Datasette

Commodore Datassette 1530

A datasette ( Commodore : Data ss ette ) is a tape drive that was widely used in the 1980s to store computer data on conventional compact cassettes (CC). Datasette is a portmanteau of Data (English for data ) and tape . The name originally comes from Commodore, but was later occasionally used for similar devices on other home computers , among others. a. used by Atari , Apple , Robotron , Tandy , Sinclair , Texas Instruments and Amstrad / Schneider .

Disk

Typical labeling of cassette inlays with the counter readings of the datasette and the corresponding computer game titles

There were cassettes specifically known as data tapes (e.g. the Magna or Computape products), but these were expensive and offered less storage space than conventional cassettes, so that usually commercially available audio cassettes were used as data cassettes. Around 100 kByte can be stored on a conventional 30-minute cassette in the standard format of the Commodore computer. By using loading accelerators such as Turbo Tape , roughly 1 Mbyte can be saved per 30 minutes of tape. The bit rate is in the range from around 300 bit / s to just under 5 kBit / s.

disadvantage

Datasettes are no longer used today because they are inferior to current data carriers by many orders of magnitude in terms of capacity and speed . In addition, almost all datasettes are purely linear media, in which the user has to search for tape locations by hand using tedious rewinding; This means that they are in principle inferior to media with random access , such as floppy disks , hard drives or CD-ROMs .

variants

Cassette drives with random access were only represented in medium-sized data technology , especially in the 1970s; the only home computers with a random access cassette drive are the 1980 Philips P2000M and the portable Epson HX-20 and PX-8. Both used the mini-cassette developed by the manufacturer for dictation machines and designed for start / stop operation . There both housed around 170 KB of data.

There were also a number of drives that used special cassettes with an endless belt, such as the MicroDrive from Sinclair, or the rather exotic Entrepo Quick Data Drive for the Commodore 64 .

Distant relatives of Datasette are, drives the data on VHS - video cassettes or video-8 save tapes. Due to their high price, these were not used in home computers, but were sometimes used in the professional sector for data backup of large archives, since they offered an extremely high storage capacity for the conditions at the time (on a 240-minute VHS tape, over 2 gigabytes can be stored, which at that time corresponded to the content of several dozen hard drives). Tape drives of this type are also known as "streamers".

The first digital synthesizers (as well as some analog synthesizers with storage options, such as the Korg Poly 61 ) often had a socket for connecting a tape recorder or a cassette recorder , via which individual presets could be stored on tape or cassette and then called up again. Since the drive itself was not controlled by the respective instrument, any medium suitable for recording audio could in principle be used. The media used in the recording studios varied over time and digital formats such as DAT , DCC and Minidisc were also widely used. Lovers of classic synthesizers today usually save their presets via the audio interface of a personal computer.

The datasettes 1530 and 1531 from Commodore weigh 0.7 kg and are 19.5 cm wide, 5 cm high and 15 cm deep. They differ from each other in terms of the color of the housing and the type of connector. The latter has an 8-pin mini-DIN round plug only for connection to the home computers of the Commodore 264 series ( C16 , C116 and Plus / 4 ), with some 1531 models a 1530-compatible adapter plug was included.

technology

Data signal from a Datassette: 15 seconds audio example (no fast charger)

Signal course of an exemplary data sequence for the Commodore Datassette

Datasette connection of the Commodore 64

The drive, the preamplifier and the tape heads of a normal music cassette recorder are used to build a datasette ; loudspeakers and microphones are dispensed with. As an additional element, it has a demodulator , which in this case is a Schmitt trigger and serves to determine the zero crossings of the received signal.

The data are usually (exception: Atari company ) stored on the tape with a modified frequency shift keying , as shown in the adjacent figure. In this recording, the amplitude carries no information. During reading and decoding, after the Schmitt trigger, the time intervals between the positive and negative zero crossings of the signal were measured using timer modules such as the MOS Technology CIA (6526) and the data information was recovered from this time information. In the sketch on the right, the negative zero crossings of the signal, which are important for data reconstruction, are drawn as black circles on the center line. Depending on the time interval, the two values can be differentiated logically 1 and logically 0 .

The bits received individually in this way are then shifted into a shift register and continuously compared with special bit sequences for synchronization at the beginning of a data transmission. The first byte of such a sequence for synchronizing the beginning of the block is referred to as the lead-in byte , followed by several sync bytes , which are used to compensate for possible fluctuations in the band's synchronization by coordinating the timers. For example, the fast charger Turbo Tape 64 used on the Commodore 64 uses 0x02 as the lead-in byte , followed by the sync sequence 0x08, 0x07, 0x06, 0x05, 0x03, 0x02, 0x01. This was followed by the actual user data.

Atari devices used direct frequency shift keying. Instead of recognizing the zero crossings of the signal and measuring their time interval, two fixed frequencies, which stood for the bit values 0 and 1, were filtered out of the audio signal by means of bandpass filters and their amplitude compared; the stronger signal determined the logic level output by the Atari datasette. Quick-loading programs could not be used with the Atari datasettes, since most of the properties of the recording format were fixed by the hardware of the devices.

For some computer models ( e.g. Apple II , the Sinclair models and the home computer KC 85 from GDR production) there were no suitable special datasets from the same manufacturer. Instead, any commercially available audio cassette recorder could be connected via the audio inputs and outputs; in this case the demodulator was located in the computer itself.

The original IBM PC and the IBM PCjr also had a data set port which, like the keyboard port , was designed as a 5-pin female DIN circular connector . However, cassettes as storage media were already considered obsolete when the IBM PC was introduced for the targeted market segment; In addition, the datasette could only be accessed directly via the internal ROM-Basic of the IBM PC, but not via the much more powerful and mostly used DOS. For these reasons, the interface was hardly used and, unlike with earlier small computers, there was no market for pre-recorded program cassettes. The interface was no longer used in its immediate successor, the IBM PC XT . On the part of the PC-BIOS, the programming interface for application programs was kept very simple and consisted of four functions of the software interrupt 15h. These made it possible to start the drive motor (function 00h, AH = 00h), to stop (function 01h, AH = 01h) and to read (function 02h) and write (function 03h) a certain number of bytes on the tape, which in CX had to be specified. In ES: BX the pointer to the memory address of the data buffer was to be named. Since DOS - in contrast to the handling of floppy drives - did not offer any further routines for the datasette, only the low-level access routines of the BIOS were available for the use of the interface by programmers and users, who find their own ways to manage the raw data on the cassettes had to. After the Datasette interface had disappeared, the PC successors used interrupt 15h for other purposes. With the PC AT , an attempt to call the datasette routines could even lead to a system crash.

In order to record and read out data, it is important that the sound head is correctly adjusted. Typically only one mono track with a typical bandwidth of around 10 kHz was used. An exception are the Atari devices, which put the data on one stereo track and a music track on the other to support the charging process. Fluctuations in the synchronism caused by the drive and the cassette were either compensated for by a correspondingly low and therefore more robust data rate or, in the case of some fast chargers, by special, continuously repeated synchronization sequences during runtime, which, depending on the method, were also repeated within data blocks.

Occasionally there were computer magazines with a sound foil enclosed between the pages , which, when transferred to a cassette, produced a recording that could be read in from the datasette, whereby a single-track sound head was better suited for monophonic recordings, provided the amplified loudspeaker output of the turntable was an ordinary audio source used as a datasette. Could replace cassette recorder. Dedicated Datasette devices had no audio connections and did not need the 44 kHz bias for premagnetization , since the modulated signals only had fixed levels without any dynamics.

Applications

It can be saved in different file formats, depending on the home computer. Self-written programs were often saved as a single BASIC file. Commercial programs and games, like floppy disk programs, usually consisted of several files (title graphics, further levels), which were then reloaded and often saved in machine language. On the back of the cassette there was often an identical copy of the game or other levels.

Emulators almost only use cassette images, such as .TAP and .T64, and more rarely also real audio files such as .WAV .

In the television program WDR Computerclub , audio signals were broadcast as so-called hard-bit rock in BASICODE , which could be recorded and read in via a datasette.