Yes, the "triplex multiplexers", or whatever they are called, are quite
expensive. Pop one open and guess what you will see? Usually multiple
Philips chips and an Averlogic. Better to just go straight to four boards or
one board with four chips and a bridge.
From: Michel Bardiaux <mbardiaux@xxxxxxxxxxx>
Reply-To: video4linux-list@xxxxxxxxxx
To: video4linux-list@xxxxxxxxxx
Subject: Re: Grabbing from multi input
Date: Mon, 17 Feb 2003 18:37:46 +0100
Fabrizio Zeno Cornelli wrote:
>
> Many cards have four or even more inputs. Anyway a single bt chip (a
> single device) can handle up to four inputs with a multiplexer.
> When an input is selected, you can gain a framerate of 25 fps (PAL).
>
> I must acquire images from multiple cameras (at least two) continuosly.
> Does anybody know hot to get the best framerate per input (ideally 25/4
> fps less switch time) grabbing sequentially from all the avaiable
> channels?
>
> With the following mode I can achieve only 4 fps per camera (with 2
> cameras):
>
> while (1) {
> switchInput(0)
> read();
> ... (process image)
> switchInput(1)
> read()
> ... (process image)
> }
>
> With the following mode (memory mapped buffers) I can achieve 12 fps per
> camera, but the images are corrupted (contains informations from more
> than a single camera) when using more than 1 buffer, and using 1 buffer
> the result is tipically the same as reading with read():
>
> while (1) {
> dqbuf();
> ... (process image)
> switchInput(1)
> qbuf();
> dqbuf();
> ... (process image)
> switchInput(0)
> qbuf();
> }
>
> I've tried inserting some usleep(), from 10 to 80 msec, in different
> places, in order to synchronize, but there are corrupted images anyway.
>
> I repeat, many cards have four inputs, so I suppose that will exist a
> method to grab from multiple cameras and gain a real-time framerate.
That they have multiple inputs does not guarantee they can switch from
one to the other quickly enough. AAMAF it is almost sure you can *not*
achieve an aggregate rate of 25fps because the synchro signals are
likely to be out of synchro (sic) w.r.t. each other, so that the TV
board will need some time to re-acquire synchro after every switch.
One solution that *would* work (we use it, although not with a BT board
or with Linux): get a so-called 'quad multiplexer' and change the
*quadrant* in your software, instead of the input. But that might be
more expensive than having simply 4 BT boards!
>
> Thanks,
> Fabrizio Cornelli and Sandro Bosio
>
> --
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--
Michel Bardiaux
Peaktime Belgium S.A. Bd. du Souverain, 191 B-1160 Bruxelles
Tel : +32 2 790.29.41
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