Friday, March 19, 2010

More on the MOTM-650 -- Channels and Voice Groups

So now that I've had a bit of time to play with it, here is some more information on the MOTM-650 MIDI interface. In this post, I'm going to concentrate mostly on how the the voice groups work and how MIDI note information is routed to the output jacks. In my next post, I'll cover the arpeggiator functions.

Output Channels and Voice Groups

The first thing I had to understand about the 650 is the difference between the four physical output channels and the voice groups. Notice that the 650 has four columns of jacks; each column is an output channel. (The MOTM documentation refers to these as "voices", but I think that's a bit misleading, since the 650 produces no audio output by itself. I'm going to continue referring to them as "output channels" even though that's a bit wordy. And yes, to avoid confusing these with MIDI channels, if I mean "MIDI channel", I'll spell that out.) Each output channel contains (from bottom to top) a pitch CV jack, a gate jack, a velocity CV jack, and an aux CV jack. (The EXT CLOCK jack at the upper right doesn't go with any output channel; it's an independent entity. More about it in the next installment.) Each output channel also has a red LED underneath the LCD display that lights up when that output channel's gate is in the "on" or "high" state; these are labeled V1 through V4 on the panel.

The voice group, on the other hand, is a software construct. Each voice group "owns" one or more output channels, depending on how many voice groups the 650 is configured to use. Each voice group listens to incoming MIDI data on one MIDI channel (which is configurable in the voice group's parameters). When the 650 receives MIDI note messages, it directs the MIDI events to a voice group according to what MIDI channel the data came in on. The voice group then transforms the MIDI note information into control voltage and gate signals, and directs these to be output on one or more of the output channels it owns. This is done according to the allocation mode, which is also a voice group parameter. The neat thing is that if the voice group owns more than one output channel, it can operate them polyphonically or in unison, depending on the chosen allocation mode.

Global Modes

To sort all this out, we need to being by introducing the 650's global mode settings. If you are at the 650 and you haven't pressed any of the buttons, the 650 LCD screen will be at its top-level display. (If you've been messing with it, press the ESC button 4 or 5 times, until the display quits changing.) The display will look something like what is pictured below. (In this article, I will show text that you should see on the 650's screen in bold.)

G1 CH=1

This summary display shows you the basic status of the voice groups. The above shows that you are looking at voice group 1, and it is set to MIDI channel 1 and is in SOLO mode. More on that in a minute. First, let's go through the global settings. Press ENTER and you will see:

Global Options

Press ENTER again and you will see something like:


This lets you select the division of output channels into voice groups. We'll come back to this in a minute. At this point, pressing INC or DEC will scroll through the available global parameters. I'll save a complete recap of the global parameters for the end of this post. For right now, there are two things you may want to change; one is the backlight setting. Press INC or DEC until you see something like:


On this setting, the backlight comes on whenever you press a panel button, and goes off 5 seconds later. You generally want to keep the backlight off as much as possible because it draws a lot of current from your modular's power supply. But for right now, you may find the auto backlight annoying. Press ENTER, and the bottom line of the display will begin flashing. This indicates that you can now use the INC and DEC buttons to change the value. You can change the setting to OFF, AUTO, DIM, MED, or BRIGHT, and as you change it, you will immediately see the change. Press ENTER again to store the change, or ESC if you decide you really didn't want to change it after all. (This will be the case for all of the parameter-changing screens.) Either way, the bottom line of the display will stop flashing, indicating that you are no longer in the parameter change mode.

Now, press INC some more until you see:

(or something else)

The three possible values are Last Note, Low Note, and High Note. Most of the time you will want this on Last Note -- I'll explain why in a bit. If it's not on Last Note now, press ENTER so that it flashes, and then use INC/DEC until it says Last Note, and then press ENTER to lock that in.

At this point, pressing INC/DEC will show other global parameters. Pressing ESC will take you back to Global Options, and pressing ESC again from there will take you back to the top level screen.

Go to the Global Options again and press ENTER. You will be at the VGrpType screen. Here you will select the number of voice groups and how they will divide up the four output channels. The 650 can be configured to have (text in brackets shows what the second line of the display shows):
  • One voice group which uses all four output channels [4].
  • Two voice groups, each of which uses two output channels [2/2]. In this mode, voice group 1 uses the two leftmost output channels, and voice group 2 uses the two right most.
  • Four voice groups, each of which uses one output channel, counting from left to right [1/1/1/1].
For the purpose of following this discussion, use INC and DEC until 2/2 is shown, and then press ENTER to lock it in.

Voice Group Parameters

Each voice group has a set of parameters that tell it how to interpret MIDI data, and what to do when the voice group receives more MIDI notes than it has output channels available. To get to the settings for each voice group, press ESC as many times as you need to to get back to the top level screen. Now, press ENTER and then INC, and you will see


You are now in the parameter settings for voice group 1. Let's step through these parameters. Press ENTER and you will see:


This says that voice group 1 expects to receive MIDI data on channel 1. Presuming that you have either a MIDI controller or a computer with a MIDI interface connected to the 650, check which MIDI channel your controller/computer is transmitting on. If it isn't channel 1, go through your steps for changing a parameter's value on the 650: press ENTER (bottom line of screen starts to flash), use INC/DEC to change the value, and then ENTER again to lock it in. Press INC again to see something like:

VG1 Allo
(may be a different word)

This is the poly mode / output channel allocation selection. We'll come back to this one. Press INC again, and you will see:


This is your glide, or portamento setting. When on, it causes pitch control voltages output by this voice group to move smoothly between note values. There are constant-time and constant-rate settings. The constant-time setting can, incredibly, be set in millseconds from 1 ms to 65.536 seconds. The constant-rate setting is in arbitrary values from 1 to 127. Experimentally, setting 127 results in portamento that moves at about one octave in 100 ms; a setting of 1 causes it to take around 22 seconds to transition one octave. Leave this alone for the time being and press INC again, and you will see:

VG1 PBend
2 (or some other value)

This sets the pitch bend response in semitones. When the voice group receives MIDI pitch wheel messages, it will increase or decrease the pitch CVs being output according to the pitch bend. (You can also get the isolated pitch bend voltage as a separate output signal from an aux jack.) This parameter sets how much the full-scale bend is, in semitones. Allowed values range from 0 (no bend) to 24 (two octaves in either direction). If you want to change it, press ENTER, use the INC/DEC buttons to change the value, and then press ENTER again. Now press INC to see:


This parameter allows you to change the function of the gate jacks in the output channels assigned to this voice group. The available setting other than Normal is S-Trig. This is a requirement for interfacing to some old Moog gear; if you have any such, you probably already know about S-triggers. Otherwise, leave this parameter alone.

The next screen is:


The Velocity jack in an output channel normally outputs a control voltage that represents the MIDI velocity of the note being played. This parameter allows you to change the jack's function to output a trigger instead. I'm sure this has a purpose, but I'm not sure what. Leave it alone and go to the next screen:

VG1 Aux
CC1 ModW
(or something else)

This allows you to select what signal will be output by the Aux jacks. The available choices are Velocity (outputs the MIDI velocity), PitchBnd (outputs the pitch wheel value independent of the pitch CV), ChAftTch (aftertouch), ClkPulse (will be covered later), Disabled, or any MIDI Continuous Contoller (CC) from 0 to 31. Use the ENTER and INC/DEC buttons if you want to change it. Then press INC to go to the next screen:

4 Volts

This sets the scaling for the aux jack; it will output the indicated voltage when the selected control signal is at its maximum value. It can be set to 1, 2, 4, or 8 volts. Depending on what you have the signal coming out of the aux jack routed to, it can be handy to be able to change this, but most of the time you will probably want to leave it on 4 volts.

Voice Group Allocation Modes

Now, press INC several more times until you cycle back around to the output channel allocation parameter:

VG1 Allo
(may be a different word)

The most complex aspect of the voice groups is the output channel allocation, and they interact with the allocation modes. The complexity comes in when the "4" or "2/2" voice group mode is selected in the global parameters, so that the voice group owns more than one output channel -- what does the group do with each of the outputs? It's somewhat like the process of allocating voices on a polyphonic synth.

To illustrate the modes, we will now do a little tutorial. If you have sufficient resources in your modular, patch up a little "two-voice" demonstration patch. You'll need two VCOs, two EGs, two VCAs, and a mixer. Note that filters aren't necessary for this purpose; you just want to be able to hear notes play. Now patch it up like this:
  1. The first output channel on the 650 has its pitch CV patched into VCO #1. The gate is patched into EG #1, and the VCO and EG outputs are patched into the signal and control inputs of VCA #1, respectively.
  2. The first output channel on the 650 has its pitch CV patched into VCO #2. The gate is patched into EG #2, and the VCO and EG outputs are patched into the signal and control inputs of VCA #3, respectively.
  3. The signal outputs of VCAs #1 and #2 both feed into a mixer.
If, back when we discussed the global settings, you set the voice group mode to 2/2, you are now good to go. Otherwise, press ESC until you get back to the main screen; press ENTER to get the global options screen, use INC to get to the VGrpType parameter, change it to 2/2, and lock that in. Now press ESC until you get back to the main screen, press ENTER to see the global options, press INC to get to the voice group 1 options, and press ENTER. Then, press INC until you see the VG1 Allo screen.

Press ENTER to change the parameter, and press INC or DEC until you see:

VG1 Allo

and then press ENTER to lock it in. This is the easiest of the alloction modes to understand. In this mode, the voice group plays in strictly monophonic fashion, and it only uses the first of whatever output channels are allocated to it. So as you play, you will see only the V1 light lighting. If you have set up the demonstration patch, you will only ever hear VCO1. Now, press ENTER, then INC twice, then ENTER again, and you will see:

VG1 Allo
Solo Uni

This mode will "play" all output channels allocated to the voice group in unison. So if the 650 were in the "4" voice group mode, you would see all four sets of CV/gate/velocity jacks will output the signals for that note, and you'll see this by way of all four of the red gate LEDs lighting up when you play the note and going out when you release it. As it is, if you are following along with this writeup and you have your 650 in the 2/2 voice group mode, the first two output channels will be active and the V1 and V2 LEDs will light when you press a note. If you have patched your modular as above, you will hear your two VCOs playing in unison (or whatever interval you have tuned them to).

What if you play more than one note in these mono modes? Remember that "Priority" parameter we looked at back in the global parameters? It determines what happens in this situation: it can be set to output either the highest note played, the lowest note played, or the last note played (that is, the note most recently pressed). If this note is subsequently released while other notes are still held, the 650 will again determine which note has priority and the pitch CV will jump to that note -- without the gate signal going off and back on. This is sometimes called "retriggering" and can be observed in many monophonic synths. If you have the gate signal patched conventionally into an envelope generator, the new note will play while the EG remains in its sustain phase, since the gate signal didn't cycle. One use of this is to simulate the "hammer" and "pulloff" techniques used by guitarists. The gate signal will not go low until all held notes are released. If you want to play with this, you can go back to the Global Options and change the Priority setting, but make sure you change it back to "Last Note" before proceeding.

Press ENTER, DEC, and ENTER again, and you will see the next mode:

VG1 Allo
Solo Rot

In the "Solo Rot" (rotation) mode, the voice group will not activate all of its output channels at once. Rather, the first note played will be output by the first available output channel. When another note is played, that note will be output by the next output channel, and so on. This is a sort-of-monophonic mode in that only one gate in the voice group will be active at a time. But. if you have set up the demonstration patch described above, turn the Release settings on both of your EGs to long values. Now play some notes. Can you hear two notes sounding at once in places? Here's what's happening: basically you have patched your modular as a simple two-voice polyphonic synth. Now, when you play a note, its pitch CV and gate will be output by output channel 1 on the 650, and you will hear the note played by VCO #1, EG #1, and VCA #1. Now you play a second note. This note will be output by the pitch CV and gate jacks of output channel 2 on the 650. The gate for output channel #1 will drop, since the 650 is still in a mono mode. However, with a long enough release time on EG #1, you will continue to hear the first note as it releases while the second note sounds. Note that the pitch of the first note remains the same -- the first output channel continues to hold the pitch CV of the previously played note, even though it has dropped its gate. It has to do so in order that the pitch of the note can still be heard during the release phase. The 650 doesn't know how the EG is set up (or anything else about how the modular is patched up), so it most hold the pitch CV until it needs that output channel for a new note. This is actually a fundamental principle of control of any analog synthesizer: the control interface, whatever type of mechanism it may be, must hold the pitch indication of the last played note even after the performer releases the note, since an arbitrary amount of time may elapse between note release and the note fading to inaudibility. Going back to our setup, when you play a third note, the rotation will return to the 650's first output channel. If the previous note is still sounding owing to a very long release time, the pitch will jump as the output channel outputs the pitch CV and gate for the new note.

And that leads us into the the polyphonic modes. Go press ENTER, INC, INC, ENTER, and you will see:

VG1 Allo

Now play first an E and then an A. Since the E is the first note played, it will be output on output channel 1. The A will then be output by output channel 2. You will hear both notes being sounded. Now let up first the A and then the E, and then play an F. Which channel outputs the F? In poly 1 mode, it's the second channel. Why? Because in this mode, the 650 tries to allow for the longest release time possible for each released note before it re-uses the output channel. Since the A was released first, and it was being output by the second channel, the 650 chooses the second channel to play the F. If we let up the F and then play F#, the F# will be output by the first channel, since that channel hasn't been used since the E was released, and that was longer ago then when the F was released. In computer science terms, this is called least recently used. The advantage of least recently used mode is that it is least likely to "cut off" a note that hasn't yet faded into inaudibility. Of course, with only two channels allocated to the voice group, it isn't going to make much difference if you are playing fast. But if you had a four-note poly setup, and were in the mode where voice group 1 uses all four channels, it might make a difference. More to the point, you can control the output channel usage by how you release notes, which opens up possibilities like patching each output to patches of unlike timbre, and then using it to create patterns of notes that vary in both pitch and timbre.

NOTE: none of the poly modes will work right unless the Priority parameter in the Global Options is set to LastNote! The Low Note and High Note priority settings interact with the poly modes in an unfortunate way: if you are holding a key on the keyboard, the voice group will ignore any lower-value or higher-value notes that are played, depending on the Priority setting. If you have your 650 in a Poly mode and it seems to be "missing" notes, check the Priority setting.

Note that this poly 1 mode will not "steal" a note; that is, if you are already holding two notes and you press a third, the third note won't be heard because there is no output channel available for it. However, if you press ENTER, INC, and ENTER, you put the voice group's VC1 alloc into "Poly1 St" mode. Now the third note will steal an output channel from a note that is still being held.

Press ENTER, and then keep pressing INC until you see:

VG1 Allo

and then press ENTER to lock it in. Poly 2 mode allocates output channels to notes according to which note was played first. As in our example above, if you hold E-A with the E being struck first, the E will be output on the first channel and the A on the second channel. If you then let up these two notes and play an F, the F will be output on the first channel. Letting up the F and playing F# will output the F# on the first channel, and so on. As long as you play only note at a time, that one note will be output on the first channel. Only when you play two notes will the second note you played be output on the second channel.

What good is this? Well, it's the bees' knees for polyphonic portamento. You can enable portamento (glide) mode for the voice group (go back to the discussion above about the voice group parameters) and portamento will be applied to each output channel as it transitions from one note to the next. In the Poly 2 mode, you have control over which output channel plays which note by slightly arpeggiating the notes you play; the first one you strike will be played by the first channel, and so on. This way, you can control the glides between individual notes in a chord and prevent them from "crossing" each other. (Or make them all cross if that's what you want to do.) As in the case of Poly 1 mode, Poly 2 is non-stealing, but there is a "Poly 2 St" mode which will steal.

Finally, we have the polyphonic unison modes. Press ENTER and then INC as many times as needed until the display reads:

VG1 Allo

This is a rather strange but interesting hybrid mode. Play one note, and both output channels allocated to the voice group will output the note in unison. However, if while holding that note, you play a second note, it takes away the second output channel, and then it behaves as if it were in Poly 2 mode. This continues until all notes are released, and then the next note will be played by both channels in unison. When the VGrpType is in "4" mode, it gets more complicated:
  • Play and hold a note, It gets all four output channels, in unison.
  • While holding that note, play and hold a second note. It will be output on output channels 3 and 4, while the first note retains output channels 1 and 2.
  • While holding those notes, play a third note. It will take output channel 2, and output channel 4 will mute (the gate will drop).
  • While holding those notes, play a fourth note. It will take output channel 4.
If that isn't all complicated enough, there is also a UnisonSt mode that will steal.

The Aux Jacks and MIDI Controller Data

Before we go, let's talk about the functions of the AUX output jacks some more. The aux jack can be made to respond to MIDI control signals coming from the MIDI channel that is assigned to the voice group. This operates independently from the note tracking and voice allocation; unless the group is set to output velocity on the aux jacks, all of the aux jacks in a voice group will output the same signal. This can be set to track pitch bend, velocity, aftertouch, or any MIDI continuous controller number in the range 0-31. (Mod wheel, a commonly used choice, is MIDI CC 1.) Another voice group parameter allows you to set the scaling of the aux jack.

Something worth mentioning is that in the "2/2" or "1/1/1/1" voice group modes, it is not a requirement that each voice group be set to a separate MIDI channel. If two or more groups are set to the same channel, they will all respond to that channel. One use of this is to be able to output multiple performance control parameters. For example, in "1/1/1/1" mode, you could set voice groups 1 and 2 to the same MIDI channel, but set the aux jack for voice group 1 to track aftertouch, and set the aux jack for voice group 2 to track CC 1 (mod wheel).

In a later post (hopefully within the next week), I'll cover the 650's arpeggiator functions.

1 comment:

Pacey (Les petits pilous) said...

Thanks a lot for these infos on the 650 !