In July 1985, Houston Lighting and Power was planning its new trunking radio system. Because the utility experiences some of the most damaging weather conditions, its trunking radio system was designed for the worst.  Part of the plan was to give the electric system managers plenty of radio resources, give them control and trust them to use the resources wisely.

The trunking system was designed in 1985 with more than 2000 radios in six fleets, 90 departmental talk groups and six all-call talk groups. Since then, it has been expanded to include 130 talk groups. The trunking system has telephone interconnect, and certain radios are programmed to allow private calls. 

Recording the power company system using the "de-trunking" method would have required 90 (now 130) base receivers capable of trunking (one for each talk group), three 40-channel logging recorders, and all of the wiring, power supplies, and ancillary equipment needed to connect this equipment together.

When the cost of 90 receivers and 3 logging recorders (each generating a reel of tape per day), as well as the inability to record private conversations, was considered, the detrunking recording method was shown to require overwhelming manpower and funds, yet it would only provide a partial solution at best. 

A further drawback of the "de-trunking" system its requirement of additional hardware a receiver and logging recorder channel each time a new talk group is added to the system. After a certain point is reached, depending on system design, a major expansion of the fixed-end equipment may also be required.

Alternative solution:

Remember, the desired result is the ability to selectively recover any radio transmission based on talk group, individual radio ID or RF channel. It is impossible to predict which transmissions will be of interest, so another solution is to record the RF channels "raw" and, using trunking control channel information, build an index of all transmissions. When it comes time to play back a particular transmission, a computer can search the index and play the desired transmissions.

One method of such recording uses an analog multitrack recorder and another uses a digital recorder. Although both methods are feasible, the digital version is preferred for several reasons.

1. Digital technology is moving ahead rapidly.
2. Digital hardware costs are decreasing.
3. A digital recording system is, in reality, a computer, and computer experience is useful in solving difficult problems.

The idea of recording the RF channels "raw" and de-trunking only when necessary significantly reduced the recording system’s hardware requirements. Instead of needing an audio source and digitizer for each talk group, only an audio source and digitizer are required for each RF channel, i.e., a 25 channel trunking system needs only enough hardware to receive audio information from the 25 RF channels plus one receiver for the control channel information regardless of the number of talk groups or subscriber units employed on the system. The only time that more hardware is required is when another RF channel is added. Adding one receiver per channel, one digitizer or digital signal processing (DSP) channel and a minor software modification activates the logging recorder system for the new configuration.

Choosing the medium for archive storage also presented a challenge. The selected medium would need enough capacity to record a "failsoft" condition of every RF channel transmitting continuously for at least 24 hours. Two commonly available computer cassette tape drives and an optical disk were selected as possible candidates.