[LAU] Audio interface latency measurements

[Len Ovens]

On Sat, 21 Jun 2014, Robert Jonsson wrote:

> Follow up question: from a theoretical perspective, is it likely a usb
> 2.0 interface would have similar transport latency as firewire? Usb 1
> I suppose would be worse due to lower clockspeed.

As was already stated, clock speed of the interface is not really 
relevant. It seems in fact that no one is really interested in USB3 
because it does not have any improvement for audio, USB2 is enough. The 
limitation with USB1 is bit depth, bit rate and channel count.

In general, throughput and latency are two different things. Larger 
packets mean better throughput, but smaller packets mean lower latency.

I am not sure, but it seems to me the USB1.1 audio standard effectively 
means that the lowest latency for USB1 is jackd set to 64/2. This is the 
smallest buffer size supported. I do not know, but it seems that fire wire 
audio is about the same from what I have read (I don't have one of my own 
to confirm).

The main trouble with USB is on the MB. Finding a USB port that is not 
shared with something else via an internal hub. I think adding a USB card 
would make things better, but trying different ports on a laptop gives 
good results too. With any audio interface, having it's own irq is 
important, I have moved PCI cards to different slots with a big 
difference. It shouldn't be, but it seems tunning a computer for audio is 
a must still for low latency. Audio is very definately _not_ plug and play 
for (semi)pro audio work. There is no silver bullet kernel or distro that 
just makes everything work. On my laptop, there is one USB port that gives 
good audio... so long as the port next to it is empty... and the wireless 
kernel module is unloaded and .... you get the picture :)

--
Len Ovens
www.ovenwerks.net