iOS UDP Server Message Processing Latency Too High ~35-40ms

Question

We have a critical need to lower the latency of our UDP listener on iOS.

We're implementing an alternative to RTP-MIDI that runs on iOS but relies on a simple UDP server to receive MIDI data. The problem we're having is that RTP-MIDI is able receive and process messages around 20ms faster than our simple UDP server on iOS.

We wrote 3 different code bases in order to try and eliminate the possibility that something else in the code was causing the unacceptable delays. In the end we concluded that there is a lag between time when the iPAD actually receives a packet and when that packet is actually presented to our application for reading.

We measured this with with a scope. We put a pulse on one of the probes from the sending device every time it sent a Note-On command. We put another probe attached to the audio output of the ipad. We triggered on the pulse and measured the amount of time it took to hear the audio. The resulting timing was a reliable average of 45ms with a minimum of 38 and maximum around 53 in rare situations.

We did the exact same test with RTP-MIDI (a far more verbose protocol) and it was 20ms faster. The best hunch I have is that, being part of CoreMIDI, RTPMIDI could possibly be getting higher priority than our app, but simply acknowledging this doesn't help us. We really need to figure out how fix this. We want our app to be just as fast, if not faster, than RTPMIDI and I think this should be theoretically possible since our protocol will not be as messy. We've declared RTPMIDI to be unacceptable for our application due to the poor design of its journal system.

The 3 code bases that were tested were:

1. Objective-C implementation derived from the PGMidi example which would forward data received on UDP verbatim via virtual midi ports to GarageBand etc.

2. Objective-C source base written by an experienced audio engine developer with a built-in low-latency sine wave generator for output.

3. Unity3D application with Mono-based UDP listener and built-in sound-font synthesizer plugns.

All 3 implementations showed identical measurements on the scope test.

Any insights on how we can get our messages faster would be greatly appreciated.

NEWER INFORMATION in the search for answers:

I was digging around for answers, and I found this question which seems to suggest that iOS might respond more quickly if the communication were TCP instead of UDP. This would take some effort to test on our part because our embedded system lacks TCP capabilities, only UDP. I am curious as to whether maybe I could hold open a TCP connection for the sole purpose of keeping the Wifi responsive. Crazy Idea? I dunno. Has anyone tried this? I need this to be as real-time as possible.

Answer 1

Answering my own question here:

In order to keep the UDP latency down, it turns out, all I had to do was to make sure the Wifi doesn't go silent for more than 150ms (or so). The exact timing requirements are unknown to me at this time, however the initial tests I was running were with packets 500ms apart and that was too long. When I increased the packet rate to 1 every 150ms, the UDP latency was on par with RTPMIDI giving us total lag time of around 18ms average (vs. 45ms) using the same techniques I described in the original question. This was on par with our RTPMIDI measurements.