Re: Thinkpad R60 for Audio Update - Firewire Conflicts with Audio

Dmitry Baikov wrote:
> On 4/10/07, Robin Gareus <robingareus.org> wrote:
>> PCMCIA is ok, but not great for low latency. (adds
one extra interface
>> between PCI bus & sound-card). - the cheap
solution is to get a PCMCIA
>> card for your hard-disk - instead of using the
built-in firewire for the
>> disk. - maybe you want to do that anyway. use the
built-in firewire to
>> an external audio-device and the PCMCIA for
storage.
> 
> Quite the opposite.
> PCMCIA is great for latency. That "extra" PCI
bridge does NOT affect
> audio latency.
One thing is of course that you should check that your
bridge's 
interrupt isn't shared either.

> Firewire latencies are bigger than PCI/PCMCIA and will
always be. Ask
> Pieter Palmers for details.
Correct. The FireWire host controller is a PCI card, meaning
that it has 
the same latency limitations as a PCI soundcard. The latency
of the 
FireWire bus itself is added to this lower limit.

Let's say that if you can achieve a certain latency number
on a certain 
configuration with a prefect FireWire setup, you will always
be able to 
achieve a lower latency on the same system with a perfect
PCI setup.

That being said, I don't think this is an issue. If we
address the 
FireWire host controller in the same way as we address the
PCI soundcard 
(i.e. DMA double buffering), it won't introduce any extra
latency. The 
extra latency that occurs, is due to the buffering on the
device. 
Therefore, with respect to latency, there is not really a
difference 
between FireWire and ADAT or MADI: It depends on the
buffering on the 
interface you hook up to it. Basically firewire can give the
same 
performance as RME+MADI, Delta+ADAT, ...

There is one problem with this: currently we can't adress
the FireWire 
host controller in this manner, meaning that we are limited
in the 
performance. It is being solved in the new juju firewire
stack.

> 
> So if you are for low latency, PCI/PCMCIA is
unbeatable.
> If you want numbers, I can provide them for Echo Indigo
IO and RME 
> MultifaceII.
> Ask somebody to jdelay their Firewire card.
Please don't . The
current system is far from 'ideal', and would 
certainly give a wrong impression on the capabilities of the
FireWire 
technology.

Results for a first generation FireWire device: 
http://freebob.sourceforge.net/index.php/ESI_Quata
fire_Latency

At 48k it comes down to the fact that the device + the
windows 1394 
stack together limit the roundtrip latency to around 250
frames. FreeBoB 
has a similar latency lower limit. Note that the audio
buffers should be 
added to this, making the actual number around 320 frames.

The newer FireWire devices (based upon DM1500 or certainly
DICE) will 
exhibit much lower latencies. I don't have figures on that
yet, I'll try 
and see if I can get some... I expect something like 50-100
frames of 
extra latency, for a total of around 150 frames. This is
quite 
comparable to the PCI cards.


Pieter
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