Re: Multiple Port Grabber Cards

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From: Thore Karlsen [MVP DX] (sid_at_6581.com)
Date: 08/02/04 

Date: Mon, 02 Aug 2004 09:40:01 -0500

On Mon, 2 Aug 2004 10:16:09 -0400, "Chris P. [MVP]" <msdn@chrisnet.net>
wrote:

>>> I want to support multi-input grabber cards (for security) and was
>>> wondering if anyone happens to know how the multi-input cards look
>>> to the directx software: In other words, do multi-inputs enumerate
>>> just like normal capture cards so that each video input looks like a
>>> separate capture-source-filter. I'm only talking about the grabber
>>> cards that provide simultaneous capture inputs.
>>>
>>> Would like to support multi-capture in a security camera prog I'm
>>> working on.

>> The ones we make come up as different devices. Ours, and most cards
>> we've looked at, seem to be just a bunch of capture chips behind a PCI
>> bridge.

>Ditto. All the good ones we've seen enumerate as individual capture
>devices. There are some cheaper cards that may only have a single capture
>chip behind a video mulitplexor - these cards cannot get full frame rate
>from all channels.

There are other problems with it as well, such as auto gain on the chip
getting confused with multiple sources, and if one camera goes down it
could take all the others on the chip with it, too. There's also the
problem with bouncing video where it doesn't capture the same field of
the frame every time. Can't change procamp properties per input, either.
In general, multiplexed inputs are a terrible solution.

>Also beware of PCI bus limitations if you start trying to stack up mulitple
>cards in a machine.

Absolutely. Server class machines generally work better for this. Do you
have any experience with PCI Express? 

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