The Kansas City standard (KCS), or Byte standard, is a data storage protocol for standard cassette tapes or other audio recording media at 300 bits per second. It originated in a symposium sponsored by Byte magazine in November 1975 in Kansas City, Missouri, to develop a standard for the storage of digital microcomputer data on inexpensive consumer quality cassettes. The first systems based on the standard appeared in 1976. One variation on the basic standard is CUTS, which is identical at 300 bit/s, but with an optional 1200 bit/s mode. CUTS is the default encoding used by several later machine families, including those from Acorn and the MSX. MSX added a higher 2400 bit/s mode that is otherwise similar. The 1200 bit/s mode of CUTS was used as the standard for cross-platform BASICODE distribution. KCS originated from the earliest days of the microcomputer revolution, among other prolific protocols. Most home computers of the era have unique formats without any intercompatibility.
History Early microcomputers generally use punched tape for program storage, an expensive option. Computer consultant Jerry Ogdin conceived the use of audio tones on a cassette to replace the paper tapes. He took the idea to Les Solomon, editor of Popular Electronics magazine, who was similarly frustrated by punched tapes. In September 1975, the two co-authored an article on the HITS (Hobbyists' Interchange Tape System), using two tones to represent 1s and 0s. Soon after, several manufacturers started using similar approaches, all incompatible. Wayne Green, who had just started Byte magazine, wanted all the manufacturers to collaborate on a single cassette standard. He organized a two-day meeting on 7–8 November 1975 in Kansas City, Missouri. The participants settled on a system based on Don Lancaster's design. After the meeting, Lee Felsenstein (of Processor Technology) and Harold Mauch (of Percom) wrote the standard, which was published in Byte magazine's first issue. A KCS cassette interface is similar to a modem connected to a serial port. The 1s and 0s from the serial port are converted to audio tones using audio frequency-shift keying (AFSK). A "0" bit is represented as four cycles of a 1200 Hz sine wave, and a "1" bit as eight cycles of 2400 Hz. This gives a data rate of 300 baud. Each frame starts with one "0" start bit, followed by eight data bits (least significant bit first) followed by two "1" stop bits, so each frame is 11 bits, for a data rate of 27+3⁄11 bytes per second. The February 1976 issue of Byte has a report on the symposium, and the March issue features two hardware examples by Don Lancaster and Harold Mauch. The 300 baud rate is reliable, but slow; a typical 8-kilobyte BASIC program takes five minutes to load. Most audio cassette circuits support higher speeds. According to Solomon, the efforts were unsuccessful: "Unfortunately, it didn't last long; before the month ended, everyone went back to his own tape standard and the recording confusion got worse." The participants of the Kansas City symposium include these:
Ray Borrill, Bloomington, Indiana Hal Chamberlin, The Computer Hobbyist, Raleigh, North Carolina Richard Smith, The Computer Hobbyist, Raleigh, North Carolina Tom Durston, MITS, Albuquerque, New Mexico Bill Gates, MITS, Albuquerque, New Mexico Ed Roberts, MITS, Albuquerque, New Mexico Bob Zaller, MITS, Albuquerque, New Mexico Lee Felsenstein, LGC Engineering / Processor Technology, Berkeley, California Les Solomon, Popular Electronics Magazine, New York, New York Bob Marsh, Processor Technology, Berkeley, California Joe Frappier, Mikra-D, Bellingham, Massachusetts Gary Kay, Southwest Technical Products Corp, San Antonio, Texas Harold A Mauch, Pronetics/Percom Data, Garland, Texas Bob Nelson, Pacific Cyber/Metrix, San Ramon, California George Perrine, HAL Communications Corp, Urbana, Illinois Paul Tucker, HAL Communications Corp, Urbana, Illinois Michael Stolowitz, Godbout Electronics, Oakland, California Mike Wise, Sphere Corporation, Bountiful, Utah
Format and enhancements The original standard records data as "marks" (one) and "spaces" (zero). A mark bit consists of eight cycles at a frequency of 2400 Hz, and a space bit consists of four cycles at a frequency of 1200 Hz. A word, usually one byte (8 bits) long, is recorded in little endian order, which is least significant bit first. 7-bit words are followed by a parity bit. As each 8-bit byte is encoded into 11-bits for transmission, the total throughput is 27 bytes per second.
Tarbell The Tarbell Cassette Interface, which, according to early PC retailer Stan Veit, "became a de facto standard for S-100 computers", supported the Kansas City Standard in addition to the Tarbell "native" mode ("Tarbell standard"). The Tarbell standard is very different than the KC standard: it records "1" as "01" and "0" as "10", and typically ran at 1500 bit/s (187 bytes per second, 3000 Hz). 300-baud KC support is implemented by recording "1" as 00000000 (hence "1010" ×4) and "0" as 01010101 ("1001" ×4) at 4800 Hz.
CUTS Processor Technology developed the popular CUTS (Computer Users Tape Standard), which works at either 300 or 1200 baud. The 1200 baud version simply reduced the number of cycles per bit, with a mark being two cycles of 2400 and a space being one cycle at 1200. It was otherwise the same as the original KC standard, with a single "0" for the start bit and two trailing "1"s as stop bits, giving an effective data rate of 873 bits per second. The company provided the S-100 bus CUTS Tape I/O interface board, which offers both CUTS and Kansas City standard support to any S-100 system.
Quick CUTS The Quick CUTS standard proposed by Bob Cottis and Mike Blandford and published in the Amateur Computer Club newsletter operated at 2400 baud, encoding "0" as a half-cycle of 1200 Hz and "1" as a whole cycle of 2400 Hz. The receiver was self-clocking using a phase-locked loop. Published in 1978, it predates the 1982 patent for the similar coded mark inversion proposal (but not its priority date of July 27, 1978).
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