I'm considering MythTV as part of a security video project and was wondering how many usb tuners the hardware could handle.

I've read that uncompressed 640x480 video is 47 mb a second- a usb2 bus can handle 480 mb/s. So "theoretically" it could handle 9 feeds...

Throw in hardware encoding to reduce the size of the tuner stream and you may be able to get more.

Anyone got any thoughts on this? What's the largest number of usb tuners anyone has gotten working?

tks mightily

Scott Syms
Halifax, NS

I think the theoretical amount and the actual amount of tuners you could use are going to be very different. All USB traffic is controlled by the processor. I don't know the details but I do know that the more you add the faster your processor and system bus are going to need to be. If you google for usb vs firewire you will find some good articles that explain that firewire is an end to end protocol that doesn't really need the CPU. USB on the other hand is controlled/trafficked by your processor and is therefore at the mercy of your CPU.

Something to think about if you do run into performance issues is building one myth backend server, then have several other systems with the tuners doing the recording but saving it to the backend server. A cluster of sorts. You could probably find some old 800 or 900 Mhz systems that could handle it just fine. I'm curious about this one so please post if you get this set up.

Matt

The usb2 chain runs at 480 megabits per second. The video you want to encode is 47 megabytes per second. Now knowing that there
are 8 bits in one byte, that makes your usb2 chain run at 60 megabytes per second.
sorry about the bad news.
you might want to try pci slot instead or with the usb2.

Luckily its not going to be uncompressed video

Yeah, we've done this math before in relationship to disk bandwidth, and for MPEG-2 compressed video you're talking more like 600 Kbytes/second. Figure it's a bit more once you add the protocol overhead for USB, but still under 1Mbyte/second. I'd be more worried about latency issues and CPU load, which is the same reason why DMA is so critical for the disk.