Finding the needle in an electronic haystack Recording and playing back voice communications from a trunked radio system requires less equipment with a method that records ‘raw’ RF channels. Digital compression techniques extend recording times to more th

So you want to record audio from a trunked radio system? Nothing to it. With a multitrack recorder and audio sources from trunked receivers or directly from the trunked radio system, you are in business.

Sometimes, though, we do not say exactly what we mean.

Before surgery begins, we say we want the anesthesiologist to put us to sleep–when what we really want is for the good doctor to wake us up afterward. We say we want a drill bit–when what we want is a hole. When we talk about recording a trunked radio system, what we really want is to be able to hear what was said in a particular conversation. When it comes time to listen to a particular conversation–especially if it had been a “private” conversation, there may be a serious problem. After a trunked radio system has changed channels a few times, finding and playing back a particular conversation is more difficult than finding a needle in a haystack.

Recording (logging) a conventional two-way radio system is easy. An ordinary, multichannel (multitrack) recorder works fine. Finding and replaying a conversation requires only that you position the tape and play the correct channel.

Logging a trunked radio system is another matter. Figure 1 below shows communications activity on a typical, small, trunked system. Routine talk group transmissions, private transmissions and a telephone interconnect call are color-coded. Notice how the individual transmissions hop from one frequency to another. Transmissions move around for several reasons.

The frequency-hopping allows the system to find an open channel. Furthermore, it ensures a more even channel-loading.

Many trunking radio systems are equipped for telephone interconnects and “private” conversations. Logging a telephone interconnect conversation is not normally a problem because the recorder can be connected to the telephone lines that are dedicated to the system controller.

Private calls, on the other hand, present a number of problems. Contrary to popular belief, a private call does not tie up a physical radio channel as a telephone interconnect does. Instead, a private conversation is, to the trunking radio system, an ad hoc “talk group” that is created on the spot for the two radios involved in the private call. There is no formal redefinition of the programmed system talk groups, and, when the private call ends, the ad hoc “talk group” vanishes.

Previously, the only way to log and record a trunking radio system was to de-trunk the talk groups prior to recording and to use a standard multichannel (multi-track) recorder. With this method, you need equipment to de-trunk each talk group, and private conversations are extremely difficult to record–if you can record them at all.

There are several solutions. 1. Do not assign many talk groups. (Unfortunately, this would defeat the effectiveness of the trunking radio system.) 2. Do not allow private calls. (Unfortunately, this would limit management and supervisory effectiveness.) 3. Do not create new talk groups until you have the necessary equipment and recording channels to capture them.

These probably are not practical solutions.

Real world example In July 1985, Houston Lighting and Power was planning its new trunking radio system. Because the utility experiences some 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 2,000 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 three 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 is 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 for 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 the control channel information, regardless of the number of talk groups or subscriber units 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.

The tape drives offer large capacity; prices for the drives themselves and the cassettes are reasonable; and they provide reasonable recovery times. One cassette system uses commonly available 8mm video cassettes and has 5 Gigabytes (GB) of capacity on a cassette. Digital audio tapes (DAT) offer 4GB of capacity at a higher cost per 4mm cartridge. Both drive systems have acceptable performance and capacity.

The compact disc, read-only memory (CD-ROM) optical drives do not have sufficient capacity yet; nevertheless, by using the small computer system interface (SCSI) buss for tape drives, disk drives or both, future storage devices and improvements can be readily accommodated.

Both the 8mm video tape drive and the DAT 4mm tape drive use helical scanning, and frequent starting and stopping is not recommended. To overcome this limitation, a hard disk drive was incorporated as a large buffer to: buffer the incoming digitized audio until enough is stored to justify starting the tape drive. make possible simultaneous recording and playback.

It is not necessary to use a separate device for playback. The simultaneous record-and-playback capability permits unattended operation and remote playback.

The breakthrough that makes it possible to record the radio system for more than 24 hours on a 4GB or 5GB tape is the use of voice compression algorithms and DSPs.

Even so, the problem is more complex than finding a needle in a haystack. You have to find a series of needles in a needlestack. You must label and file the needles in an organized way when you store them and create a directory and an index for finding the specific needles you want.

How it works The logging recorder has a digitizer and compressor operating continuously for each RF channel, outputing one-second packets of digitized audio. During periods with no activity on the trunked system–intervals with no active channel grants–these packets are discarded. When the trunking control channel issues a channel grant, the recorder generates a header that contains the date, time, RF channel, talk group and unit ID. The logging recorder’s main processor commands the DSP to save the packet and to append the header. The main processor then writes the completed packet to the hard disk and writes an index record in two locations. When a “save set” of about 50,000 channel-second voice packets (about 50MB) is stored on the hard disk, the file is closed, and a new “save set” is opened. The processor then transfers the closed “save set” to tape for permanent storage. Depending on capacity, a number of “save sets” are retained on the hard disk until space is needed. When space is needed for record or playback, the oldest “save set” is erased, and a new “save set” takes its place.

The multitasking computer can record, play back and transfer voice packets among the DSPs, disk drives and tape drives and still have time to drive a simple video display. Even so, the advantages of this technology would be limited if the system were difficult to use. To speed the training and to minimize the complexity, two computers are used. One runs the recorder, and another serves the people using the system. The “people server” is a Windows-based PC that provides local control, a local area network (LAN) interface or modem access to the recording system.

The people server’s Windows program has an attractive, easy-to-use search capability that can recover radio conversations by talk group, radio ID or RF channel while providing system control and operating updates. To minimize search time, the system requests a time-and-date window to limit the extent of the search. If the conversation sought remains on the hard disk and if the time window is not too large, search results and playback usually are available in less than 30 seconds.

The people server provides three important displays. The primary screen, shown in Figure 2 on page 38, provides system status information including RF channel activity and tape status. A mouse can be used with this screen to select “live” radio system monitoring by talk group, radio ID or RF channel.

Figure 3 on page 40 shows the search screen used to specify searches by talk group, individual ID or RF channel. Several options are available to control the date and time limits for the search. A smaller range results in faster searches.

Playback audio can be delivered as local analog audio, remote analog audio (via phone) or as a digital sound file with the .WAV extension for playback on a multimedia-equipped PC. Figure 4 on page 44 illustrates the two playback modes. The real-time mode plays individual transmissions in sequence with the original interval between transmissions. The compressed-time mode plays individual transmissions in sequence with an interval of 1-2 seconds between transmissions–a great convenience when a series of transmissions occurred over a 10-to-15-minute period.

Playback security can be provided by both password and hardware if desired. Physical and performance security can be provided by redundancy.

One frequently asked question is “What happens if the desired ‘save set’ has been transferred to the tape and erased from the hard disk?” The system checks its directory for the tape volume where the “save set” is stored. If the tape is still on the machine, the oldest “save set” on the hard disk is erased, the desired “save set” is copied from the tape back to the hard disk, and the desired conversation is played. If the tape has been removed, a request will be displayed to load the needed tape volume. Another optional capability is to use a “carousel” or “juke box” system available in several sizes with as many as 100 tapes. With the carousel or juke box, the system automatically loads the needed tape, if it is available.