GE Mark V

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GE Mark V is a historic U.S. format or protocol of trunked, two-way radio introduced by General Electric Mobile Radio in the early 1980s. This equipment was also sold in Australia. The product name looks and sounds similar to GE-Marc V, a turbine controller made by General Electric.

Contents 1 Details 2 How it worked 2.1 Two-tone sequential 2.2 Continuous tone squelch 2.3 Transmitter tests 2.4 Operator view for trunked radio historians 2.5 Missed calls 3 Tech trivia

[edit] Details

These radios are seen as obsolete and there are no known instances of these systems operating in the U.S. today. The general category of this kind of trunked system is called, "Scan-based trunking." In the U.S. and Australia, these systems used analog FM, operated in the 806-869 MHz band, and were primarily used for commercial, non-public-safety trunking. Some systems offered half-duplex, (push-to-talk) telephone interconnect. This feature was popular before the rollout of analog cellular telephones.

Radio models used in these systems included the names, "Classic", "Centura", "Centura II", "Corona," and "Centura TC". While most radio models were sold with a speaker and microphone, some had a telephone handset and cradle attached to the front of the dash-mount radio housing. There was an earlier trunk-mount model that looked like a MASTR Executive II but its name is not known. It is believed that an MPR-series hand-held model may have been offered for GE Mark V trunked systems.

A large system operator might own their radio backbone (repeaters). GE Mark V Specialized Mobile Radio systems, where subscribers paid a company to operate the trunked radio system, were present in major U.S. cities. The operators typically charged a monthly fee for each radio plus airtime charges.

Some mobile units could be programmed to work on more than one GE Mark V trunked system. For example, if the SMR operator had a system in Bloomington and another covering Chicago, many radio models offered an "area" switch. This selected the transmit area priority. The radio would automatically scan all programmed areas unless an option was set to restrict scanning to the current area setting. Different radio models had 29- to 100-channel capacity and there were limitations on the number of areas and the number of channels per area. In its default configuration, the radios function in the same way a talk-back-on-scan option works: regardless of which area was selected, the radio would join a conversation on the system where it was taking place.

In early radios, a 32x8 (not 32Kx8) PROM defined frequencies and option settings. In later radios, a programming box set the radio's RF channels and options. In U.S. systems, the set of channel frequencies for a system or area were usually unique to each trunked system. The radios were not capable of roaming. (Roaming in this use means working with an unknown GE Mark V system the user happened to run across in their travels).

[edit] How it worked

In a GE Mark V system:

• electronics that determine which channel a conversation will occur on are inside each radio. Radios scan all available channels in the system to find an unused channel for a conversation. They also scan to look for a tone sequence initiating a conversation from another radio in their own group.
• repeaters are essentially stand-alone repeaters, except for tone handshaking electronics, deadbeat disable computer, telephone patch equipment, or equipment used to bill air time, (talking time). Some of these were options.
• because call set up was slow, repeaters had a long hang time in order to hold a group's mobile radios on a channel until a conversation was finished.
• a trunked system or area could have up to 29 channels. Some 100-channel models, for example, could accommodate three 29-channel areas plus one 13 channel area.
• the user would hear a low-pitched error tone if they took the microphone off hook and no channels were available or the radio was out-of-range.

[edit] Two-tone sequential

GE Mark V used a two-tone sequence to identify a group: what modern systems call agency-fleet-subfleet or talk groups. Each radio had at least one tone pair, which identified the group of radios it could talk with. It was similar in format to two-tone sequential paging codes except that, in a GE Mark V system, the first tone was much longer than the second. This long first tone gave a bigger time window for all the scanning radios to find and decode a two-tone sequence. The first tone was lengthened for systems with more channels.

It was possible for radios to have several tone pairs or groups. These could be used to make phone calls over a patch without all other users in the same group having to listen to the call. Some systems had hierarchies: manager groups could talk amongst one another without going out over everyone's radio.

[edit] Continuous tone squelch

Systems used a continuous tone like CTCSS but in the 2,900-3100 Hz range. A low-pass filter eliminated most of the tone from speaker audio. The 3 KHz tone was present through the transmission but cut off just before the repeater dropped in order to eliminate the squelch crash. There were two programmable continuous tones: all radios on a single system used the same tone. These were used to distinguish between sites in the same way SAT tones were used on AMPS cellular systems. Radios using the system had to have their continuous tone set to match the desired system and have a compatible two-tone sequence for their group.

[edit] Transmitter tests

Since the radio would not transmit unless the ready light was lit, checking reflected power, channel frequency, or deviation outside the system coverage area, (or for a system that was not built yet), required the technician to attach a test box. The box bypassed the logic that required a handshake with the trunked repeater in order to transmit.

[edit] Operator view for trunked radio historians

If the operator desired to make a radio call, they would pull the mike out of the hang-up box. This would cause the radio to look for a channel with no carrier present. When a vacant channel was found, the push-to-talk relay would chatter and the transmit indicator would flicker. The radio would handshake with the repeater using a single analog tone. The handshake would identify the repeater as "in range" and the channel as "not busy." If the radio successfully performed the busy handshake, its two-tone group sequence was transmitted over the air. As soon as the tones were sent, the receiver audio turned on and the radio made a doorbell-like "ding dong" chime to tell the operator the channel was available to talk. A green "ready-to-talk" light-emitting diode on the radio lit. The speaker turned on and the operator heard idle channel noise on a carrier.

Other radios in the same group would scan until they heard a first tone matching their own two-tone sequence. Hearing a matching first tone, the scanning would stop and wait to determine if the second tone matched its group. If the tone didn't match, the radio would silently go on searching. On hearing a matching tone, every radio in the group would do the doorbell chime, display a green, ready indicator, and the speaker audio would turn on. The user would hear the open carrier until someone talked or the repeater carrier dropped.

Since the doorbell sound was annoying, users tended to try to hold the repeater carrier on until the called party answered so they wouldn't have to listen to continual ding-dongs. If base was slow to answer, the transmission might sound like, "Unit four to base," <squelch tail>... <squelch tail>... <squelch tail>... <repeater drops>. On some radio models, the doorbell sound was programmable but it's not clear if those responsible for systems knew this.

[edit] Missed calls

A major drawback to this architecture is the fact that a missed two-tone sequence at the beginning of a transmission, or a lost signal during a transmission, causes the rest of a transmission to be missed. If the mobile receiver gets interference or loses the repeater signal for the moment the second selective calling tone is sent, it remains muted, missing the entire conversation. The same is true if the repeater signal is lost at any time during a transmission. Only another two-tone sequence being sent will allow it to rejoin the conversation.

A modern trunked system with a control channel is more costly and complicated, but sends continual messages for all in-progress conversations. If your radio is in a talk group conversing on channel 3, the control channel continually sends "<go to channel 3>" messages over and over until the transmission ends. Suppose the user is driving through a tunnel with no signal then exits and acquires the signal. If the mobile receiver acquires the control channel signal any time during the conversation, it immediately decodes the channel assignment and switches over to join the conversation.

[edit] Tech trivia

System documentation shows at least some radio models, including Classic and Centura, were not capable of being programmed for areas near the Mexican Border. Federal Communications Commission channel assignments along the border follow 25 KHz offsets (example: 809.775 MHz) unlike the rest of the US where they follow 12.5 KHz offsets, (example: 811.1625 MHz).