I have been working on this for a long time. The problem really never goes
away. You get the top/bottom switch, and also frame tearing intermittently
when consecutive signals go really badly out of sync. The external switcher
is the solution I will be looking at next. My stable framerates are
considerably lower than you are achieving as I have low quality cameras to
deal with that drift in and out of sync. Without the external switcher, I am
getting 6 to 8 *total* fps. Basically, every time I switch inputs, I have to
wait a while for the chip to stabilise. My guess is it is turning its ADC
off and on or something... maybe it just has really poor syncing..
The cards I have seen that achieve closer to 25/x fps per camera all have
LM1881 and a video mux. And those components are cheap, and the driver
changes aren't too bad from what I can see.
From: Gabor Kerenyi <wom@xxxxxxxxxxx>
Reply-To: video4linux-list@xxxxxxxxxx
To: video4linux-list@xxxxxxxxxx
Subject: multiplexing 4 channels
Date: Mon, 17 Feb 2003 17:06:40 +0100
hi!
I have a serious problem with the BT8x8 chip. I have to
grab four channels at the highest framerate. I don't need
the full frametrate (25 fps), but I need as high as possible,
the higher the better.
I looked for this problem in the archive and there was a
thread about it but no solution.
I use 4 not synced video inputs (PAL).
There are two problems:
1. If I change the channel after each field then the BT chip
sometimes swaps the top and bottom field. I don't know why.
The BT documentation says it determines the top/bottom field
from the first HSYNC after the VSYNC rising edge (in phase
with the VSYNC or not).
I can get the video signal stable if I use a second read and
drop the first picture. But the framerate is too low this time.
2. The framerate independently from the stableness is low.
8 fps/channel for two channels. Well there is a Piccolo (Eurosys,
using BT chip) card and a windows driver and it can do
about 16 fps for two signals and 4-5 for 4 signals (and
of course it never swaps fields).
So how that card and driver can do it? OK, there is
an external muxer for the inputs, so only one input is
used of the BT chip. I think this is not a big difference since
the muxer in the BT chip is also an "external" device.
Does anyone have ideas about it? Why does it swaps
fields at all???
thoughts from me:
it swaps because the BT chip has an internal sync to
correct bad V&HSYNCs, such as VHS video. Because the
mux is independent from the chip and when another
source is selected it simply follows its internal timer and
doesn't care of extra SYNC pulses.
Would it help if we reseted the chip after the channel switch?
Or simply force the chip resync (if it's possible)
I can get fields swapped if I grab full size picture (768x576)
in interlaced mode as well.
Thx,
gabor
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