On 21/07/17 08:18, Clemens Ladisch wrote:
What latency you can reach depends on how much other applications and drivers interfere with the scheduling; on the Pi, typical culprits are WiFi, ethernet, or USB.
Indeed. Any serial and/or packetising interface will certainly add _some_ latency. The drivers need to collect and slice up the data, put it into those packets and send them over the medium. Then the packets must be unpacked and the data stream put back together.
IIRC (and I may not, so feel free to correct), USB 2.0 and lower will cause 1ms of latency all on it's own, just because of the time-slotting used in the serial stream.
Ethernet can certainly be quicker, but can also be less predictable if there's any other traffic or you use large packets.
I imagine, but do not know, that WiFi may be slower. A quick 'ping' test here shown two to fifteen ms, but that may be caused by other factors. That compares with a fairly steady 0.5ms over Ethernet.
If you are looking for total latency of 1ms, that it likely over-optimistic, though it might _just_ be feasible. Just one lost packet, though, will likely cause an audible click (those are fun on a multi-killowatt PA).
Safer would be, say, 5ms.3ms is probably OK as it gives enough time to re-send a missing packet _and_ work towards recovering the lost time.
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