On Distributed Communications Series
IV. Priority, Precedence, and Overload
Appendix D
Digital Communications Media
Teletype
Although a wide variety of teletype codes is in use, we will probably most often encounter conventional five-unit and eight-unit start-stop teletype signals. These signals are convertible to binary-stream transmission with buffering storage or by sampling at a rate much higher than the length of a single teletype bit.
Voice
Digital voice transmission can be performed at a wide variety of data rates. Seven-sample pulse-code modulation (PCM) using 8000 samples per second gives excellent voice quality, but requires 56,000 bits/sec. Differential PCM can be performed with 38,400 bits/sec. Recently, this writer heard tests of High Information Delta Modulation (HIDM) using a data rate of only 19,200 bits/sec.[1] The quality was good and the intelligibility excellent. HIDM uses relatively simple analog-digital-analog conversion equipment and appears to meet the requirements for military voice transmission, including good dynamic range. The use of this type of modulation has been assumed in the discussion in the text of this Memorandum.
Another class of digital voice equipment is vocoder equipment. In the vocoder, voice energy is frequency-division separated by a bank of band-pass filters; the output signal strength of each filter is measured and transmitted as a digital signal. Vocoder equipment is generally expensive and the reconstructed voice is of low quality. Its chief virtue is that it makes extremely efficient use of bandwidth, requiring less than 2400 bits/sec. There are indications that very-high-quality, high-intelligibility vocoders might be built in the future having a data rate of 5-10 kilobits/sec.
(One development that occurred after the preparation of this Memorandum will change the loading figures for voice by a large factor, and should at least be mentioned here. In the description of the Multiplexing Station (ODC-VIII) it is pointed out that it is easy to suppress blank spots in a voice stream without losing quality or breaking synchronization. This would probably reduce the average voice-conversation data rate, when using HIDM for example, from 19,200 bits/sec to about 5000 bits/sec without any drop in quality.)
Facsimile
Facsimile transmission, normally an analog signal, can be converted by conventional analog-to-digital means to operate at about 9.6 kilobits/sec.
Computer Data
A wide variety of manually and semi-manually operated data generating devices are used with computers. These include typewriter keyboards, coded insertion card interrogators, Hollerith card readers, punched paper tape readers, and high-speed teletypewriters. These devices are generally characterized by low bit-rate requirements.
In the future, we may also find cases where computers might "talk" to one another. Here, the amount of data exchanged can be small, if only processed data is exchanged or if humans are involved. But, there are also applications where it is desirable to exchange raw data between machines. Here, the assumption of equal information per unit time is not applicable, because we are not limited by the human factor in such a loop. One application might be a dump or transfer of the major parts of the high-speed core memory of a computer into a remote computer at a high bit-rate--on the order of one million bits/sec. Magnetic tape can also be read at one station and rewritten into a remote tape unit for later processing, perhaps at rates up to 250,000 bits/sec.
[1]Winkler, M. R., "High Information Delta Modulation," IEEE International Convention Record, Paper 47.3, New York, New York, March 28, 1963.
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