Getting started: Transport

In the past, we’ve talked about clock signals and how they work in Eurorack. However, clocks are only one part of the puzzle. Transport controls allow you to start and stop the music – if you’ve ever used a DAW, a drum machine, or another complex sequencing device, you’ve probably encountered transport. 

Today, we’ll explore the important bits of transport in Eurorack, and demystify the different signals and controls you’ll need in your system if you want to be able to press play.

Generating the signals

If you want to have a dedicated transport control in your system (as in, a button you press to make things go or stop) you’ll need* a module to perform that duty. That can come in the form of a dedicated clock module, like Horologic Solum or the ever-present Pamela’s [iteration] Workout, or a sequencer with a start/stop control and analog clock and rest outs.

This will give us the interface for interacting with the transport state, as well as generating the two signals that we’ll need to make things work correctly.

*You could totally patch this up from scratch with a manual trigger button, a pulse generator, and a sequential switch, but at that point you might as well buy a clock module, right?

Starting simple: analog clock and reset as transport analog

Traditional analog sequencers (and a majority of fun digital sequencers, like our Mimetic Digitalis or Numeric Repetitor) don’t have a concept of transport, per se. When they receive a clock pulse, they advance a step, and if they receive a reset signal, they start at the beginning.

This simplicity is useful because we can do fun things like sequence clock signals, or use a separate clock divider module to change the rate that our sequencers advance.

Transport for these types of sequencers is rather simple. If you want them to play, you turn on the clock signal. If you want them to stop, you turn off the clock signal. 

However, that doesn’t cover phase, and that’s where reset comes in (and makes things just a little bit more complicated). If we only used clock signals that we could turn on and off, we could start and stop our sequencers at will, but when we resumed our clock signal, our sequencers would continue from the step they last played instead of starting from the beginning. 

For example, let’s say we have separate sequencers for our drums and a melody line. If both sequencers don’t always restart to step 1, we could end up with a kick on the first beat happening an 1/8th note too late, making our entire patch sound off. The sequencers would still advance at the same rate, but they would be out of phase, and potentially sound different than we intended. This becomes particularly prevalent when dealing with sequencers with different pattern lengths (6 steps long versus 8 steps long, for example), or when manually navigating a sequencer when a patch is paused. 

And this is where reset comes in. A separate reset trigger from your transport module will ensure that your sequencers start from step 1 when you press play. 

Generally, this reset trigger will fire when you press play. There are two reset schemes that you’ll run into with traditional sequencers: they’ll either reset to step 1 as soon as they receive a reset, or they’ll reset to step 1 on the next clock pulse after receiving a reset trigger. If you send both a clock and reset signal simultaneously when playback starts, a majority of sequencers will jump to step 1 and begin playback as you’d expect. (That said, it’s useful to be able to generate a reset trigger at both playback start and at playback stop – some sequencers are a bit finicky, and may prefer one type of reset.)

DIN Sync/Sync24 clocking has advantages

DIN Sync (aka Sync24) is another type of analog clock that you may run into. It’s a slightly more sophisticated scheme, both in how it’s generated and how it’s interpreted, and is my preferred way of clocking things whenever applicable. 

DIN Sync also consists of two signals: a clock signal and a run gate. The clock signal is a constant 24 PPQN clock, meaning that if you want your sequencer to advance at, say, one step per quarter note, it needs to do some division itself – you can’t really use clock divider modules with DIN Sync in the same way you can with standard analog clock schemes. 

The run gate is the interesting part: when the run gate goes high, playback starts from the beginning, and continues while the gate remains high. When the gate goes low, playback stops. 

DIN Sync is my preferred method of clocking for a few reasons. First, the run gate must lead the clock by 9mS, which eliminates potential phase issues caused by clock vs. reset trigger latency (at least, in theory… much to my dismay, some devices ignore this part of the DIN Sync spec). 

The use of a run gate and clock signal also allow for three different states of playback from an intelligent transport source: play, when the run gate is high and the clock signal is active; pause, when the run gate is high and the clock signal is not active; and stop, when the run gate is low. And since the run gate resets all sequencers that follow it upon playback start, everything will start in phase, too.

Analog clock + run gate is a simple and common strategy in Eurorack

A handy middle ground for Eurorack that’s becoming more common is the use of run gates along with simple one-pulse-equals-one-advance interpretations of clock triggers. This allows for fun patches with, for example, a trigger sequencer sequencing a CV sequencer, while still using a simple-to-interpret gate signal for transport state. 

This sort of clocking scheme has been adopted as a configurable option by a few complex sequencers like the WMD Metron, Intellijel Metropolix, and the upcoming Mimetic Digitwolis from yours truly.

Addendum: Using MIDI for clocking

The MIDI spec has its own clocking standard. It’s somewhat complex (and outside of the scope of this post) and has both advantages and disadvantages. 

If you have a number of sequencers that all have MIDI clock, it can be a great choice for simple plug-and-play transport. Generally, you’ll want to use one device as the main clock source, run its output through a MIDI thru device (effectively a mult for MIDI), then patch the outputs to the inputs on your other sequencers. 

MIDI, unfortunately, has the highest likelihood of adding latency to your system. Daisy chaining multiple devices from MIDI in to MIDI thru will add some amount of delay, and, being a serial connection, a certain amount of latency when using things like note messages as well as clock is generally small but unavoidable.