There is number of DAW-sequencer plug-ins that can work with control voltages. For example: Expert Sleepers Silent Way and M.O.T.U. Volta. Also, any specific CV source can be made with SynthEdit, SynthMaker, Pure Data, Max/MSP etc. In order to output CV to a hardware, an audio-interface with DC-coupled analog outputs is required. (Audio-interface should work correctly with direct current) Most of today’s audio-interfaces have protecting HP filters, placed right before analog outputs. These filters let only alternating current to be passed trough (outputs are AC-coupled) and also provide good dynamic range. Few old professional interfaces (MOTU 828 and others) has DC-coupled outputs. Also, Expert Sleepers has the whole range of specialized audio-interfaces and DACs with DC-coupled outputs. These are good interfaces, but a bit expensive.
The ESI Gigaport HD is known as inexpensive, multichannel 24/96-ready USB audio-interface. Out of the box this interface has AC-coupled outputs but circuit can be easily modified to support CV.
There is eight electrolytic capacitors inside the device: C36, C41, C46…C71 (marked red). All we have to do is to short capacitor’s terminals together. The first option is to solder a jumper wires around each cap. The second option is to insert switches that could toggle DC- or AC-coupling for individual outputs.
Finally we got an interface that supports both CV and audio.
Measured specifications
- Channel number: 8
- Amplitude: 6v p-p max. (−3…+3 volts)
- Maximal positive sample value (32767 или 8388607) inverts (Version 3 drivers)!
- Channel DC cross-talk: <0.03% (<146dB)
Use carefully! Unprotected inputs/outputs are sensitive to low-impedance current sources. Inaccurate connection may cause damage to interface or degrade it’s specs.