Latency

The digital audio processing on Axoloti runs with a buffer size of 16 samples, at 48kHz this corresponds to 0.333 milliseconds. This is not the complete system latency however, as the analog-to-digital and digital-to-analog conversion itself adds latency too.

The audio latency from analog in to analog out is measured to be around 2.04 milliseconds. On the scope image, the top yellow track is a pulse fed into the analog audio input. The blue track is measured at the analog output.

AudioThroughLatency

The latency from MIDI input to analog output is measured at 1.2 milliseconds worst case – measured from the end of the MIDI data. A full MIDI note command takes 0.96 milliseconds by itself. On the oscilloscope image: the yellow track is the incoming MIDI signal, the blue track is the generated output pulse. This image is made with persistence set to on, so you see overlapping traces from multiple measurements. The jitter in latency is around 0.6 milliseconds.

MidiAudioLatency

Note that sound travels through air at around 340 meter/second. So one millisecond corresponds with 34 centimeters distance of air-travel. Getting closer to your speakers or switching to headphones is a good way to reduce latency further!

As a reference I measured my PC audio latency. I used a RME Hammerfall DSP Multiface audio interface, with the driver set to 32 sample buffer size – the lowest available. I made an audio-through patch in Max/MSP 6.1, using the Hamerfall ASIO driver. Measured total audio latency is 3.64 milliseconds at 48kHz. The Hammerfall can also make a hardware mix path from inputs to outputs without going through software, measurement shows that its convertor latency (excluding driver and software) is about 1.5 milliseconds.

Testing with Pure Data with PortAudio-ASIO at 32 sample buffer size – but Pure Data running at 64 sample block size, less is not available – on the same hardware, gives 4.3 milliseconds latency.

While the Hammerfall DSP is over ten years old, I believe more recent audio interfaces have no smaller buffer size available.

* Bizarre, the oscilloscope screenshots dumped on a memory stick show a battery level icon, never visible on the oscilloscope screen itself (Rigol DS1042)?

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