[mythtv] dvb-t confusion

Jukka Tastula jukka.tastula at kotinet.com
Thu Jul 8 16:12:13 EDT 2004

Some observations and questions about installing a dvb-t card into mythtv. 
It's a long and boring story of unending failure, sorry for wasting your 

I have been trying to install a dvb-t card since tuesday evening now and so 
far I've yet to see mythtv show a single frame off the card.

I've looked through all the faqs and howtos I could find with google and 
the one on mythtv.org but there are still some things I can't understand 
about setting it up.

I started by recompiling mythtv with dvb support, which was a bit confusing 
as the howto still doesn't say much about editing settings.pro for very 
basic things like lirc support which is something I'd expect pretty much 
everyone to want. I figured I'd just get the cvs version while at it as 
reading the list I've seen great many tweaks done there since 0.15.1.

Now I'd like to know why the setup/setup binary is not in any of the 
install rules? I'd think everyone wants to use it at least once so why not 
install it? Any other neat programs/files the install rules don't install 
by default I should know about?

After this nonsense I tried to just wipe out my old (analog) card 
configuration by running the setup program. Last time I did it was such a 
long while ago I couldn't remember how it's done so I must say the process 
really isn't very clear if you're totally clueless and just follow the 
menus. What really got me the first time around was the "video sources" 
menu. I did see it said "Configuration will run in the terminal window" 
but I really didn't understand it meant the terminal window I started the 
setup program from ... so from my clueless point of view it really felt 
like it was frozen at "50%". I actually did that a couple of times before 
I figured it out. 

Then it asks me about channels. I got to where it asks me about channel 
number "(just like xawtv)" ... now what exactly do I answer there if I'm 
using a dvb-t card? It doesn't really have channels like analogue cards 
had and I can't use it with xawtv anyway... I'm confused so I answer 
nothing there.

Next I got back to the setup program and start the channel editor. Now on 
the first page, what is the "Channel number" actually used for? Is it just 
an identifier for the channel or is it used in tuning the channel?
I select the tv format PAL as that seems to be the most likely candidate 
(does it even matter in dvb?) and got to the next page. Now here it seems 
that whatever I enter in "FEC" gets copied to "HP Coderate" and other way 
around, same for "modulation" and "constellation" .. is this intended or 
am I/the program doing something wrong? I understand FEC is only used for 
cable and not dvb-t but then I have this confusing channels.conf entry 
that "scan" gives me and it says FEC_2_3:FEC_NONE ... so which one is it?
Should I just leave all these settings at "auto"? I do know what the 
correct values for all the options are but these programs don't seem to be 
so sure which option is which.  

Next page is the pids, I filled out for each channel the video, audio, 
teletext, subtitle and service id. This should be enough?

Decided I was done with the setup program and started up mythfrontend and 
opened the setup menu. There in tv settings recording profiles I see a new 
entry: "Hardware DVB Encoders"... sounds good. It has all the same 
profiles as the analog card I had previously installed had. I open up the 
one titled "Live TV". Now I'm really confused because the "image size" 
page has settings for width and height??? What exactly are these for? Then 
when I enter the next page I'm even more confused as it's the exact same 
"video compression" page as for analogue cards. I can select a codec mpeg4 
or rtjpeg and bitrate etc ... What good are these for live tv when the 
card pushes out perfectly good mpeg2 already? Same for all the other 
recording profiles, none of them seem to let me just record the mpeg2 the 
card pushes out natively? If I'm supposed to ignore all these settings 
it's certainly not very clear.

Now when I selected live tv I got what I pretty much expected after all 
this guessing. A black screen. It stayed there, frozen for a few seconds, 
10 maybe. Not a sound or frame came out of it during this time. So I look 
in the logs ...

>02:23:00 Changing from None to WatchingLiveTV
>02:23:00 DVB#0 Using DVB card 0, with frontend DST TERR.

Looks like it actually knows what my card is.

>02:23:00 DVB#0 WARNING - Unsupported code_rate_hp option.
>02:23:00 DVB#0 WARNING - Unsupported constellation parameter.

What are these two about? 2/3 should be very much legal value for HP 
Coderate and QAM 64 for constellation.

>02:23:00 DVB#0 Successfully tuned to channel .

Too bad it does't tell me which frequency that is

>02:23:00 DVB#0 Recorder: Card opened successfully.
>02:23:00 DVB#0 WARNING - Uncorrectable error in packet, dropped.
>02:23:00 DVB#0 WARNING - Uncorrectable error in packet, dropped.
>02:23:00 DVB#0 WARNING - Uncorrectable error in packet, dropped.
>02:23:00 DVB#0 WARNING - Uncorrectable error in packet, dropped.
>02:23:00 DVB#0 WARNING - Transport Stream Continuity Error. PID = 512

I goes on and on with the same last 5 lines with different PIDs till 15 
seconds into it I get something new

>02:23:15 Couldn't read data from the capture card in 15 seconds.  Game 
over, man.

Then a couple of lines later

>Videostream: ASPECT: 4:3  Size = 720x576  FRate: 25 fps  BRate: 10.00 

Then it keeps printing out those errors (~60 lines per second) till

>02:23:33 Changing from WatchingLiveTV to None
>2004-07-08 02:23:38 Closing DVB recorder
>2004-07-08 02:23:38 Closing DVB channel

The frontend prints some stuff on the terminal it is run from also

>22:36:59 Changing from None to WatchingLiveTV
>[mpeg2video @ 0x40802490]skiped MB in I frame at 4 10
>[mpeg2video @ 0x40802490]ac-tex damaged at 31 35
>[mpeg2video @ 0x40802490]Warning MVs not available
>[mpeg2video @ 0x40802490]00 motion_type at 14 11
>[mpeg2video @ 0x40802490]ac-tex damaged at 24 16
>[mpeg2video @ 0x40802490]ac-tex damaged at 40 21
>[mpeg2video @ 0x40802490]slice mismatch

There are 1000s of lines of these mpeg2video errors

>22:37:07 prebuffering pause
>22:37:09 Waited 2 seconds for data to become available, waiting again...
>22:39:22 Waited too long for ringbuffer pause..
>22:39:28 Changing from WatchingLiveTV to None
>22:39:28 Changing from None to None

After this I'm back at the tv menu.

Now I figured these errors and continuity errors and packet drops are 
because of poor signal quality. Trying with tzap ; dvbstream |mplayer - I 
sure can see lots of errors again, lots and lots of errors but once in a 
while there's a few seconds or almost watchable video. Bad signal or not 
there are frames there, broken or otherwise why isn't mythfrontend showing 
any of them? Is it being totally braindead and trying to encode the mpeg2 
into rtjpeg or mpeg4 first and the encoder can't make any sense of it/the 
cpu is too slow for that?
How good a signal strength or signal/noise ratio should I have for the 
stream to be usable? At present looking at the setup programs "Card 
verification wizard" I seem to be getting 32% signal strength and 20-35% 
signal/noise. Bit error rate and uncorrected blocks stay at zero which of 
course can't be right. But anyone have a clue on how high should the 
numbers go?

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