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:

1. Buffer the incoming digitized audio until enough is stored to justify starting the tape drive.
2. 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 4 or 5 GB 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 must find is 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, outputting 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 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 50 MB) 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 hard 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, local area network (LAN) interface or modem access to the recording system.

The people server’s Windows program has an attractive runs provides an attractive, easy-to-use search capability that can recover radio conversations by talk group, radio ID(s), or RF channel while providing system control and operating updates. To minimize search time, the system will 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 are usually available in less than 30 seconds.

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

Figure 3 (previous page) shows the search screen that is 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 search 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 multi-media equipped PC. 

Figure 4 illustrates the two playback modes. The real-time mode plays the individual transmissions in sequence with the original interval between transmissions. The compressed-time mode plays the individual transmissions in sequence with an interval of 1-2 seconds between transmissions a great convenience when a series off 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 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.