[mythtv-users] SD DVB-T in the UK: Surround Sound?

[Paul Gardiner]

On 14/01/2011 12:46, John Pilkington wrote:
> On 14/01/11 12:17, Paul Gardiner wrote:
>> On 14/01/2011 12:06, Alex Butcher wrote:
>>> On Fri, 14 Jan 2011, Paul Gardiner wrote:
>>>
>>>> On 14/01/2011 10:33, John Pilkington wrote:
>>>>> My recording of the first episode of the new series "Human Planet"
>>>>> from
>>>>> BBC1 (SD DVB-T) is marked as being in "Surround Sound". I don't think
>>>>> I've seen this before. I've now deleted the original recording but
>>>>> ffmpeg -i looks as usual:
>>>>>
>>>>> Stream #0.0[0x258]: Video: mpeg2video, yuv420p, 720x576 [PAR 64:45 DAR
>>>>> 16:9], 15000 kb/s, 25 fps, 25 tbr, 90k tbn, 50 tbc
>>>>> Stream #0.1[0x259](eng): Audio: mp2, 48000 Hz, 2 channels, s16, 256
>>>>> kb/s
>>>>> Stream #0.2[0x25a](eng): Audio: mp2, 48000 Hz, 1 channels, s16, 64
>>>>> kb/s
>>>>> Stream #0.3[0x25d](eng): Subtitle: dvbsub
>>>>> Stream #0.4[0x261]: Data: [5][0][0][0] / 0x0005
>>>>>
>>>>> Is this a real New Thing or just a mistake in the flags?
>>>>
>>>
>>> We're talking about the 256kbps stereo stream and the 64kbps mono
>>> streams,
>>> right? I think I see these a lot. This is why I added logic to pick the
>>> 'best' (um, biggest!) audio stream to mythburn.py.
>>>
>>>> Might it be flagging the use of pro logic?
>>>
>>> Or a Welsh soundtrack? Or a descriptive narration for the
>>> visually-impaired?
>>
>> No, I mean marking it as "Surround Sound" may just mean the main
>> stereo stream encodes a pro-logic back channel. BBC have been
>> using pro-logic for ages.
>>
>> Paul.
>
> I don't think the 64kb/s channel has anything to do with this: it's just
> the usual 'audio for the visually impaired' track and may or may not be
> always active; but is the main stereo channel now including new info
> that could be accessed, and am I throwing that away by treating 'Stream
> #0.1' as pure stereo - remembering that this is an SD transmission?
> Might 're-encoding to ac3' (which is the default in MythArchive)
> actually recover something or is it just there because equipment for the
> American market doesn't understand mp2?

If it's pro logic, then you wont gain anything by recoding it as ac3,
unless you wish to play it with something that understands ac3 and
not pro-logic (unlikely I'd have thought).

You wont be throwing the information away: pro-logic works at the
analogue level. Nor should you be surprised that ffmpeg does not
report it because it would need to decode the stream and do
quite complex analysis on the result to be able to tell pro-logic was in
use.

Paul.