[PD] Can anyone tell me not to buy a Quattro or a UA-20?

[Frank Barknecht]

Hallo,
Dan Wilken hat gesagt: // Dan Wilken wrote:

> >PCI is much more efficient than USB (1.0).
> >
> But does it reduce CPU load, as in, might I be able to get more of my 
> tracks playing at once if I have a good PCI sound-card?

Audio data intended to go to a USB device still has to go over PCI to
the USB host controller. From there it additionally will have to be
shoveled to the USB card.  And you have a bandwith restriction, so you
can only squeeze so much data over the USB cable.  USB devices are
generally very stupid, most work still has to be done by the computer.
They just play back what comes in. This is in contrast to say the RME
cards, which do a lot of things like mixing on the card itself and
thus spare some CPU cycles. But this only gets relevant when doing,
say, 12 channels of 96/24 data. The bandwith restriction alone might
be in favour of getting PCI. On Linux with USB you also will be
restricted in what samplerates are good to use. As USB has a
resolution of 1000 frames per second, you need to choose the rate so
that it can be divided by 1000. 

Example (as remembered from a list mail by Clemens Ladisch, I cannot
find anymore, and thus prone to errors on my side): 

Samplerate is 44100 Hz, which is one sample every 1/44100 second. You
need to match this with 1000 USB frames per second or one every 1/1000 second. 
So one USB frame needs to transport 44.1 samples. As there are no
fractional samples, it will transport 44 samples most of the time,
but every 10th frame will transport 45 samples. 

But ALSA only supports constant period size, so this will not work
very well and you'll have to crank up period size. For example on my
laptop I have to use a period size of 2048 in Pd to get almost click
free audio with a Quattro at 44100. Even I who is just using some
midi sliders, can feel this amount of latency.

Thus it's better to use a sample rate of 48000, where you get constant
48 samples every frame. But then the UA-20 only does 44,1 kHz
samplerate. Bad kitty. All this isn't a problem with PCI or PCMCIA
cards. CPU load is really not an issue in the decision, it is not
really important in the context of other things that are much worse in
the comparison.

Conclusion: If you don't *have* to use USB audio - because you run a
laptop - avoid it like the plague.

ciao
-- 
 Frank Barknecht                               _ ______footils.org__