Equipment - intercom

(photo of intercom)
Base unit, with two stations

A custom-built system that's a party line system (e.g. all parties can talk to each other simultaneously).  There's a base unit which provides power and acts as a hub between all stations (the base also includes two in-built stations), several remote stations, and a collection of headsets.  This is part of the equipment we use when doing multi-camera video recording.

Features


Pinouts

3-pin XLR intercom line

  1. Cable shield and power ground
  2. Intercom audio signal
  3. +12 volts DC power

The XLR connector shells on the cables can be connected to the ground pin inside the connector, our system isn't upset by that.  Since most of our XLR cables are used for microphones, we ground the female connectors, that leaves any floating cables with shielded sockets.  The male shell is grounded by whatever it's plugged into.

Using a 3-pin cable allows the use of ordinary microphone leads.  This intercom is different from various other intercom systems, I haven't tried to make it compatible, I don't have the information I need to do that well, it's not really needed for our own work, and often not practical (there's too many different systems).  If we have to tie two, or more, intercom systems together, there's better ways to approach it than simply joining the party line buses together.

8-pin intercom line

Most stations are mono, with these connections:

  1. +12 volts DC power
  2. Power ground
  3. Intercom audio signal (approx 1 Vp-p with moderately loud audio)
  4. Intercom audio ground
  5. Program audio signal (mono)
  6. Program audio ground
  7. Tally signal
  8. Tally common

Stereo stations use this variation:

  1. +12 volts DC power
  2. Power ground
  3. Intercom audio signal
  4. Intercom audio ground
  5. Left program audio signal
  6. Left program audio ground
  7. Right program audio signal
  8. Right program audio ground

For either case, audio wiring could be shielded, or twisted pair (tested with up to 100 metres of CAT5 cable as the patch lead, having acceptable cross talk between audio signals and acceptable levels of hum pickup—both significantly quieter than the desired signal, and almost unnoticeable under most working conditions).  And the program audio feeds can be used for any audio signal (e.g. monitoring another talkback system, one-way).

5-pin XLR headset

  1. Microphone shield
  2. Microphone audio signal
  3. Earphones common ground
  4. Earphones left signal
  5. Earphones right signal

An alternative, no-longer used by us, but recorded here for those who might try connecting a different (and incompatible) headset:

  1. Microphone shield
  2. Microphone + audio signal
  3. Microphone - audio signal (connected to pin 1 if unbalanced)
  4. Earphones + signal
  5. Earphones - signal

The XLR connector shells on the headphone cables are not connected to any pins inside the connector.  The shell is grounded by the chassis of whatever you plug it into.

We originally used the second arrangement, for various reasons:  Most of our headphones came wired that way, none were wired the other way.  The stations were built for video production work, not theatre staging, and using a balanced microphone signal was more desirable than two-channel earphone audio.  Mono earphones allow headsets to cover one or both ears without missing out on hearing the audio from an earphone that's not placed over an ear, it provides redundancy for an earphone that goes open-circuit, and it's easier to understand speech when you can hear it in both ears.  The use of one headset connector is more convenient than dual connections (e.g. one three-pin XLR for the mike, and one 6.5 mm connector for stereo earphones).  And it would have been far more expensive to use seven-pin connectors on all headsets, which would have allowed balanced microphone audio and independent stereo earphones, with complete electrical isolation between each transducer.

We switched to the first method after getting some DT-109 beyerdynamic headsets with unbalanced microphones, and good-enough earphones to use for audio monitoring as well as intercom.  Now all the headphones are wired for stereo earphones and unbalanced microphones, so any headset can be used on any station, and other equipment; and this wiring layout is commonly used by other some system's headsets.  Most remote stations are still only mono audio, with the same sound in both ears (left and right being joined together in the equipment socket, now, rather than the headphone plug).  Stereo stations can monitor program sound in stereo, but intercom is still monophonic to both ears at once.  Balanced audio, when needed, is provided by the circuitry in the station, rather than the headset.

I mention this saga for anyone else contemplating designing an intercom, and wondering which might be the best way to go.  The benefit of balanced microphone connections was marginal when there's only a couple of metres of flex between the headset and the intercom station (there are more noise problems from a headset microphone being near equipment, than its cabling).  But the convenience of being able to use any headset, anywhere, without needing adaptors or different cables is significant.

7-pin XLR headset

  1. Microphone shield
  2. Microphone + audio signal
  3. Microphone - audio signal (connected to pin 1 if unbalanced)
  4. Left earphone - signal
  5. Left earphone + signal
  6. Right earphone + signal
  7. Right earphone - signal (connected to pin 4 inside stations not requiring completely separate left/right speaker wiring)

The XLR connector shells on the headphone cables are not connected to any pins inside the connector.  The shell is grounded by the chassis of whatever you plug it into.

Currently, we have one stereo headset on a 7-pin plug.  This one's intended for an audio operator to use as a combined talkback and audio monitoring set, so they only need to have one set of headphones for both tasks.  Most of the crew do not need stereo monitoring, so the extra expense of more seven-pin connectors (about four times the cost, or more), and additional audio amplifiers per station, is avoided.

The intercom system tests well with 200Ω dynamic microphones, cheap electret microphones of unspecified impedance, and 8Ω–400Ω earphones (various beyerdynamic, Sennheiser, and other headsets).  Higher impedance headphones (e.g. 1kΩ) were a bit too quiet for some environments.


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