video4linux, bttv, vbi and missing lines in teletext...

video4linux, bttv, vbi and missing lines in teletext...

[Manuel Reimer]

Hello,

I want to view teletext using my bt878 card.

I tried with mtt, but I always had missing lines in the teletext 
display. As I have been unsure, which is the reason, I also downloaded 
and compiled alevt. Here, the same lines are missing.

Now I used the test tools from the zvbi library, to capture the teletext 
page to a file. I never got any errors while capturing. After capturing 
for a while, I sorted the result by line and found out, that line 1 and 
line 13 are always missing on this teletext page (Page 116 on german 
channel "Sat1"). In the whole logged file, I never have those two pages, 
but have all the other pages several times.

Which may be the reason? Why I'm unable to receive those two lines? I 
connected a TV to the same AV cable, I usually have on my PC, and the TV 
displays the teletext without any problems...

Is someone here, who may help me with getting this problem fixed?

Today I'll visit someone, who has a Windows PC. I've already removed the 
bt878 card from my PC. I'll try at his PC, if I get the same problem 
with the original driver and TV watching software. If this works, I'll 
boot Knoppix from his PC to get sure, that I also get the same problems 
on his PC with his video source. I'll tell you about the results of my test.

For the case, it's important for the discussion:

manuel@linux2:~$ uname -a
Linux linux2 2.6.21.5 #1 Tue Jun 19 15:10:54 CDT 2007 i686 Pentium III 
(Coppermine) GenuineIntel GNU/Linux

The "card=" value for the bttv module is currently 54. I also tried 65 
with the same results. Watching TV works pretty well, but in teletext, I 
miss one or two lines in nearly any page.

Thank you very much in advance

Yours

Manuel

Re: video4linux, bttv, vbi and missing lines in teletext...

[Manuel Reimer]

Manuel Reimer wrote:
> Today I'll visit someone, who has a Windows PC. I've already removed the 
> bt878 card from my PC. I'll try at his PC, if I get the same problem 
> with the original driver and TV watching software. If this works, I'll 
> boot Knoppix from his PC to get sure, that I also get the same problems 
> on his PC with his video source. I'll tell you about the results of my 
> test.

I tried my card on the windows PC, but we wasn't able to install the 
original driver, as it is outdated and didn't install on Windows XP. So 
we used:

http://sourceforge.net/projects/btwincap

Using this driver, I had exactly the same problem, I also have on Linux. 
I was able to get a second card, which is the same model, I have. I 
tried the second one, too and also have the missing lines with the 
second card.

Now I don't know, if this is a driver problem, or if this model of bt878 
card is just cheap shit.

CU

Manuel

Re: video4linux, bttv, vbi and missing lines in teletext...

[Mauro Carvalho Chehab]

Hi Manuel,

I noticed you've opened a thread about this at both V4L ML and LKML. The
better place for you is to discuss this issue at V4L ML. LKML is very
generic and just a very small group of people working with those devices
could help you. Those are also listening at V4L, among with several
other guys.

Em Qui, 2007-09-06 às 17:25 +0200, Manuel Reimer escreveu:
> Manuel Reimer wrote:
> > Today I'll visit someone, who has a Windows PC. I've already removed the 
> > bt878 card from my PC. I'll try at his PC, if I get the same problem 
> > with the original driver and TV watching software. If this works, I'll 
> > boot Knoppix from his PC to get sure, that I also get the same problems 
> > on his PC with his video source. I'll tell you about the results of my 
> test.
> 
> I tried my card on the windows PC, but we wasn't able to install the 
> original driver, as it is outdated and didn't install on Windows XP. So 
> we used:
> 
> http://sourceforge.net/projects/btwincap
> 
> Using this driver, I had exactly the same problem, I also have on Linux. 
> I was able to get a second card, which is the same model, I have. I 
> tried the second one, too and also have the missing lines with the 
> second card.

Maybe the windows driver you took may be using the same VBI algorithm as
the Linux driver.

> Now I don't know, if this is a driver problem, or if this model of bt878 
> card is just cheap shit.

I suspect it is just a matter of proper settings.

The bttv hardware only provide "raw VBI". It means that it will return
to a program whatever stuff were sent during VBI (*). 

(*) VBI is a timing interval that exists at analog transmissions between
the end of one line and the start of another. It were meant to give some
time to the electron beam to move from one line to another.

For BTTV driver, you just set-up to adquire VBI or not. There are some
standard-based definitions, like the expected VBI intervals, and the
interval where VBI collect starts.

The hard work is doing at software decoding level, where a filtering
process takes place, to analise if the patterns makes some sense,
according with some specification, like teletext. If it makes sense, the
decoding occurs.

Maybe your board is collecting VBI sooner (or later) than expected, and
this is causing the missfunction of your userspace app.

Anyway, Michael Schimek send you some glues at V4L ML. He is one of the
authors of those VBI decoding tools. So, I'm sure he can guide you to a
proper solution.

Cheers,
Mauro