From the Summer 1965 issue of the Collins Signal magazine.

Checkout of the moon-bound Apollo spacecraft will involve the communication of thousands of critical details by hundreds of NASA engineers and technicians working in many different locations.

Not only must these personnel be able to talk to one another, but the test supervisor and test conductor must be able to talk to all of them.

A conventional intercom system is not suited for this type operation because such a system including hundreds of stations would require a prohibitive maze of wiring. Even if wiring for this many stations were practicable, voice quality would be unsatisfactory because of the sound degradation inherent in a conventional intercom system.

These disadvantages have been eliminated in a unique interior communications system being installed by Collins Radio Company in the Manned Spacecraft Operations Building, part of NASA's sprawling John F. Kennedy Space Center at Merritt Island, Florida. The MSO Building will be used for assembly and checkout of the Apollo spacecraft which will carry three astronauts to the moon and back.

Termed RADIC (RADio Interior Communication), the Collins-developed system was designed and is now being installed under supervision of the Canaveral District of the U.S. Army Corps of Engineers.

Dual-station RADIC unit is housed for operation in launch area environment.

RADIC, a closed-loop, r-f communication system, utilizes single-sideband, suppressed-carrier, frequency-division multiplex techniques. A RADIC unit, actually a multichannel transceiver, provides a single audio circuit that can use any of the available frequencies for communication. Initially, the RADIC units being installed have a 112-channel capacity.

Besides providing a large station capacity with a minimum of wiring and negligible sound degradation, the RADIC system is an extremely flexible one that facilitates expansion. RADIC units are linked to trunk lines by a quick-connect/disconnect pressure tap. Adding or removing a unit requires only a few minutes and does not disturb the rest of the system.

In addition to the RADIC units in the MSO Building, Collins will furnish RADIC units for the gigantic Vehicle Assembly Building, now under construction. When completed, the V AB will be the world's largest structure. It will be used for assembling the Saturn-V booster for the Apollo spacecraft.

NASA's huge Manned Spacecraft Operations Building under construction.

Cabling required for the RADIC installation is simplified. An individual station is connected by a single ½-inch coaxial cable to the trunk line. The trunk line will include a 50-ohm coaxial cable for transmission, one for receiving and two power supply lines. By confining radio signals to well shielded cables, the RADIC system is not susceptible to outside interference, nor does the system radiate interference.

The initial RADIC installation will include six branches, each of which can accommodate 83 dual stations with a maximum signal attenuation of only three decibels.

Branches will be summed at a RADIC system center by the use of feedback amplifier techniques. Removing or adding branches will vary the signal level only one db. The system center will include a line amplifier for transmit-receive connections, power supplies, interface modems to connect the RADIC system with telephone lines and distribution facilities for test conductor and test supervisor stations.

For utmost reliability, the RADIC system center will have fault-sensing networks that automatically switch to back-up facilities in the event of trouble. In the case of the power supply, the system will switch over to batteries should a failure occur in the normal 220-volt power source.

Each station in the Merritt Island installation will be a dual unit providing independent capabilities for two persons. Station users will be equipped with headsets for hands-free operation.

Common to the dual stations are a regulator/4-500 kc generator and a transmit/receive r-f amplifier. Digital frequency synthesizers and audio i-f amplifiers are independent in dual RADIC stations.

Colossal Vehicle Assembly Building under construction will be RADIC equipped.

The frequency synthesizer generates signals in the 516-960 kilocycle range for the carrier injection signal. The frequency selected in this range is counted down to 4 kc, then compared with a 4 kc reference frequency for phase lock and oscillator control.

After amplifying the microphone input signal, the vox-operated transmitter mixes the audio with a 500 kc signal derived from the 4 kc reference oscillator. The audio response is 300-3000 cps. Then the modulated signal, 500.3-503 kc, is again modulated with the frequency synthesizer oscillator frequency. The translated carrier covers the spectrum of 13-460 kc.

Although each modulator is balanced and suppresses the carrier, the first modulator actually determines the transmitted carrier suppression. This enables the second injection signal level to operate at a high level in order to enhance the transmitted signal.

The r-f output is transmitted from an impedance level of 75 ohms for proper match into the line tap coaxial cable. The line tap contains circuitry to bridge the transmission line and to preserve an impedance match under all conditions.

Audio side-tone is provided in conjunction with the vox operation to allow RADIC users to self check station units. The side-tone is applied to the headset at a constant, reduced level via the receiver.

At the receiver, the r-f spectrum described for the transmitter is demodulated, and the 300-3,000 cps audio is amplified and applied to headset connectors. The audio amplifier incorporates an AGC section with a 15 db range to compensate for line level variations and the difference in talker levels.

The regulator compensates for the voltage drop due to the long power distribution line.

When no signal is being received on a RADIC unit, the receiver is disconnected from the headset, eliminating all background noise. The receiver is activated upon reception of an incoming signal.

Each station unit will have a paging device that uses one of the 112 channels for entrance into the building's audio paging system.

An optional feature on RADIC units is an all-call circuit that allows a unit to transmit on the i-f to all other units in the system, regardless of whether they are busy.

Thirty mv-rms is the nominal test tone level per channel on the RADIC transmission line. Intermodulation distortion is -40 db, and audio distortion is less than three per cent.

The initial RADIC installation is designed for a transmission line frequency of ±3 db over a range of 4 kc to 3 mc. This will allow channel capacity to be increased from 112 to 600 channels.

Collins engineer Ed Mitchell checks dual-station RADIC unit destined to be installed in MSO Building.
Collins Signal, Issue 56, Volume 13-1, 1965 - Pages 25-27