david.zanetti at catalyst
Jan 19, 2006, 9:07 PM
Post #2 of 4
On Fri, 2006-01-20 at 17:43 +1300, criggie [at] criggie wrote:
Re: Ratios, Was Building a new box using an ASUS A8N-VM CSM board
[In reply to]
> 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
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 [at] catalyst>
Team Leader, Systems Administration
Catalyst IT Limited