Topic: Digiface AVB: Presentation Time Offset
while I was working with a Digiface AVB and an M32-DA Pro sending audio from the Digiface to the M32 DA Pro, I wanted to adjust the Presentation Time Offset to 0.5 ms. I didn't look close enough at the dialog in the RME AVB Controller (which states nanoseconds as unit) and set it to 500 (assuming the unit was microseconds). This is of course way too short. I think, what somehow confused me were the "," used as digit group separator.
After changing the presentation time offset to this value neither the Digiface nor the M32 DA Pro complained. Audio was playing, but only every now and then I would hear some audio artifacts on the speakers connected to the M32 DA Pro. Neither the Digiface nor the M32 DA pro showed any error. Only after looking at Hive I was able to find the issue.
I found it interesting, that the M32 DA Pro kept playing audio, although all packets were too late. If the M32 DA pro would have refused to play audio after I made the change, I would have a clear indication, that I did something wrong. The intermittent audio artifacts had colleagues already telling me, that AVB would obviously not be as good as Dante, when it wasn't possible to operate it safely in such a situation. :-( This is bullsh*t, of course, but they had a point there.
Although this is clearly a user error (won't happen to me again), I would like to suggest a few changes to the dialog in the RME AVB Controller, to make it more robust and prevent such types of user errors:
the digit grouping separator should adhere to the system's locale.
let the user choose from a set of presets in a dropdown plus an option to set it manually.
show the entered value in milliseconds next to the input text field. Most audio folks probably are familiar with milliseconds for delays and such things.
This is probably controversial, but don't accept values below a certain value. Due to AVB's observation window, values smaller than 125 000 ns only work when two devices are connected directly to each other without a switch and when the aren't too many streams around. This would need really careful tuning and a user running Hive on the computer where the Digiface is connected to. When a switch comes into play, this value is increased by 0.125 ms per switch hop. With a switch in the path, MSRP accumulated latency is closer to 300 000 ns, e.g. Digiface AVB to Meyer Sound Galaxy 816 with an Extreme Networks X440-G2 is 287401 ns. By the way: when connecting the Digiface AVB directly to the GALAXY 816, the GALAXY reports an MSRP accumulated latency of 150 000 ns.
It might be helpful for users, that they can select, whether they are operating with two devices connected directly or through a switch and limit the selection in the dropdown accordingly.