Mailing List Archive

On Fri, 2006-01-20 at 17:43 +1300, criggie@criggie.dyndns.org wrote:
> Steve said
> > I've got one too, had no problems getting it working. I've also got a
> > tuner-less video capture card for Sky and with my current capture
> > settings (576x576 mpeg4 + mp3)
>
> This one has been bugging me for weeks. Why do many of the mythtv howtos
> (and the default) use a 1:1 ratio of 480x480?
>
> Surely a 4:3 ratio like 640x480 or 320x240 would be best?

The upper limit for horizontal resolution is quite low, somewhere around
400 pixels across on a good TV. Cables (esp composite) limit this
further. And colour is even lower resolution than that.

Vertical resolution comes from the hsync, so it's more or less fixed.

When I had my PVR based on an SAA7134, I recorded everything in 352x576.
No-one seemed to notice. And if you can throw away half the pixels with
no-one noticing, it's going to improve what the encoder bitrate has to
encode.

480x480 is probably more horizontal than is possible over a cable (480
vertical is visible for NTSC), but there's no real harm in scaling it
back up to 4:3 when you can't resolve that level of detail anyway :)

--
David Zanetti <david.zanetti@catalyst.net.nz>
Team Leader, Systems Administration
Catalyst IT Limited
+64-4-8032233 +64-21-402260

David Zanetti wrote:

>
>When I had my PVR based on an SAA7134, I recorded everything in 352x576.
>No-one seemed to notice. And if you can throw away half the pixels with
>no-one noticing, it's going to improve what the encoder bitrate has to
>encode.
>
>
>
I used 352x576 in my early days as it is a DVD compliant format (1/2 D1).

Yes often no one could notice :)

Steve


>480x480 is probably more horizontal than is possible over a cable (480
>vertical is visible for NTSC), but there's no real harm in scaling it
>back up to 4:3 when you can't resolve that level of detail anyway :)
>
>
>


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On 1/20/06, criggie@criggie.dyndns.org <criggie@criggie.dyndns.org> wrote:
> Steve said
> > I've got one too, had no problems getting it working. I've also got a
> > tuner-less video capture card for Sky and with my current capture
> > settings (576x576 mpeg4 + mp3)
>
> This one has been bugging me for weeks. Why do many of the mythtv howtos
> (and the default) use a 1:1 ratio of 480x480?

IIRC, in my case one of my cards won't seem to do 640x576. 576x576 is
the highest resolution my cards both seem capable of. But as others
have mentioned broadcast horizontal resolution isn't even that high.
So if you capture at say 720x576 you are simply doing some scaling in
the capture device.

> Surely a 4:3 ratio like 640x480 or 320x240 would be best?

You don't want to sacrifice vertical resolution just to maintain a
"correct" aspect ratio - you're far better off doing 320x576 than
320x240.

Steve

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