stuart,

Detecting AC3 in received DVB seems a bit tricky. Have a Google for
KaxTV, which is a tool that can do it.

As to Myth misdetecting the audio as Layer 1, I'd wonder whether that
matters in practice? Does it alter the data stream, or just stream it to
disk anyway?

Cheers
Hamish
--
Hamish Moffatt VK3SB <hamish [at] debian> <hamish [at] cloud>

lists at kenneth

Jun 23, 2004, 10:13 AM

Post #3 of 3 (553 views)
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Re: Problem recording AC3 from DVB-T [In reply to]

Hi Hamish,

On Wednesday 23 June 2004 16:49, Hamish Moffatt wrote:
> Detecting AC3 in received DVB seems a bit tricky. Have a Google for
> KaxTV, which is a tool that can do it.
>
> As to Myth misdetecting the audio as Layer 1, I'd wonder whether that
> matters in practice? Does it alter the data stream, or just stream it to
> disk anyway?

I'm thinking this might be caused by a quickfix that was put in for the sake
of bringing the mythtv player through channel changes painlessly.

The function below will incorrectly label an ac3 stream in the audio pids
field as a normal PES audio stream. The problem with moving the ac3 pid
elswhere is that a patch went in a long time ago, and that was not written
with a missing audio pid in mind, so it'll segfault. If you could test either
ifdef'ing out the audio stuff below, then test a channel with ac3 as audio
pid. Or have one mpeg2 audio pid in the audio pid field and a ac3 pid in the
other field (here libavcodec should at least list the ac3 stream). Or is
there more to it than this?

Kenneth

void DVBRecorder::CorrectStreamNumber(ipack* ip, int pid)
{
for (unsigned int i=0; i<chan_opts.pids.audio.size(); i++)
if (chan_opts.pids.audio[i] == pid)
ip->cid = 0xC0 + i;

for (unsigned int i=0; i<chan_opts.pids.video.size(); i++)
if (chan_opts.pids.video[i] == pid)
ip->cid = 0xE0 + i;
}
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