Re: firewire bus/card counter

Dan Dennedy wrote:
> On Tue, Jun 30, 2009 at 3:54 PM, Carl Karsten<[email protected]> wrote:
>> Also, is there a spec that says 1 dv cam per buss, or is that some
>> side effect of typical implementation?
> 
> No limitation in the spec.

Well, there is of course the general limitation of bus bandwidth, 
particularly the portion of it which is available to isochronous 
transmissions on one hand (that's 80% of total bandwidth), and the 
actual bandwidth utilization by a DV streamer on the other hand, 
according to the stream format.

> I have been able to get 2 DV at S100
> working on 1394a. I have been able to get at least 4 DV analog
> converters that have dip switches to set S400 working on 1394a. I
> think it might be possible to get more on gap-optimized bus, but
> Stefan knows more. It is has been a long while since I looked at that
> math. The trick to make it work is in IEC 61883 Connection Management
> Procedures, which I tried to implement in libiec61883 and the dvgrab
> -guid option. However, I ran into difficulty with devices behaving
> differently - even different models from the same manufacturer. Your
> mileage may vary, and I am no longer in a position with multiple
> supporting devices to do more work on it.

I always forget what the bandwidth of a single DV stream is.

25 Mbit/s net, but due to occasional zero packets 30...31Mbit/s in practice?
http://lists.apple.com/archives/Firewire/2008/Jul/msg00001.html

29 Mbit/s?
http://www.adamwilt.com/Tidbits.html  (section "FireWire Frustrations")

I think this post by Eric Anderson is the canonical answer:
http://lists.apple.com/archives/firewire/2004/Oct/msg00033.html

Gap count optimization doesn't influence the issue per se, because it 
doesn't change how much bandwidth is available for isochronous I/O.  But 
I presume that gap count optimization could help with reliability of 
accompanying asynchronous traffic (camcorder control and status) if 
isochronous bandwidth exceeds the allowed 80% of total bandwidth.

(I have no own practical experience in this area and don't know a lot of 
the 61883 and AV/C specs).
-- 
Stefan Richter
-=====-==--= -=== ----=
http://arcgraph.de/sr/

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