TheTarbell Cassette Interface is an expansion card for use with theAltair 8800 early personal computer, or other systems using the Altair'sS-100 bus. It was designed by Don Tarbell[1] and sold by Tarbell Electronics starting in 1976.[2] At the time, it was considered to be fast, reliable, and popular, and became ade facto standard forcompact cassette data storage.[3][4]
The Tarbell format is essentiallyManchester encoding where a "1" bit in the original data is output as "01", and a "0" bit is output as "10". This is accomplished by reading a byte into ashift register which is periodically triggered by a555 timer. Although the system can be run at any audible frequency, it is normally set to 3,000 Hz. As each data bit requires two output bits, the effective data rate is 1,500bits per second, or 187 bytes per second. On reading, the system usesclock recovery to calculate the original data rate to account for tape stretch and other effects. This means the data can be recorded at any hardware-supported rate, and the manual suggests up to 560 bytes per second.[5]
At the time of its release in 1976, the 300 bits per secondKansas City standard was also in use. Tarbell later modified the system to support these tapes. This increases the main oscillator from its normal 3,000 Hz to 4,800 Hz, and then writes either 00000000 to produce a single "1", or 01010101 to produce a "0". Because the system divides the basic rate by two, and each bit requires eight outputs, this produces the standard 300 bits per second output of the Kansas City standard.[5]
Tarbell also sold other products, including TARBELL CASSETTE BASIC in 1978[6] and aShugart Associates-compatible dual disk drive subsystem. The latter includes a Tarbellfloppy disk interface, said to plug into any S-100 bus computer, introduced in 1979.[7]
My name is Paula Rouse. I worked for the "Famous" Don Tarbell at Tarbell Electronics in Carson, CA from 1976 to 1983. I was his first employee when he began selling cassette interface kits and assembled units. He was a terrific boss and his wife Brenda and I have been best friends for all these years. She called me this evening to say that Don passed away this morning, May 19th, 1998, after a long bout with cancer.