Interfacing to Sony CCU-M3
We found three issues to deal with when interfacing our Sony CCU-M3 to Clearcom-type systems. Please review the circuit description and schematic diagram for the Clearcom-type system.
- Balanced vs. Unbalanced.
The Sony CCUs we have dealt with (CCU-M3 and CCU-M5) use balanced audio and the Clearcom analog system uses unbalanced. This was solved very simply with a small 1:1 audio transformer. We used a small (< 1-inch cube) surplus transformer we had, the kind that are used in dial-up modems. They don't have very good frequency response at the low end or the high end, but then that is actually desirable in an intercom. Limiting the bandwidth to voice frequencies reduces incidence of interference, noise, and distortion. transformers like this are commonly available from places like All Electronics, Digikey and Mouser.
The Clearcom system uses the same wire for both the intercom audio, and for signaling. The audio is AC coupled, and the signal is a DC component on top of it. We decided that we didn't need signaling out to the camera operators since: a) the camera operators are supposed to have their headsets on all the time; and b) the tally circuits built into the cameras and CCUs provide a "call" function. We just used a capacitor to AC-couple the Clearcom audio line to the transformer to keep DC out of the transformer.
- Audio Levels
This was the biggest problem. The Clearcom system runs at a significantly lower level than the Sony intercom. When we first devised the interface (just the capacitor and the transformer), it worked, but the receive level at the cameras was too low for practical use in noisy environments (like musical programs). And the microphones on the cameras were way too loud compared to the Clearcom intercom stations.
We use these cameras and CCUs almost exclusively in a production unit where all the control-room personnel (director, AD, TD, shader, tape operator, etc.) are using Clearcom-clone intercom stations. Because of this, we decided to modify the cameras to boost the audio level to the earphones in the camera op's headset, and to attenuate the mic signal. We made a very small (postage stamp size) circuit board with an op-amp booster and a volume control to knock down the mic level.
This has worked out quite well for us, but it might not be practical for everyone, and especially not for rental equipment(!) Actually, it wouldn't be hard to package the earphone boost and microphone attenuator into a very small external package which could be used with any camera, but it is not the elegant, no-modification solution we would have preferred.
So what it comes down to is an interface unit which connects between the Clearcom bus and the CCU bus (simply the transformer and capacitor); and another unit on each camera which boosts the earphone level and attenuates the mic level.
Sony CCU-M3 Interface
Camera Amplifier/Attenuator Circuit