BYTE Magazine sponsored a symposium on November 7 and 8, 1975 in Kansas City MO
regarding the interchange of data on inexpensive consumer quality audio cassette
drives. These drives may be used as one of the mass storage devices in the first
generation of personal computers, and will retain importance for some time to
come as a means of interchange of software between computer enthusiasts who
purchase products of the small systems industry. In order to promote the growth
of the industry, BYTE sought to achieve an industry standard on audio cassette
data interchange through a working conference.
We extend our greatest appreciation to the 18 people who worked very hard until
late Friday night and Saturday morning to discuss the multitude of problems and
solutions associated with digital recording on auto cassettes. The names of the
participants are listed in table 1. In spite of the short time available, the
participants were able to draft a set of provisional standards which seems to
promise great reliability and is rather inexpensive to implement;
implementations may be entirely in hardware, or may require a mix of software
and some minimal hardware. Considerations were given to the problems of speed
variation among recorders and playback equipment, start and stop delays,
recording density (or speed) versus reliability, and recording frequencies to
avoid interference with the telephone network in case some users plan to
transmit the tones of the cassette over the phone lines.
On Saturday afternoon, Mr. Felsenstein and Mr. Mauch volunteered to write up the
consensus among the participants as to a provisional standard which has been
reproduced below.
Provisional Audio Cassette Data Interchange Standard
The consensus among the participants of the audio cassette standards symposium
at Kansas City MO sponsored by BYTE Magazine is as follows:
The proposed standard centers around the use of a frequency shift modulation
method from which serial clock data can be extracted at rates of up to 300 baud.
The system is intended to be used with low to medium cost cassette recorders
incorporating electrical stop and start capability which may be operated under
program control.
The technique proposed provides for long and short term tape speed variation,
limitations in bandwidth due to effects such as tape misalignment, and the
necessity to retain low cost and low complexity of the hardware. The technique
allows for potential operation at higher tape speed than the nominal 1.875
inch/s (4.75 cm/s).
A mark (logical one) bit consists of eight cycles at a frequency of 2400 Hz.
A space (logical zero) bit consists of four cycles at a frequency of 1200 Hz.
A recorded character consists of a space as a start bit, eight data bits, and
two or more marks as stop bits.
The interval between characters consists of an unspecified amount of time at the
mark frequency. In this respect the data format is similar to that of
asynchronous data communication.
The eight data bits are organized least significant bit first, most significant
bit last, and followed (optionally) by a parity bit. The total number of
significant bits and the parity bit cannot exceed 8.
Where less than eight data bits are used, the unused bits (following the
optional parity bit) at the end of the character are mark bits (2400 Hz).
Data will be organized in blocks of arbitrary and optionally variable length,
preceded by a minimum of five seconds of marks. To avoid errors due to splice
and wrinkle problems common at the beginning of tape, the beginning of the first
data block will occur no sooner than 30 seconds from the beginning of clear
leader.
The contents of the data block are not specified.
The data block ends after the stop bits of the final character.
Bit clocking information may be extracted from the recorded waveform, which is
always an integer multiple of the bit rate, regardless of tape speed. This
permits the recovery and retiming of data by means of a UART, which requires a
clock of sixteen times the bit rate, although other simple circuitry may be
used.
A reliable bandwidth of 3000 Hz was assumed in choosing mark and space
frequencies due to the head misalignment expected between various cassette
recorders. The recording technique is a redundant form of Manchester or
bifrequency code which has a long history of reliability in the computer
industry. In its present form it was proposed by three independent manufacturers
at the conference. One cited reliability rates of one error in 10**7 characters
for 200 passes.
Table 1: Participants at Audio Cassette Symposium.
Ray Borrill, 1218 Prairie Dr, Bloomington IN
Hal Chamberlin, The Computer Hobbyist, P 0 Box 5985, Raleigh NC 27607
Tom Durston, MITS, 6328 Linn NE, Albuquerque NM
Lee Felsenstein, LGC Engineering, 1807 Delaware St, Berkeley CA 94703
Joe Frappier, Mikra-D, 32 Maple St, Bellingham MA
Bill Gates, MITS
Gary Kay, Southwest Technical Products Corp, 219 W Rhapsody, San Antonio TX
78216
Bob Marsh, Processor Technology, 2465 Fourth St, Berkeley CA 94710
Harold A Mauch, Pronetics, 4021 Windsor, Garland TX 75042
Bob Nelson, PCM, San Ramon CA
George Perrine, HAL Communications Corp, Box 365, Urbana IL 61801
Ed Roberts, MITS
Richard Smith, The Computer Hobbyist, P 0 Box 5882, Raleigh NC 27607
Les Solomon, Popular Electronics, 1 Park Av, New York NY 10016
Michael Stolowitz, Godbout Electronics, Box 2355, Oakland Airport CA 94614
Paul Tucker, HAL Communications Corp
Mike Wise, Sphere, 791 S 500 W, Bountiful UT 84010
Bob Zaller, MITS