The audio system interfaces with the caution and warning system for reception of C/W (tone) signals; with UHF, S-band and Ku-band systems for transmission and reception of external signals (air-to-air and air-to-ground); and with the three tactical air navigation sets for receiver selection and signal monitoring. The network signal processor supplies S-band and Ku-band signals to the audio system for transmission and/or reception at various flight crew stations in the spacecraft. The attached payload (Spacelab), payload bay, launch umbilical and the operations recorders all interface electrically with the audio systems for transmissions and/or reception of signals.

The eight loops in the audio system are (1) air-to-ground 1, (2) air-to-ground 2, (3) air-to-air, (4) intercom A, (5) intercom B, (6) paging, (7) C/W and (8) TACAN. A/G 1 and A/G 2 are used to communicate with the ground through the S-band PM and Ku-band systems. In the low-data-rate mode or while the teleprinter is being used, A/G 2 is not available for voice communications. A/A is used, by convention, to communicate with the ground and with EVA astronauts through the UHF system. Intercoms A and B are used to communicate from station to station within the orbiter and Spacelab. The paging loop allows one crew member to send his voice to all active stations. The C/W loop sounds different tones for different malfunctions or emergencies. The TACAN loop, accessible only at the commander's and pilot's crew stations, is used to identify TACAN ground stations for navigation. Six audio terminal unit panels are in the orbiter at the following crew stations: commander, panel O5; pilot, panel O9; mission station, panel R10; payload station, panel L9; middeck, panel M042F; and airlock, panel AW18D.

The audio distribution system is a digital system that greatly reduces the number of wires necessary to carry electronic signals between system components. Electronic impulses can be given identifying characteristics and sorted into groups of signals. The coded impulses, called bits of information, are generated by the particular position of each switch on the various spacecraft control panels. Several bits can be reduced in number by a multiplexer to a particular identifying impulse. Bit groups from several sources are reduced so that a large number of signals can be sent along a single wire. Up to 128 bits of information can be encoded by the audio terminal unit into a serial-data word and sent along one wire to a decoder in the audio central control unit, where the bits are identified and separated by their original characteristics. Audio signals then are distributed by the ACCUs to the appropriate ADS components. There are no digital voice signals in the ADS, only digital enable signals. All ADS voice signals are analog (audio).

During launch and entry, each flight crew member wears a crew altitude protection system. The enclosed environment of the helmet lessens the severe noise levels encountered at launch and allows intelligible air-to-ground communications. For communication capability, a headset containing a microphone and earphone fits over the crew member's head, and a connector and cable interface with the headset interface unit, connected through communications cables to respective ATUs. The microphone can be positioned to suit the individual flight crew member. For emergency egress, a pull-away connection is used between the CAPS and HIU, in addition to the standard CAPS/HIU twist-on connector.

The headset interface unit has separate push-to-talk buttons for transmit and intercom modes and a volume control that determines the level of sound heard through the ear piece (microphone sound level is determined by automatic gain control circuitry within the ATU). In addition, the commander and pilot have push-to-talk switches on their rotational hand controllers for the transmit mode. Push to talk means that a push button must be depressed to allow a flight crew member to talk through the system. The headset interface unit is a portable microphone switch that is considered part of the communications carrier umbilical. It provides volume control and push-to-talk capabilities to the CCA Snoopy cap used for EVA, to the CAPS and to the wireless crew communication unit as a backup. The Snoopy cap integrates the communication carrier assembly into a skullcap. The HIU has a clip that attaches to the crew's flight suits. The unit has a three-position rocker switch and a volume control knob. The switch positions are xmit ; icom; and an unlabeled, spring-loaded-off center position. The xmit position allows access to intercom and external circuits, while the icom position is for intercom only. The volume control knob acts in series with the volume controls on the associated ATU.

The communication cables vary in configuration depending on seat location. Each seat has two 4-foot communication cables or a 14-foot length, as required. One 4-foot cable is flown as a spare. The cables connect to CCU outlets at various locations in the crew compartment. Each CCU has a specific ATU that controls communication loop configurations. CCUs are located at panels L5, R6, A11, A15 and M039M; two are located in the airlock on panel AW82D.

The CCU outlets and power switches provide electrical connection to headset and CAPS cables; an on/off toggle switch at each outlet controls electrical power to the respective cables. The commander's and pilot's stations each have three-position CCU switches labeled CCU, off and suit. The CCU position permits power flow to a WCCU or HIU. The off position blocks microphone power from a WCCU, HIU or LEH. The suit position permits power to reach the CAPS microphone. All other CCUs are two-position switches labeled CCU and off , which function the same as the commander's and pilot's switches. The CCU/EMU switches are located in the airlock below the EMU outlets on the panel labeled power/battery charger . The individual three-position switches are labeled EMU 1, EMU 2 and bus select . Each switch allows the crew to select main A or main B dc power for the EMU or to turn power on and off to the EMU. For entry communication configurations, the power controlled by the switch is used for microphone-associated circuitry only. Leaving CCU power off confines that individual to a listen-only mode, independent of ATU configuration.

A multiple headset adapter installed on the middeck ceiling is plugged into the CCU outlet on panel M039M. Its three CCU outlets allow up to three crew members seated on the middeck to share the one available outlet. When any one person connected to an MHA keys (in PTT mode) or activates the voice-operated transmitter, all three individuals' microphones will be keyed, and individuals sharing the MHA will hear each other talking only on side tone (up to 20 decibels below other loop levels). One MHA is stowed in a locker for on-orbit or backup use. Voice-activated transmission means that if a crew member's voice reaches a certain volume level (which is adjusted by a dial on each ATU), the system will transmit that crew member's voice. Hot mike means that the microphone is always keyed.

Once the shuttle reaches orbit, the crew altitude protection systems are stowed, and wireless communication units are used in place of HIUs or communication cables to allow the flight crew freedom of movement around the crew cabin.

The WCCU consists of a wall unit and a leg unit that is worn by each flight crew member in the crew compartment during orbital operations. The wall unit connects to a CCU outlet and remains attached to the crew compartment wall by Velcro until stowed for entry. Each wall and leg unit transmits on a unique pair of UHF frequencies; therefore, wall and leg units must be used together. Each set is identified by a letter on the set (e.g., A ). The wall unit is identified further by enclosing the letter in a box. Each unit is stowed with its cabling attached. The wall unit has a 23-inch cable to interface with the CCU outlet, and the leg unit has a 22-inch cable attached to a lightweight headset.

When the WCCU is unstowed, the only assembly necessary is to insert and tighten the flexible antenna in the bottom of each wall and leg unit. Since the wall unit receives power from the CCU outlet, the on/off/volume knob is not used and the battery pack is empty. The master vol control is set to full volume. All other switches are set as required; typically, the individual communication loops are used. The leg unit is stowed with battery pack installed and is attached to the crew member's leg with a wrap around elastic strap. The rotary on/off/volume knob (unlabeled) is turned clockwise past the on/off detent, and the volume is set as desired. Batteries are changed by depressing the battery pack latch push button lever (unlabeled) and sliding the battery pack off the unit. Expected use from one battery is three days (with the unit turned off during sleep periods). Sliding the new battery pack into the unit causes both the electrical connector and mechanical connector to latch.

The very lightweight headset is the interface between the leg unit and crew member. A single-strand wire headband holds the earphone against the ear and supports a thin boom holding a noise-canceling microphone near the mouth and a cable connector to the crew member's leg unit. The lightweight headset cable and connector also can interface with the HIU.

The audio central control unit, the heart of the audio system, is located in the crew compartment middeck forward avionics bay. The ACCU identifies, switches and distributes signals among the various audio distribution system components. Both digital and audio signals are received and processed by the ACCU, but the ACCU transmits only audio signals. It polls the ATUs to determine the panel configuration and connects the selected loops to the ATUs. It is also the point where the audio system is connected to the S-band PM, Ku-band and UHF systems for external communication. The selection switch for the two ACCUs is on panel C3.

The ACCU circuitry activates signals from the launch umbilical connection intercom A and B channels. Any crew station ATU then can be configured to transmit and receive intercom signals from the ground through the umbilial. (Only intercom signals are processed through the umbilical.)

Eight ATUs in the crew compartment are audio control panels used to select access by the CCU jack at each station and to control the volume of various audio signals. An ATU at each flight crew station, two in the middeck and one in the airlock control signals to headsets or CAPSs through the crew member CCU. The middeck and mission specialist crew stations have an ATU to control signals through a speaker/microphone unit. Signals to or from ATUs are processed by the ACCU. The vox, PTT, hot mic, ATU and master vol controls are controlled by circuitry within the ATU. All other knobs or switches on the face of the ATU send digital enable signals to the ACCU (except the audio function of the ATU power switch, which supplies power to the ATU circuits).

A redundancy feature of the four switches on the four ATUs allows control of a particular panel to be switched to another ATU. The left may be switched to the right commander's and pilot's ATU, and right pilot ATU control may be switched to the left commander's ATU. The commander's and pilot's control knobs are located on the respective panels. Mission specialist ATU control may be switched to the payload specialist's ATU, and airlock ATU control may be switched to the middeck and payload specialist's ATUs. The control knob for the mission specialist's ATU is located below the CRT 4 keyboard beneath the mission specialist's ATU. Control of these latter two ATUs is not reversible as commander and pilot ATU control is. Airlock CCU/ EMU 1 control is switched to the middeck CCU ATU, and airlock CCU/EMU 2 control is switched to the middeck speaker/microphone unit ATU. Both functions are switched with the single control knob in the airlock ATU. In the norm position, control of the ATU is from the panel to which the knob belongs. The other position of the knob indicates the ATU to which control can be transferred. The ATU control knob changes all ATU functions to the alternative ATU except the master volume control. This redundancy protection is used in the event of a failure or malfunction of any of the four ATUs that have an ATU control knob.

Each ATU has its own three-position power switch to control all signals to or from the ATU. The switch positions are aud/tone, aud and off. In the aud/tone position, all available functions of the ATU are armed; and transmission and receptions may be made through the ATU depending on the position of other switches on the ATU. C/W tone digital enable signals are sent to the ACCU to allow C/W audio to reach the ATU, thus the CCU or SMU. The aud position has the same functions as aud/tone except that C/W signals are blocked from the ATU. The off position shuts off power to the ATU power supply, for the ATU amplifiers. Siren ( P) and klaxon (fire) C/W signals go directly to an SMU, even with the SMU and ATU power off.

Each ATU has a two-position, spring-loaded-off paging switch that must be held in the page position to activate the circuit. When activated, the switch enables the ATU to transmit to all other ATUs, the EVA transceiver and the attached payload circuit (Spacelab). Any number of stations may use the paging circuit simultaneously, and the circuit may be used regardless of the position of the various individual channel control switches.

On all ATUs, the two air-to-ground channels, the air-to-air channel and intercom channels A and B have individual three-position control switches for selecting access to particular channels for transmission or reception. The switch positions are t/r, rcv and off . The t/r position permits transmission or reception over the selected channel. The rcv position deactivates transmission capability on the selected channel and permits only reception of signals. The off position deactivates transmission and reception on the selected channel. These control switches do not turn on any transmitter or receiver but allow access to a transmitter or receiver.

Each channel control switch has a thumbwheel volume control to adjust signal intensity on the related channel. The thumbwheels are labeled from zero (lowest volume) to nine (highest volume) and cover a range of approximately 27 decibels in 3-decibel increments. There is a volume control thumbwheel for TACAN signals on the commander's and pilot's ATUs.

The xmit/icom knob controls four combinations of external and intercom transmissions. The four knob positions are labeled PTT/hot, PTT/vox, PTT/PTT and vox/vox. In each case, the first set of initials indicates the method of external transmission activation, and the second set indicates the method of intercom transmission. In the PTT/hot position, external transmissions are made through (1) push-to-talk activation of a rotational hand controller at the commander's or pilot's station, (2) an HIU (any CCU station) ATU or (3) an SMU (middeck). Hot mike is activated, and the intercom is continuously live from the selected station ( HIU or SMU xmit must be keyed to enable external transmission). In the PTT/vox position, external transmissions are made by the transmit function of an HIU, SMU or RHC PTT; and intercom signals are voice activated. The PTT/PTT position provides access to external and intercom channels through the PTT of an RHC and HIU or an SMU, and the RHC PTT will activate any external and intercom channels selected. Vox/vox provides access to external and intercom channels and is voice activated.

Vox sensitivity regulates the loudness of the signal required for activation. The max setting requires a higher decibel level to activate the circuit than the min setting.

Master speaker volume control of all incoming signals to earphones or speakers is adjusted with the master vol knob. It acts in series with the volume control wheels for the individual channels-A/Gs 1 and 2, A/A, intercoms A and B and TACAN. Master volume knobs are located on the commander's, pilot's and airlock CCU ATUs and on the middeck SMU ATU. Two master volume knobs labeled 1 and 2 on the airlock ATU control volume to the respective CCU/EMU outlets in the airlock; the knobs are labeled from one (minimum volume) to nine (maximum volume).

The middeck ATU and the middeck speaker audio panel control operation of the SMU located at that station. In addition to the features of the other ATUs, the speaker/microphone unit ATU has a three-position power switch labeled off, spkr and spkr/mic . In the off position, no signals go through the ATU. In the spkr position, the SMU operates as a speaker only. In the spkr/mic position, the SMU can be used as either a speaker or a microphone. The SMU is located in the middeck ceiling and its power switch is located on the ATU. Signals to or from the SMU are selected on the ATU. A three-position, spring-loaded-off switch on the face of the SMU operates in conjunction with the PTT function of the ATU. The positions are xmit for access to external transmissions, icom for internal communications and the unlabeled off position for blocking outgoing PTT signals from the SMU. The xmit position sends signals over selected intercom circuits and to any external transmitters selected by the SMU ATU. The icom position excludes signals to external transmitters and allows signals to be sent over the selected intercom channel (A or B or both).

Keying the icom or xmit switch overrides the speaker, except for C/W emergency signals. In the voice-activated mode, the first signal to activate the circuit, either microphone or speaker, has priority, except for emergency C/W signals. The mic key light is a two-position, adjustable-intensity light that operates in conjunction with SMU transmissions. The intensity of the top half of the light is adjustable by the mic level knob; the brighter the light, the louder the signal. The bottom half of the light (key) is illuminated when a PTT function is selected and the circuit is keyed. Siren and klaxon C/W tones go directly to the speakers, even if the speaker power switch is off.

The audio center panel, located between the two aft viewing windows at the aft flight station, has three functions: UHF control, electrical interface capability with external vehicles and the payload bay, and operations recorders selection. All switches on the audio center panel send digital impulses to the ACCU, enabling the selected functions to communicate with Spacelab and the payload bay. Sets of on/off toggle switches labeled Spacelab and PL bay outlets electrically connect the particular function to the audio distribution system. The Spacelab subpanel has seven switches to enable the following functions: A/A, A/G 1, A/G 2, intercom A, intercom B, page and tone (C/W). The payload bay outlets subpanel has two on/off switches, one for intercom A and one for B.

Two rotary knobs labeled voice record select control various audio signals to be sent to the operational recorders through the NSP. A/G 1, A/G 2, A/A and intercom A or B audio can be sent to either recorder. Any two signals may be recorded at the same time, one on channel 1 and the other on channel 2. Either channel may be turned off. Signals to the operational recorders cannot be monitored by the flight crew.

The commander's and pilot's ATUs are the only units for controlling access to TACAN signals. There are two TACAN switches: the two-position on/off switch either allows reception or blocks incoming TACAN signals; the other three-position switch, labeled 1, 2, 3, allows selection of the particular TACAN set to be monitored. There are no transmission capabilities over the TACAN channels. TACAN is a polar coordinate system that provides distance and bearing information to a selected TACAN station. It operates in a band of frequencies from 962 to 1,213 MHz. The TACAN ground station signal identification call letters are repeated in Morse code every 40 seconds.