[linux-audio-dev] Re: [linux-audio-user] Re: Firewire, what's the story? - Linux-audio-dev

9 Dec 2003

Hi,

tisdagen den 09 december 2003 12.22 skrev Steve Harris:
...
  On Tue, Dec 09, 2003 at 09:25:42AM +0100, Clemens
Ladisch wrote:
    I seem to recall that it does, but that the packet
size is smaller,
 making the latency problem easier to deal with. 
 Isoch - 1394 has a timer operating on the bus. This timer happens
 (roughly) every 125uS. 
 USB isochronous transfers happen once per millisecond. 
 OK, so its the difference between 44 or 45 samples per packet at 44.1kHz
 (USB) and 5 or 6 (1394).

 So, a plausible jack period of 64 samples will be 11 or 12 1395 packets,
 but always 125uS of jitter of course, (10 or 11 packets at 48k).

 I dont really have any idea how bad that is, its 8% of the availble time
 slot for processing the 64 samples.

 You could lessen the effect of the jitter by having triple-buffering in
 software, as in a 3x64 PCI soundcard - I dont know wether thats better
 than just going to 128 samples or not.

 FWIW, for my needs I rarely go below 256 samples/period anyway, even
 though my setup is capable of it, but I understand there are people who
 really need very low latency.

 At 256 samples/period the jitter is only 2.1% of the time slice, which is
 unlilkly to matter - cache temperature and so on has a far greater effect
 than that. 
Though it is interesting debating the technical merits of firewire, for me, 
atleast, it would be far more interesting to get it actually working. I'm 
_very_ interested in utilizing these hardwares.

I googled a little last night, some people we all know popped up here and 
there (Hi Steve, Mark and Bob).
There was a LAD message from 2001, someone who had been in contact with Yamaha 
and it seemed they(Yamaha) where working towards making mLan a part of the 
A&M standard (I think I got that right...not sure)... now... I'm not entirely 
sure what this means. It seemed as the A&M standards also cost a lot of 
money?
Do we have enough information to implement mLan support? 
As I understand it, Bob Ham had/is working on implementing 61883 support for 
Jack, which seems like it's needed for mLan, a layer below it perhaps? How 
far has this come, is there something one can test somehow, what hardware do 
one need?

Regards,
Robert
ps.
I'm cc:ing LAD as I'd like to move the discussion there. Remove LAU if you 
respond to this.
ds.