DEPontius at edgehp
Nov 29, 2009, 2:49 PM
Post #7 of 7
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> 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.
>>>> 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"
> 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
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
> 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.
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