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Kansas City standard

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Title: Kansas City standard  
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Subject: Tarbell Cassette Interface, Processor Technology, IBM cassette tape, Commodore Datasette, Magnetic tape data storage
Collection: Computer Standards, Early Microcomputers, Tape-Based Computer Storage
Publisher: World Heritage Encyclopedia

Kansas City standard

The SWTPC AC-30 Cassette Interface implemented the Kansas City standard. It sold for $80 in May 1976.

The Kansas City standard (KCS), or Byte standard, is a digital data format for audio cassette drives. Byte magazine sponsored a symposium[1][2] in November 1975 in Kansas City, Missouri to develop a standard for storage of digital microcomputer data on inexpensive consumer quality cassettes, at a time when floppy disk drives cost over $1000 USD each (today over $4400 USD).[3] Although the standard had existed from the earliest days of the microcomputer revolution, very few systems are said to have used it as their standard.[4]


  • History 1
    • Early cassettes 1.1
    • Kansas City symposium 1.2
  • CUTS 2
  • Floppy ROM 3
  • 300 baud 4
  • 1200 baud 5
  • Computers using the Kansas City standard 6
    • Early microcomputers 6.1
    • Home and personal computers 6.2
    • Programmable calculators 6.3
    • Other devices 6.4
  • See also 7
  • References 8
  • External links 9


Early cassettes

Early microcomputers generally used 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 a number of manufacturers started using similar approaches, although each of these systems were incompatible.[4]

Kansas City symposium

Wayne Green, who had just started Byte magazine, wanted all the manufacturers to get together and produce a single cassette standard. The site picked was Kansas City, Missouri. The two-day meeting was attended by 18 people who settled on a system based on Don Lancaster's design, published in Byte magazine's first issue. After the meeting, Lee Felsenstein (Processor Technology) and Harold Mauch (Percom) wrote the standard.

A 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 start bit (a '0') followed by eight data bits (least significant bit first) followed by two stop bits ('1's). So each frame is 11 bits, for a data rate of 27 311 bytes per second.

The February 1976 issue of Byte had a report on the symposium and the March issue featured two hardware examples by Don Lancaster[5] and Harold Mauch.[6] The 300 baud rate was reliable but slow. (The typical 8-kilobyte BASIC program took five minutes to load.) Most audio cassette circuits would 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."[4]

The participants of the Kansas City symposium include the following.


Processor Technology developed the popular CUTS (Computer Users‍ '​ Tape Standard) which works at either 300 or 1200 baud. They 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. Processor Technology also sold many programs on cassette tape, with the CUTS format on one side, and Kansas City standard on the other side.

Floppy ROM

Interface Age magazine May 1977 issue, with a Kansas City standard flexi disc floppy ROM.

In August 1976 at the Personal Computing show in Atlantic City, Bob Marsh of Processor Technology approached Bob Jones, the publisher of Interface Age magazine, about pressing software onto vinyl records. Processor Technology provided an 8080 program to be recorded. This test record did not work and they were unable to devote more time to the effort.[7]

Daniel Meyer and Gary Kay of Southwest Technical Products arranged for Robert Uiterwyk to provide his 4K BASIC interpreter program for the 6800 microprocessor. The idea was to record the program on audio tape in the "Kansas City" standard format and then make a master record from the tape. Eva-Tone made Soundsheets on thin vinyl that would hold one song.[8] These were inexpensive and could be bound in a magazine.

Bill Turner[9] and Bill Blomgren[10] of MicroComputerSystems Inc. worked with EVA-TONE and developed a successful process. The intermediate stage of recording to tape produced dropouts so a SWTPC AC-30[11] cassette interface was connected directly to the record cutting equipment.

The May 1977 issue of Interface Age contains the first "Floppy ROM", a 3313 RPM record with about six minutes of Kansas City standard audio.

The September 1978 Floppy ROM Number 5:. Side 1 Apple Basic "the automated dress pattern". Side 2 IAPS format "A program for writing letters".

300 baud

The original standard recorded data as "marks" (one) and "spaces" (zero). A mark bit consisted of eight cycles at a frequency of 2400 Hz, while a space bit consisted of four cycles at a frequency of 1200 Hz. A word, usually one byte (8 bits) long, was recorded in little endian order, i.e. least significant bit first. 7-bit words were followed by a parity bit.

1200 baud

Acorn Computers Ltd implemented a 1200 baud variation of CUTS in their BBC Micro and Acorn Electron microcomputers, which reduced a '0' bit to one cycle of a 1200 Hz sine wave and a '1' bit to two cycles of a 2400 Hz wave. Standard encoding includes a '0' start bit and '1' stop bit around every 8 bit piece of information, giving an effective data rate of 960 bits per second.

Also, these machines record data in 256-byte blocks interspersed with gaps of carrier tone, each block carrying a sequence number, so that it is possible to rewind the tape and resume at the proper block when a read error occurs.

Computers using the Kansas City standard

See also


  1. ^ Virginia Peschke (February 1976). "BYTE's Audio Cassette Standards Symposium". BYTE (BYTE Publications) 1 (6): 72–73. 
  2. ^  
  3. ^ The MITS 88-DCDD Altair Disk (controller board, 8 inch disk drive, case and power supply) sold for $1480 kit and $1980 assembled. (December 1975 BYTE magazine, page 45). It could store 300,000 bytes on a disk. The Shugart SA400 514 inch drive became available in late 1976 for $450 USD. It could store around 90,000 bytes on a disk. By 1977 this became a popular option for hobbyist computers.
  4. ^ a b c Les Solomon, "Solomon's Memory", Digital Deli, 1984
  5. ^  
  6. ^ Harold A. Mauch (March 1976). "Digital Data on Cassette Recorders". BYTE (BYTE Publications) 1 (7): 40–45. 
  7. ^ Jones, Robert S. (May 1977). "The Floppy ROM Experiment". Interface Age (McPheters, Wolfe & Jones) 2 (6): .pp 28, 83. 
  8. ^ Penchansky, Alan (November 10, 1979). "New Building for 'Soundsheets' Firm". Billboard (New York: Billboard Publications) 91 (45): p. 88.  
  9. ^ Turner, William W. (May 1977). "Robert Uiterwyk's 4K BASIC". Interface Age (McPheters, Wolfe & Jones) 2 (6): .pp 40–54. 
  10. ^ Blomgren, William (May 1977). "Platter BASIC: The Search for a Good, Random Access, Record Cutting Juke Box". Interface Age (McPheters, Wolfe & Jones) 2 (6): pp. 29–36. 
  11. ^ Gary Kay (December 1976). "The Designer's Eye View of the AC-30". BYTE (BYTE Publications) 1 (16): pp. 98–108. 

External links

  • The original Byte Magazine article
  • Sound sample of stored KCS file
  •'s article on the AC-30 cassette interface
  • Percom Data CIS-30 Cassette Interface Brochure
  • Kansas City Tape Decoder
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