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Re: Sequence Grabber: subsampled images with multiple cameras on different FW ports?

Hi,

It's probably less to do with simple bandwidth, but rather the speed that the devices are capable of running at.

Most DV A/V devices do not run at maximum bus speed, i.e. S400 or S800. Many force the bus to run at a much slower speed when communicating with them, e.g. S100 (100Mbps) or possibly even lower (required bandwidth for 'normal' DV is about 25 Mbps AFAIR?). The DV cameras and DVdecks I've seen (at consumer level anyway) run at S100, although I've got some newer DV-Analog converters which run at S200.

Because the bus has to 'gear down' to talk to these devices, other devices, even faster ones, have less actual time on the bus per second, and this can mean that the effective bandwidth of the bus is much reduced.

In my experiments (i.e. actually trying to get the damn things to work!) I've been only been able to get 2 S100 DV devices to work reliably on the same FW400 bus, connecting a 3rd DV device, even an S200 one, results in none of the devices being able to keep the video in sync. The theoretical bandwidth of the bus has not been exceeded, or even approached, but the slower devices drag the actual available bandwidth down.

Possibly what is happening with your iSight, which requires alot more bandwidth than the DV camera, but which also runs at a much higher bus speed. is that the iSight (or driver) is detecting the actual available bandwidth due to the lower speed device, and intelligently gearing down itself so that it can send a reliable stream. Connecting it to a second bus (e.g. a cheap third-party Firewire card) should enable both to communicate at full quality. I've certainly had three DV devices, two S100 and one S200 on two separate buses, with no problems, using Isynchronous transfer to stream data to all three at once. You may even have more luck with two iSights on the same FW800 bus, because although both are bandwidth hungry, they are also high-speed devices, and so will share the bus much more efficiently.

To see what speed your devices are running at, look the Firewire bus in Apple System Profiler, it should list one of the S speeds next to each of your devices.

Hope that helps!

Ellis.


Kevin Quennesson wrote:

Hi,

I based a project upon the WackedTV sample code.

However when I am trying to use multiple cameras - and camcorder connected to the front port of my g5 and an iSight connected to the back FW800 *or* back FW400, the resolution seems to be downsampled by 2 automatically, or more exactly the calls CVImageBufferGetCleanRect(pixelBuffer) and CVImageBufferGetDisplaySize(pixelBuffer) in the displayData callack of SGVideo.mm returns a size subsampled by two.

I understand why this would be on the same FW400 port, but in my mind the FW800 and front FW400 were independant? Why does this happen? Do I need to change some code to avoid that? 

Thanks for any help!

The subsampling happens as soon as I *plug* a second camera, I do not even need to start capturing.

Kevin
CVI

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References: 
 >Sequence Grabber: subsampled images with multiple cameras on different FW ports? (From: Kevin Quennesson <email@hidden>)



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