Prophet-6 and Logic Pro: Doubled Notes, Arpeggiator Sync and Recording Arpeggios

The setup

A producer had connected a Sequential Prophet-6 to Logic Pro through an iConnectivity mioXL for MIDI and a MOTU UltraLite mk5 for audio, with the synth coming in on line inputs 3-4. The cabling was correct. He was playing the synth through an External Instrument track in Logic, and several things about how it behaved made no sense to him.

Over one remote session it turned out to be three separate problems, each with a different cause. Each one looked like a Logic problem, and none of them really was.

Why does my synth sound doubled when I play it through Logic?

When the External Instrument track was active, every note sounded twice: once straight away, then again a moment later. Played standalone, the Prophet-6 sounded fine. The doubling was there both when he played live and when Logic played his recorded MIDI back to the synth. Both of those produce live audio at inputs 3-4.

Test it yourself

Play the synth and mute its track in Logic. If you can still hear it, your audio interface is monitoring the input directly, and you’re hearing two copies of the same signal.

The cause

The synth’s audio was taking two routes to the speakers. CueMix, the UltraLite’s internal mixer, was passing inputs 3-4 straight to the outputs with no latency. At the same time, Logic’s External Instrument plugin was taking the same input, running it through the track, and sending it out after the round-trip delay of the audio buffer and converters.

When two copies of a signal are a few milliseconds apart, you get comb filtering, which sounds hollow and phasey. At tens of milliseconds, it becomes a distinct flam. This was a flam, which tells you roughly how large Logic’s round trip was.

The fix

One of the two paths had to go. We kept Logic’s, for two reasons. The External Instrument plugin always passes its input through, so Logic’s path is the one that’s awkward to switch off. Monitoring through Logic also means you hear any effects on the track while you play.

1. Pull inputs 3-4 down in CueMix. The UltraLite mk5’s front panel can do the same job, but CueMix is on screen, so I can see and change it over the remote session instead of talking someone through buttons on a video call.
2. Turn on Low Latency Monitoring Mode in Logic. This bypasses any plugins on the monitored signal path whose latency exceeds the limit set in Logic’s audio preferences.
3. Check the I/O buffer size. Low Latency Mode only deals with plugin latency. The buffer size sets the rest of the round trip, so drop it as low as the session will run without clicks while you record.

With a single monitoring path, the doubling disappeared. The same fault turns up on any interface with direct monitoring, whether that’s Focusrite, RME TotalMix, Universal Audio Console or anything else, and in any DAW.

How do I sync the Prophet-6 arpeggiator to Logic without starting its sequencer?

He wanted the Prophet-6’s arpeggiator to follow Logic’s tempo. In Logic, go to Project Settings, then Synchronization, then the MIDI tab. There we set Logic to transmit clock to the mioXL port feeding the synth. The arpeggiator locked to Logic’s tempo straight away.

It also created a new problem. The Prophet-6 starts its internal sequencer when it receives a MIDI Start message, so pressing play in Logic now set the sequencer running. It only happened when playback began from the start of the project, because Logic’s clock mode was set to send Start from there and not when playing from other positions on the timeline.

The right setting didn’t work

The Prophet-6 has a MIDI clock mode made for exactly this situation: No Start/Stop, shown on the display as nSS. It follows incoming clock but ignores transport messages. We set it, and nothing changed.

We went through every clock option we could think of. Then we put a MIDI monitor on the signal to see exactly what Logic was sending. Clock was arriving, and Start and Stop were arriving exactly when they should. Logic was behaving correctly, so the problem had to be in the synth.

The fix: firmware

No Start/Stop had been a known problem in early Prophet-6 firmware. His synth was running version 1.20, and the current release was 1.78. We downloaded the firmware file from Sequential’s website, sent it to the synth with SysEx Librarian, followed the update procedure and restarted the synth. After that, nSS worked. The arpeggiator followed Logic’s clock and the sequencer stayed put.

The general lesson: once a MIDI monitor proves the sender is doing the right thing, stop changing settings and look at the receiver. Checking the firmware version first would have saved us the time we spent working through clock options.

How do I record the Prophet-6 arpeggiator into Logic as MIDI?

With clock sorted, he tried recording an arpeggiated part. He could hear the arpeggio, but Logic recorded long held notes, which were the chords he was holding down rather than the pattern the synth was playing.

Why the obvious approach fails

The usual way to use a hardware synth with a DAW is to turn Local Control off. The keyboard then sends MIDI to Logic, Logic records it and sends it back, and the synth plays what it receives. With that setup, the arpeggiator is applied to the notes coming back from Logic, after recording has already happened. So Logic records the notes before they’re arpeggiated.

The arpeggiated notes only reach the Prophet-6’s MIDI Out when Local Control is on. In that case, the arpeggiator sits between the keys and the output.

The fix: a recording track that talks back to nothing

1. Leave Local Control on on the Prophet-6.
2. Create a second External Instrument track with no MIDI output. Use it only for recording arpeggios.
3. Record on that track. The synth arpeggiates what you play, sends the arpeggiated notes to Logic, and you hear it directly through Local Control. Logic sends nothing back.
4. Drag the recorded region up one track onto the normal Prophet-6 playback track. Press play and the arpeggio is now written into Logic as MIDI, playing the synth.

This is a workaround, but it follows how the synth is built, and it’s much quicker than the alternative. That would be switching Local Control and the External Instrument’s MIDI settings every time he moved between normal recording and arpeggio recording. Here, the only extra step is dragging one region.

What the three had in common

All three problems showed up in Logic, but the causes were elsewhere. The first came from two mixers both doing their job. The second was old firmware. The third was where the arpeggiator sits in the synth’s signal chain.

In each case, the way forward was to establish what was actually being sent before changing anything. With hardware synths and a DAW, most problems come down to MIDI or audio going somewhere you didn’t expect.