[LAU] MOTU AVB discussion from LAC

[Fernando Lopez-Lezcano]

On 3/31/19 7:53 AM, Len Ovens wrote:
> On Sat, 30 Mar 2019, Fernando Lopez-Lezcano wrote:
...
> My comment about AVB and the motu devices is that the HW in these 
> devices does not seem to be up to the latency AVB requires. 

Or up to USB either. The thread in linuxmusicians suggests that the 
internal processing of the interfaces is very sensitive to interruptions 
in the USB packet stream. Anything dropping or being delayed messes up 
the card.

Before reading that thread I was also pretty sure (but just a guess) 
that the processor inside the card was on the edge of not being fast 
enough so any glitches would affect sample recovery or processing.

In particular at some point I was running jackd with "-s" (I had enabled 
that for other reasons and then forgot about it). With that option it 
was rather easy to get the USB interface to introduce weird periodic 
glitches (once a second?), high frequency noises and distortion that 
would NOT stop and were transmitted through AVB quite nicely - the 
recovery of samples in the card seemed to be really messed up which is 
to be expected to a degree if jackd does not properly recover from 
xruns. I don't know if this happens in other USB interfaces as well or 
is a "feature" of the Motus.

> One thing 
> the LAC talk on the AVB jack backend pointed out to me is the 6 sample 
> buffer size/irq rate. Can these devices be set to run at 12 samples? Or 
> does that drop the channel count as well due to packet size? (I wouldn't 
> think so as each stream is up to 8 channals) From what I have read, the 
> motu devices are just barely AVB compliant and do not allow all the 
> various AVB rates. There is one that is aes67 compatable with 1ms
> packets that may work better for PC (any OS) to AVB network.

Do you know which one? The Motu FAQ still states that the cards are not 
aes67 compatible.

In our big system (8 audio interfaces + 3 AVB switches) we have seen, 
very occasionally, a few AVB streams drop out, I think always related to 
the interface that connects to external laptops (through USB). The AVB 
streams blink in the connection gui and cannot be reconnected - only a 
reboot of both cards involved seem to fix this.

I have to do a thorough cleanup of overall AVB streams, we may be 
running into limits on how many streams the AVB switches can reliably 
handle - see below - the system grew "organically" and needs some love 
(in short, it is a mess! :-).

There is a very nice list of switches (and their AVB capabilities) here:

https://support.biamp.com/Tesira/AVB/List_of_AVB-capable_Ethernet_switches

> Also, for high channel work on the motu devices, I would suggest no 
> internal audio processing be used aside from routing. The internal mixer 
> with eq and effects may be what has caused trouble and decided people 
> fewer channels was a better choice for the firmware.

Definitely. I turn off anything in the card that is not needed, 
including the whole mixer. I don't know if it really helps, but I 
presume the main processor will have less stuff to do.

-- Fernando