a similar way to the IBM PC. According to some sources (see below), more than 190 different models of Apple II clones were manufactured. Most could not be legally imported into the United States. Apple sued and sought criminal charges against clone makers in more than a dozen countries. Data storage Cassette Originally the Apple II used Compact Cassette tapes for program and data storage. A dedicated tape recorder along the lines of the Commodore Datasette was never produced; Apple recommended using the Panasonic RQ309 in some of its early printed documentation. The uses of common consumer cassette recorders and a standard video monitor or television set (with a third party R-F modulator) made the total cost of owning an Apple II less expensive and helped contribute to the Apple II's success. Cassette storage may have been inexpensive, but it was also slow and unreliable. The Apple II's lack of a disk drive was "a glaring weakness" in what was otherwise intended to be a polished, professional product. Recognizing that the II needed a disk drive to be taken seriously, Apple set out to develop a disk drive and a DOS to run it. Wozniak spent the 1977 Christmas holidays designing a disk controller that reduced the number of chips used by a factor of 10 compared to existing controllers. Still lacking a DOS, and with Wozniak inexperienced in operating system design, Jobs approached Shepardson Microsystems with the project. On April 10, 1978, Apple signed a contract for $13,000 with Sheperdson to develop the DOS. Even after disk drives made the cassette tape interfaces obsolete they were still used by enthusiasts as simple one-bit audio input-output ports. Ham radio operators used the cassette input to receive slow scan TV (single frame images). A commercial speech recognition Blackjack program was available, after some user-specific voice training it would recognize simple commands (Hit, stand). Bob Bishop's "Music Kaleidoscope" was a simple program that monitored the cassette input port and based on zero-crossings created color patterns on the screen, a predecessor to current audio visualization plug-ins for media players. Music Kaleidoscope was especially popular on projection TV sets in dance halls. The OS Disk Apple and many third-party developers made software available on tape at first, but after the Disk II became available in 1978, tape-based Apple II software essentially disappeared from the market. The initial price of the Disk II drive and