Doepfer CTM64 Contact to Midi Interface (main board) - Manual (Page 8)

page 8 CTM64 Version 4 - User's Guide
Potentiometer connectors (6,7,8,9)
The four 3-pin connectors (6,7,8,9) are used to connect up to four potentiometers (rotary
potentiometers, faders, wheels). The middle pin of each connector is the slider pin (or control voltage
pin), the left one is GND, the right one is +5V. As the potentiometers are used as voltage dividers the
resistance value of the potentiometer is not critical. Any value between about 5kOhm and 250kOhm will
work without problems (linear response recommended).
The additionally available CTM64 connector set includes 4 suitable female connectors with assembled
cables to connect the potentiometers (length about 30cm, potentiometers are not included).
The four connectors are assigned to the following MIDI events:
• ST1 pitch bend (with middle plateau around MIDI data 64)
• ST2 modulation (MIDI controller #1)
• ST3 volume (MIDI controller #7)
• ST4 monophonic after touch
The actual control voltage range used for ST1 and ST2 is 0... +2.5V. 0V corresponds to MIDI data 0,
+1.25V corresponds to MIDI data 64 (middle position) and +2.5V corresponds to MIDI data 127. This
voltage range was chosen as in case of wheel potentiometers usually not the whole rotation angle is
available but only about 50%. If the full voltage range 0...+5V would be used the wheels could not cover
the complete MIDI data range 0...127 but only a small fraction (e.g. 30...100). If a standard
potentiometer with whole rotation angle is used a pre-resistor with the same value as the potentiometer
should be used (see appendix: "connection sketch" for details) to obtain a control voltage range of
0...+2.5V. For pitch bend there is a small plateau around +1.25V (MIDI data 64) so that it is easier to
find the neutral position of the pitch bender. For most of the spring-loaded pitch bend wheels this
plateau is required as the mechanical neutral position sligthly differs depending on the resetting
direction.
The control voltage range for ST3 and ST4 is 0 ... +5V. 0V corresponds to MIDI data 0, +2.5V
corresponds to MIDI data 64, +5V corresponds to MIDI data 127. This voltage range was chosen as for
these controls usually standard potentiometers with the whole rotation angle are used.
Unused potentiometer connectors have to be jumpered to GND (factory setting). Senseless MIDI data
will be sent if one of the potentiometer connectors is left open. If a potentiometer connector is not used
it has to be jumpered to GND, i.e. a jumper has to be installed in the left position (in direction to the 1/4"
jack socket).
The resistor values for the potentiometers used should be in the range of about 5k...250k. Instead of
the usage of potentiometers also control voltages may be applied to the middle pins of ST1...4 provided
that the applied voltages are in the range of 0....+5V.
Attention: Negative control voltages (< 0V) or voltages higher than +5V applied to middle pins of
ST1...4 will destroy the CTM64 ! Please pay attention to the warranty notes on page 1. Ignoring these
notes causes warranty loss and the right to return the goods.
The scan rate for the potentiometers (i.e. how often a new value is transmitted via MIDI out) is fixed to a
suitable value so that each control change is recognized very fast but the MIDI line is not blocked by
too many data. From our experience many MIDI devices would have problems to process the incoming
MIDI data in case of a too high scan rate.
We are planning an additional small hardware that converts voltages beeing not in the required range
to the necessary 0...+2.5V range (e.g. +2...+4V
→ 0...2.5V). Please ask if you are interested in this
supplement board to CTM64. It will probably be available early in 2001 and consists of a small pc board
with 2 trimming potentiometers that are used to adjust voltage offset and scale/amplification. One side
of the board will be connected to ST1 (or ST2), the other side to the controller that outputs the voltage
range.