Thursday, October 25, 2012

Wired remote volume control for ClearOne Gentner AP800 matrix mixer

How to use the Arduino microcontroller to remotely control this matrix mixer.

The Gentner AP800 has 12 balanced inputs and 12 balanced outputs. Each of the 12 inputs can be assigned to one or more of the 12 outputs. The level of each input and output can be adjusted from -20dB to 20dB. In addition each input can be muted. However, the level at which an input is sent to a particular output is not adjustable. An input's level will determine its volume across all of the output channels it is assigned to.

This unit will work fine for my particular application, Halloween sound effect playback. I am using this matrix mixer to control the relative levels of different kinds of wind and music played through two sets of stereo speakers.

But I need a way to fine-tune the relative levels of sound effects while listening to them, rather than going back and forth between the sound rack and the listening location. Additionally, since the sound effects are stereo, I want a single knob that adjusts both the left and right volume simultaneously.

After reading the manual for the AP800 I discovered it can be completely controlled through its serial port. I decided to build a wired remote control that would use an Arduino to send serial commands to the AP800 as I adjust various potentiometers.

View Schematic

View Arduino Code

The Arduino's TTL serial output needs to be converted to RS-232 serial (different voltage) before it can be interfaced to the AP800. I am using the NKC RS232 to TTL converter (DTE).

There are two components: A "base station" and a "remote" that are connected together over standard CAT5 ethernet cable. If a crossover cable is used accidentally, nothing will be damaged, but the remote will not work.

The remote has a dim red LED to indicate that it is connected to the base station and is receiving power. An "unlock" button, while pressed, initiates a process on the Arduino to read the potentiometers and send the correct serial commands to the AP800.

I made this mostly out of spare parts I had laying around the shop. If I had a bigger remote box I would have added a third potentiometer and moved the mode selector to the remote. However, keeping the mode selector at the base station allows me to leave the volume remote out in one of the listening areas where I want to easily adjust the levels for that area without accidentally adjusting those for a different area.

The Arduino program has four sets of variables - ModeX_InputYZ - one set for each of the four modes selected by the rotary switch. These determine which inputs are controlled by the two potentiometers for each mode select setting.

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