[mythtv-users] vertical shakeing, odd/even confusion svideo out ATI9100

Dennis Tell dennis.tell at gmail.com
Sun Jul 17 07:20:58 EDT 2005

Hi all,

I have a bit of a problem with TV-out and deinterlacing, and would
appreciate input from someone who's been working on that part of the
code base.

I am using mythtv with a motherboard with integrated ATI 9100 pro
graphics and svideo tv out. The tv out is handled by ATI's propriatory
driver. The driver lets you choose synchronization of the buffer swap
with the screen refresh. I am using the "bob" deinterlace mode with a
screen at 50Hz (PAL), which if I understand the source correctly
alternates output of odd and even fields, scaled to full screen
resolution. The ati driver then reinterlaces the picture by picking
even lines from even fields and odd lines from odd fields.

My problem is that 50% of the time odd and even seem to get mixed up,
resulting in a tv image that shakes vertically. If I hit pause,
rewind, fast forward or anything else that causes changes in the
display, the problem is usually resolved..

Sometimes the problem occurs spontaneously while I'm watching tv, even
though I did nothing to "provoke" it. I'm guessing this is due to
mythtv being a little bit late or early with the front buffer update,
so that the graphics card misses one image or display the same one

My questions are as follows:
1. Is this a known issue?
2. Would we require source code for the ati driver to resolve this
issue, or are there other solutions?
1. I can understand why the problem occurs when starting to watch tv,
but why does it occur spontaneously while I'm watching TV? Slight
frequency mismatch between tv-out and incoming  dvb stream? CPU spikes
on my machine causing one field to be dropped?

For reference, I'm using a computer with:
Gigabyte 8TRS350MT
Celeron D 2.4
512 mb ram
Aver 771 DVB-T PCI

I live in Australia, so am using PAL-B with the ati fglrx driver. My
screen is set to 800x600 at 50Hz. I would like to set it to 720x576,
but can't as the driver doesn't like it. Would I perhaps have less
problems if I managed to set the resolution to 720x576? I've heard
people saying that what your screen resolution is set to is irrelevant
as the driver rescales and outputs at PAL resolution anyway. Seems
reasonable to me. 800x600 should be fine if that's the case.


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