[ivtv-users] NTSC centering, "itchy" top line

[Dale Pontius]

Andy Walls wrote:
> On Sat, 2009-11-28 at 10:31 -0500, Dale Pontius wrote:
>> Devin Heitmueller wrote:
>>> On Wed, Nov 25, 2009 at 12:59 PM, Dale Pontius <DEPontius at edgehp.net> wrote:
>>>> Using hvr-1600 capture cards on the NTSC side.  All works well, but when
>>>>  playing back, or especially on live TV (In MythTV) the image centering
>>>> is a bit off - down and to the right.  In addition, the top line of the
>>>> image is "itchy", flickering and so forth.
>>>>
>>>> Are there knobs somewhere to recenter the image?
>>>>
>>>> I presume the itchy top line has something to do with interlacing...
>>>> It is present in MythTV, but disappears after transcoding.  Is this
>>>> something the right de-interlacer would fix?  What should the
>>>> de-interlacing situation be when I'm using NTSC TV-Out on the video card?
>>>>
>>>> Should this question really be asked on the MythTV list, also?  I know
>>>> the re-centering question is probably more appropriate here, and moving
>>>> the image up might hide that top line, but perhaps the interlacing
>>>> issues are better asked there.
>>>>
>>>> Thanks,
>>>> Dale Pontius
>>> I can comment better if you provide a screen capture of the issue.
>>> However, it sounds like you are seeing the VBI information at the top
>>> of the screen, which is usually hidden by a television's "overscan"
>>> area.
> 
> 
> Hi Dale,
> 
>> I'll have to look into how to get a screenshot.  I normally run
>> fullscreen, which makes using screenshot tools (which I don't often do
>> anyway) kind of tough to use.
> 
> Could you tell me what your analog source is:  broadcast cable, VCR,
> Set-Top-Box, etc. and on what input: analog tuner, or SVideo or
> Composite?
> 
This is the analog tuner input, connected to broadcast cable.
> 
>> But that's also why I was mention looking
>> for a knob to recenter the image.
> 
> "Knobs" to muck with HCENTER and VCENTER in the cx18 driver invariable
> begin to affect capturing VBI data properly and maintaining HSYNC.
> 
>> There's black to the left and top of
>> the image, and it flows all the way to the right and bottom sides.
>> Seems to me a little recentering plus a little overscan could take care
>> of it just fine.  But with the image shifted down as it is, enough
>> overscan to take out the "itchy line" takes out too much else.
> 
> Where is this itchy line?  If it is in the Vertical Blanking interval it
> should be mostly black with white flickering portions with the right
> side of the line flickering more than the left.
> 
Given what people have said here, I've looked again at the itchy line.
It's not VBI.  It appears to be an interlace problem, as best I can tell
The top line of the picture appears to come and go, very quickly.  It
doesn't seem that fast, and I would swear I couldn't really see 1/30
second, but that's kind of what it seems like - the top line present on
every other frame, going black on the alternate frames.  There is no
bright white coming and going - it's the image line flickering.

I'm guessing that someone's going to tell me to check my de-interlace
option in MythTV.

> The menu in MythTV has a "zoom" option IIRC correctly and it lets you
> pan around the centering of the image with the arrows when enabled.
> 
I recently fiddled with this just a little, while getting lirc working
again after upgrading to 0.8.6.
> 
>> Quantitatively, the down-right shift is on the order of the width of the
>> typical single-height taskbar of the typical window manager.  (In the
>> current case, xfce - taskbar on top.)
> 
> A full NTSC line is 286/4.5 MHz = 63.556 microseconds.  Around 10.9 usec
> are in horizontal blanking, leaving about 52.656 usec of active line
> video.  Sampled at the pixel clock of 13.5 Mpixels/sec * 52.656 usec =
> 710 pixels.  You can rely on about 3 pixel times being dark on both
> sides of the line, so that means the visible, bright pixels on a NTSC
> line is 704.
> 
I've looked a little more at the centering.  There is more space on the
left, but still some space on the right.  There is space at the top, but
none on the bottom.

> (That's of course if you told MythTV or mplayer to capture at 720x480)
> 
> Get out a ruler and measure your screen.  Knowing that the bright part
> of a line is 704 pixels, you can estimate the pixel times that are dark
> on the left and right side of the screen.
> 
I'm set up for 720x480, since I was trying to set the capture native for
the card.  I have yet to do more of this - I just got my main backend
machine up - it was offline while my daughter was home for Thanksgiving.
She had a paper, and had her machine home with her.
> 
> 
>> I'll find the time to look harder at the screenshot issue.  When I've
>> needed to do it before, I've used "xv" - I just don't do it often.
> 
> I only have a STB to generate analog for me anymore.  It's reconstructed
> analog waveform is rather ideal.  I'll still try to reproduce the
> problem.
> 
> But a window or screen shot would help.
> 
> $ /usr/bin/mythfrontend -w --geometry 720x480
> 
> will open the Myth frontend in a window.
> 
I'll give this a try - I didn't know you could put the whole thing in a
window with a command line option like this.