CV tutorial 1: Graphic display of control voltages (CV) in Reason

A tutorial series on Propellerhead Reason music production software. Revised 2012-07-28.

Most Reason rack devices have input and output connections for control voltage (CV) signals, which can interconnect devices to synchronize their operation. The Reason Rack can be regarded as one big modular synthesizer, where you can link components together into one big throbbing, pulsating sonic thang.

This tutorial assumes you're familiar with basic uses of CV, including Sequencer Control, which uses note/pitch and gate/velocity signals to play synths monophonically, and Curve CV, which sends continuous signals such as LFO or Envelope curves between devices. We'll start with Curve CV, and come back to Sequencer Control in a later tutorial.

CV behavior can sometimes be hard to understand (and to measure), so it will be useful to display CV signals graphically. This tutorial will demonstrate a safe way to record CV into an audio clip, so you can have a look at the actual waveform, such as the following CV signal generated by a synth's LFOs:

CV graph from Thor

You will also be able to look at CV signal levels on audio level meters. CV signal levels can usefully be measured in dB, just like audio.

 

CV and audio are (almost) interchangeable

Starting with Reason 5, Propellerhead introduced the Thor synthesizer, which treats audio and CV signals as essentially equivalent. For example, audio oscillators and LFOs (CV sources) both put out continuous, periodic waveforms. Thor's audio oscillators operate with fundamental frequencies in the 10 Hz to 10 kHz range, while Thor's LFOs operate between 0.07 Hz and 99.6 Hz. In other words, the audio and LFO frequency ranges overlap. Low-frequency audio oscillators can easily be used as LFOs, and higher-frequency LFOs can easily be used as audio sources.

This trend continues with rack extensions such as Pulsar, whose LFOs go from 0.06 Hz to 1.05 kHz, and FXpansion's EtchRed, whose LFOs go from 0.25 Hz to 1.024 kHz. At the low end, these rates correspond to extremely slow musical tempos (0.25 Hz = 15 bpm), while at the high end, these LFOs can be used as audio sources, or for amplitude modulation (AM) or frequency modulation (FM) of other audio signals.

The main difference between audio signals and CV signals is that CV signals run at "control rate" — 1/64 the sample rate of audio signals. For example, if audio rate is 96000 Hz, then control rate is 1500 Hz.

 

Converting CV to audio and vice versa

Thor is the tool to use for this. Thor's Modulation Bus Section provides a patch bay that can be used to mix and match audio and CV signals. Figure 1 shows an instance of Thor in which everything is turned off, except a patch of CV In 1 to Audio Out 3:

Thor converting CV to audio

A CV signal plugged into Thor's CV 1 Modulation Input will be converted to an audio signal on Audio Output 3:

CV-to-audio connections on back of Thor

The reverse is just as easy. With the following routing, Audio In 1 is converted to a control signal on CV Out 1:

Audio-to-CV routing in Thor

Of course, Thor's internal audio oscillators and modulators can be used as sources here, too, in place of the external inputs:

Routing Thor's internal CV sources

 

Recording CV into an audio clip

You generally don't want to listen to CV signals unless they are bipolar and audio rate. For example, it is not a good idea to route a full-volume, 8 Hz LFO square wave to your studio monitors. Nor do you want to listen to a signal from a unipolar envelope generator. We don't want to actually hear CV signals, only view them onscreen. So this is the setup I use for recording CV into audio clips:

Rack setup for recording CV to audio

You can download this CV testbench setup here:

CV-testbench.reason.zip (zip archive of Reason song file)

Revision 2012-07-27: Throughout the examples and downloads, I changed the VU Offset to 0, so that all VU meters are referenced to 0 dBFS (dB Full Scale) instead of -12 dBFS, which is the default setting. That way, 0 dB will always refer to the digital ceiling — the level above which signals will clip if played through a DAC or recorded to disk. Full-amplitude CV signals are 0 dBFS (peak). Setting VU Offset to 0 changes the look of all the channel VU meters — the dB scale on the right now reads from -inf to 0 instead of -inf to +12 — but doesn't change the VU meter response, which is still an RMS (time average) value. The VU Offset can be changed by opening the Hardware Interface at the top of the Rack, clicking on Big Meter, and turning the VU offset knob all the way to the right. This change affects only the current Reason song file:

Setting VU offset in the Big Meter

The mix channel "CV Mix" receives the CV signal from Thor, expressed as audio. I'll get to the Thor setup in a minute. For now, notice that the Rec Source button (blue LED) is clicked on the front of the "CV Mix" channel, enabling it to be recorded to an audio track. The audio track, "CV Audio", specifies "CV Mix Left" as its input source (blue pull-down menu in the middle of the audio track rack device).

Here is the back of the rack, showing the CV and audio connections:

CV and audio connections

The mix channel and audio channel Direct Outs are routed to a pair of Spider Audios, which serve as "bitbuckets" for discarding the audio output of these devices — there are no outgoing connections from the Spiders. By connecting the Direct Outs, we break the default P-LAN connections to the Master Section, ensuring that these channels will never be heard in the final audio mix, and will not be hazardous to your audio system or your ears.

Here is the Thor setup. In addition to piping CV In 1 to Audio Out 3, we can assign CV In 1 to modulate Osc 1 pitch. That way, we can both hear and see what the CV signal is doing.

Thor modulation section routing

Thor's audio output is routed to a mix channel that we can monitor in the usual way. We do want to hear the modulating effects of the CV signal, even though we don't want to hear the CV signal itself.

Thor's audio output routing

Now connect a Malstrom to Thor's CV In 1,

Malstrom as CV modulation soource

and set up a simple square wave on Malstrom's Mod A LFO. Notice that everything except Mod A is turned off on the Malstrom:

Malstrom settings

Once the CV connection is made (and 1-shot mode is turned off on Mod A), you will be able to see the CV signal lighting up the VU meters of the "CV Mix" channel and "CV Audio" track — in the rack, main mixer, and sequencer.

CV signal appears on audio level meters

We also want to put Thor's sequencer in Manual Rec mode (the green button at upper left) so that we can change focus among sequencer tracks without changing their record-enable status. For example, we might want to click on the Thor track to assign master keyboard input without record-enabling it. I have set up Thor's sequencer track to play one long note, so that the CV modulation of Osc 1 can be heard.

Make sure the "CV Audio" track is record-enabled, then rewind the transport and press record. You will see a graph of Malstrom's LFO 1 square waveform, which looks like this in Edit mode:

Square LFO wave

The square waveform is, of course, bipolar. Other LFO waveforms, such as Malstrom's waveform 5, are unipolar. Select this waveform, rewind the sequencer, and press Record. Now you get the following graph:

Square unipolar LFO

With the bipolar square wave, both the positive and negative phases of the waveform read maximum (0 dB) on the channel VU meters. With the unipolar square wave, the positive phase reads maxumum and the zero phase reads minimum (-infinity dB). In some contexts, unipolar parameters are described as ranging from 0 to 127, and bipolar from -64 to +63. We will see in a later tutorial that a bipolar -64 exactly cancels out a unipolar +127. For such reasons, it is more useful to think of these signals as normalized to the range -1.0 to +1.0, and to think of full-scale signals such as the above as having amplitude 0 dBFS (dB Full Scale).

Now let's take a look at the CV output from Malstrom's filter envelope. Reconnect Malstrom's CV mod output so that it comes from the Filter Envelope,

CV output from Filter Envelope

and set the Filter Envelope somewhat as follows:

Filter Envelope settings

The envelope needs to be triggered, which you can do by creating a sequencer track for Malstrom that plays a note. The envelope graph is unipolar:

Filter envelope graph

These examples are not intended to be sonically interesting. The goal here is to demonstrate some tools for analyzing and troubleshooting CV configurations, especially those that are a little more complicated, perhaps as in the following. Here we use both of Malstrom's LFOs and sum them using a Spider CV with the merger Trim knobs set to 64:

Summing CVs using Spider CV

I'm going to set up Malstrom's LFOs with a square wave on Mod A and a faster triangle wave on Mod B:

Malstrom LFO settings

Rewind the sequencer and press Record. Again, we get a full-range, bipolar waveform, but more complicated than before. This is just the sum of Mod A and Mod B:

SUmmed LFO waveforms

 

CV signals have virtually unlimited headroom

Let's try one more thing. The trim pots on the Spider CV were set to 64. Let's see what happens if they are turned all the way up to 127. If we record the waveform, now it appears to clip:

Clipped waveform

However, the CV waveform is intact all the way through the signal chain up until it is recorded to the audio track. CV signals, like audio signals, have unlimited headroom internally. This can be demonstrated by turning the "CV Mix" channel fader down to -6.05 dB.

Trimming the signal at the audio end

If we record again, the intact waveform reappears.

Intact waveform

This shows that Reason handles CV signals similarly to audio signals, as streams of floating-point numbers — although at 1/64 the current audio sample rate. (If you were to open the above "CV Audio" clips in a third-party audio editor such as BIAS Peak and zoom way in, you would see that the CV signal stairsteps every 64 samples.) When CV signals are summed or scaled, there is no risk of clipping. Of course, the destination device receiving the CV signal may not know what to do with values above some maximum, so may "clip" those values. This can usually be remedied by turning down the Trim knob at the CV destination.

Feel free to try hooking up different CV sources to Thor's CV In 1 and recording them. Keep in mind that some CV sources need to be triggered.

 

Thor as CV source

You can also use Thor itself as a CV source. For example, you might use LFO 1 as a CV source (instead of CV In 1) as follows:

Thor as CV source

Here the modulation is somewhat more complex, with LFO 2 modulating both the LFO 1 rate (FM) and amplitude (scale amount).

LFO FM in Thor

 

Deleting unwanted recordings

As you play around with this and record many different CV signals, keep in mind that you can easily discard these recordings simply by deleting the clips from the CV Audio track. You can keep the song file small by making sure that unused audio recordings are not saved with the file. After deleting unwanted audio clips, go to the File menu and select Save and Optimize.

Next: CV tutorial 2: Control voltages (CV) by the numbers. Calibrating unipolar and bipolar CV signal levels, and mixing them in Spider CV.

 

Images in these tutorials refer to Reason 6.5 user interface elements, which are, of course, Copyright 2012 Propellerhead Software.