Re: [libdv-dev] Delaying a video signal: Some basic questions.

[Sergio R. <sr...@ya...>]

First of all, thanks Dan por your answer.
Then some more questions:

At 04:17 p.m. 05/07/2004 -0400, you wrote:
>On Mon, 2004-07-05 at 13:49, Sergio Roysen wrote:
> > Hi, hope some of the expert in this forum can help me.
> >
> > For a video art presentation involving several video signal (both recorded
> > and live), the specification for one of the live signals calls for a delay
> > of around five minutes before been exhibited. The video artist (a friend)
> > called me for help but I'm afraid I'm way out of my league on this (I do
> > mostly C programming for specialized databases engines under Linux). I
> > thought I could use dvconnect to capture the signal (the camera, a SONY
> > PC150 has a firewire connection) and playdv to show it five minutes later
> > in a scheme like this:
> >
> > dvconnect - | delay_five_minutes | playdv
> >
> > Here, delay_five_minutes should be a program (easy to write) that takes 
> its
> > input from stdin, stores it in a disk buffer and begins to output it to
> > stdout five minutes later (I also have plans to use two disks for these
> > buffer, one for getting the data in and one for getting the data out and
> > switch its use every five minutes).
> >
> > The input should be taken through firewire, and the output must be SVHS.
> > Video quality is not an issue, so I can live with some dropped frames.
> > The signals will all be PAL, but the only testing environment I has to
> > develop the solution is NTSC.
> >
> > I have several questions regarding this rather crude solution.
> >
> > 1) I don't see any parameter in the man page for playdv to take the video
> > stream from stdin. Is it posible?
>
>Yes. playdv with no args will play from stdin. Of course, without that
>you could have tried /dev/stdin or a fifo. The key thing, however, is to
>use option --no-mmap. However, playdv is not a very player and certainly
>not good for this application. For one, it does not resample to square
>pixels as used on your computer monitor if you try to play through X...

You mean I'll get a wrong aspect ratio? If that's the case, do you know 
about any other player that can take the raw dv output from dvconnect and 
play it properly?

> > 2) How to use the TV-out connector found in NVIDIA and ATI cards for the
> > delayed signal? What parameters should playdv be feed?
> > Guess it has something to do with the --display=(0|1|2|3) parameter but I
> > have not the slightest idea about the differences between gtk, Xv, and 
> SDL,
> > and, unfortunately it's beyond the amount of effort I can put in this
> > project to study them in detail. (also, since I do all of my work in
> > console mode, I'd love to see the solution implemented without having 
> to go
> > into X-Windows or depend on some of its libraries).
>
>Trying to output a NTSC or PAL video from the computer using a TV-Out
>feature is frought with problems. The biggest problem is that there is
>no vertical blanking/retrace interrupt with which to synchronize the
>video playback: how will you prevent "tearing" and make sure that the
>proper field (interlaced video) is displayed at the same field the TV
>Out is playing? Therefore, you pretty much have to deinterlace while
>resampling non-square pixels to square-pixels and hope something like
>XV_DOUBLE_BUFFER can handle the vertical blanking interrupt issue.
>
>Your best bet is to use an inexpensive DV/Analog converter box or the DV
>camera! Try using dvconnect to playback a raew DV stream you captured.
>You won't have much to worry about the issues above as long as it works.
>However, you might see some instability in the output due to interrupt
>and scheduling latency. So you will want to do a little read-ahead
>buffering on the disk I/O.

Unfortunately, the remaining budget is very tight and it would be next to 
imposible to rent 'just another piece of hardware'.
Can't use the same video camera because it's the only one available and 
it's required to take the video in signal I have to delay _all the time_.
If I cannot get a proper PAL or NTSC signal from the TV-out connector, What 
is it good for?
This signal is going to be entered in a switcher for more processing 
(chroma key and mix with another signal). The switcher is a PANASONIC 
WJ-MX20 model (in case you know something about it; may be there is 
something in it I can use for proper synchronization).
As you can see I'm pretty much stuck with that TV-out connector. Any other 
suggestions?

> > 3) Will it work? (the most I could get for the PC is a 2.4 GHz Pentium 4
> > with 256 Mb of RAM).
>
>yes

Again, thanks for you knowledge and time,

Sergio Roysen
Buenos Aires,
Argentina