[linux-dvb] Re: atscscan & azap

Patrick Boettcher patrick.boettcher at desy.de
Sat Aug 27 11:27:59 CEST 2005


On Fri, 26 Aug 2005, Alan Fry wrote:
> Hi,  some questions that I hope someone can clarify for me.  I'm using an 
> Fusion HDTV5 gold card, connected to cable. I think almost all the channels 
> are encrypted, all but one (so far) do not have a picture in Mplayer. Maybe 
> the answers are in a FAQ somewhere, but if so I haven't found them.
> 1.  Using atscscan I can:
>      - detect many channels, usually video pid is zero
>       - detect channels using QAM256, QAM64, and QAM32
> 2. using azap:
>      - I cannot zap QAM32 channels

QAM32 is not part of the ITU-T J.83 AnnexB. Thus a broadcaster 
transmitting in this modulation does not stick to the ATSC standard. That 
this works for you is probably also a undefined conincedence. The LG 
DT330x driver bails out (Have a look in syslog), when setting something 
else than 8VSB, QAM64 or QAM256.

I assume the result tuning to QAM32 is the same as with the modulation 
type tuned before? (Same channels?)

>      - I thought/think that one should:
>           a) specify video and audio pids = zero in azap, then use 
> "dvbtraffic" to find the true audio & video pids

No. Normally VIDEO and AUDIO PIDs should be found while parsing the 
MPEG2-TS sections PAT and PMT (and sometimes ATSC PSIP tables).

dvbtraffic is actually just for debugging or for getting more information 
out of the stream.

That atscscan delivers zero vpid, can have two reasons (or more):
1) it is a radio channel
2) atscscan finds an unhandled stream type when receiving the PMT (which 
contains all PIDs of a service along with stream_type).

Did you get any "WARNING: unhandled stream_type: ..." messages while 

If not, can please try dvbsnoop on any PMT for a service, which has no 
VPID. (you can get the PID of the PMT from the PAT (PID: 0x0))

>        i)    Why is ony a subset of QAM supported in azap? For example why 
> is QAM 32 not supported in azap?

see above, QAM in the USA (Digital Cable) is only defined for QAM64 and 
QAM256, although some demodulators support other QAMs too, but not the one 
you are using.

>         ii)   When I do 2(c) I can still see all the sub channels traffic 
> with dvbtraffic so it appears that azap does not filter the pids?


>         iii)   It appears that I could specify the audio and video pids when 
> I use mplayer (-aid and -vid). would this accomplish the same thing as 
> specifying them in the azap channels.conf file? Can I use one or the other, 
> or should I use both?

You only get the PIDs in the resulting TS-Stream from dvr0 you are 
requesting when running azap, which uses the PIDs from its channels.conf.

Or you use mplayer dvb://"channel name" when having a valid channels.conf 
in ~/.mplayer/.

>         iv) in almost all cases in 2(c) Mplayer detects a "TS" stream, fills 
> the cache to 20%, probes and is unable to find whatever it is that it needs 
> and then exits. Does this signal "encryption"?


>         v) is it necessary to keep azap running ?  When I kill azap,  things 
> seem to keep going at least from the point of view of dvbtraffic.  It looks 
> like azap would close various devices if it ever exited gracefully. Would a 
> version that simply tries for FE_HAS_LOCK(for a specified number of times) 
> and then exits be useful?

Would be possible, but when the frontend-node is closed, the dvb-core in 
the kernel puts the demod to sleep (if the demod supports it). Unless you 
set dvb_shutdown_timeout to 0 when loading dvb-core.ko. The lgdt330x does 
not implement a sleep method, thus it is still tuned after quitting azap. 
So for _you_ it is working, but the correct way would be to use 

HTH a little,

   Mail: patrick.boettcher at desy.de
   WWW:  http://www.wi-bw.tfh-wildau.de/~pboettch/

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