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__