Digital Compact Cassette

Digital Compact Cassette
A Digital Compact Cassette sent by Q-magazine to its readers.
Media type Magnetic tape
Encoding Precision Adaptive Sub-band Coding (MPEG-1 Audio Layer I)
Capacity 105 minutes
Write mechanism multi-track stationary head
Developed by Philips & Panasonic
Usage audio
Extended from Compact Cassette

The Digital Compact Cassette (DCC) is a magnetic tape sound recording format introduced by Philips and Matsushita in late 1992 and marketed as the successor to the standard analog Compact Cassette. It was also a direct competitor to Sony's MiniDisc (MD), but neither format toppled the then-ubiquitous analog cassette despite their technical superiority. Another competing format, the Digital Audio Tape (DAT) had by 1992 also failed to sell in large quantities (although it was established in recording studios)—DCC was envisaged as a less expensive alternative to DAT. DCC shares a similar form factor to analog compact cassettes, and DCC recorders can play back either type of cassette. This backward compatibility allowed users to adopt digital recording without rendering their existing tape collections completely obsolete.

The other side of the DCC shown above

History

Philips DCC player

DCC signalled the parting of ways of Philips and Sony, who had worked together successfully on the standard Compact Disc, CD-ROM, and CD-i before. Based on the success of Digital Audio Tape in professional environments, both companies saw a market for a new consumer-oriented digital audio recording system that would be less expensive and perhaps less fragile. Sony decided to create the entirely new MiniDisc format (based on their experience with magneto-optical recording and Compact Disc) while Philips decided on a tape format that was compatible with their earlier analog Compact Cassette format.

DCC, initially referred to as S-DAT (stationary-head digital audio tape, as opposed to R-DAT—rotary-head digital audio tape),[1] was developed in cooperation with Matsushita, and the first DCC recorders, were introduced at the CES in Chicago in May, 1992[2] and the Firato consumer electronics show in Amsterdam in September 1992. At that time, not only Philips and Technics (brand of Matsushita) announced DCC recorders but also other brands such as Grundig and Marantz (both related to Philips at the time).

More recorders and players were introduced by Philips and other manufacturers in the following years, including some portable players and recorders as well as in-dash DCC/radio combinations for automotive use.

At the HCC-dagen computer fair in Utrecht, Netherlands, on November 24, 25, and 26, 1995, Philips presented the DCC-175 portable recorder that can be connected to an IBM-compatible PC using the "PC-link" cable. This was the only DCC recorder that can be connected to and controlled by a computer, and it was only ever available in the Netherlands.

Philips marketed the DCC format in Europe, the United States, and Japan. According to the newspaper article that announced the demise of DCC, DCC was more popular than MiniDisc in Europe (especially in the Netherlands).[3]

DCC was quietly discontinued in October 1996[3] after Philips admitted it had failed at achieving any significant market penetration with the format, and unofficially conceded victory to Sony. However, the MiniDisc format had not done very well either; the price of both systems had been too high for the younger market, and audiophiles rejected MD and DCC because in their opinion, the lossy compression deteriorated the audio quality too much.[4]

Technology

Hi-Fi-system-sized DCCs and recorder

Magneto-resistive stationary heads

DCC uses a 9-track magneto-resistive (MR) head for playback. The head is fixed to the mechanism of the recorder, unlike rotary heads that are used in helical scan systems such as DAT or VHS to increase head-to-tape speed. Because of the reduced number of moving parts, DCC decks are less sensitive to shock and vibration. And because of the dimensions that are so similar to analog compact cassettes, existing auto-reverse audio cassette recorder mechanisms can easily be adapted for use in DCC recorders simply by mounting a DCC head instead of only an analog head. In fact, Philips did this during development.[5]

Magneto-resistive heads do not use iron so they do not build up residual magnetism. They never need to be demagnetized, and if a cassette demagnetizer is used on MR heads, they are easily damaged or destroyed.

Various head assemblies were used, according to the service manuals:

  • Stationary DCC recorders (i.e. recorders meant for use in home stereo systems) such as the DCC-900 use a head assembly that has 9 (MR) playback heads and 9 (coil) recording heads for DCC, and two (MR) heads for playing analog compact cassettes. This type of head assembly was designed to be rotated by the mechanism when the recorder/player switched from side A to side B.
  • Playback-only portable players such as the DCC-130 and DCC-134 used head assemblies with 18 MR heads, nine for each side of the cassette. When playing analog cassettes, two of the MR heads are used. The head assembly is fixed to the mechanism and does not need to rotate for side B.
  • Portable recorders such as the DCC-170 and DCC-175 use head assemblies with 18 MR heads for DCC playback, 18 coil heads for DCC recording, and 4 MR heads for analog playback (a total of 40 heads in one head assembly). This head assembly is fixed to the mechanism and does not need to rotate for side B.

Tape specifications and PASC audio compression

The tape speed of DCC is the same as for analog compact cassettes: 1 7⁄8 inches (4.8 cm) per second, DCCs use tape that is the same width as that from analog compact cassettes: 1/8 of an inch (3.175 mm). The tape that is used in production cassettes is chromium dioxide- or cobalt-doped ferric-oxide, 3-4 µm thick in a total tape thickness of 12 µm,[6] identical to the tape that was widely in use for video tapes.

Nine heads are used to read/write half the width of the tape; the other half of the width are used for the B-side. Eight of these tracks contain audio data, the ninth track is used for auxiliary information such as song titles and track markers, as well as markers to make the player switch from side A to side B (with or without winding towards the end of the tape first) and end-of-tape markers.

The (theoretical) maximum capacity of a DCC tape is 120 minutes, compared to 3 hours for DAT; however, no 120-minute tapes were ever produced. Also, because of the time needed for the mechanism to switch direction, there is always a short interruption in the audio between the two sides of the tape. DCC recorders can record from digital sources that use the S/PDIF standard, at sample rates of 32 kHz, 44.1 kHz or 48 kHz, or they can record from analog sources at 44.1 kHz.

Because of the low tape speed, the achievable bit rate of DCC is limited. To compensate, DCC uses an audio compression codec based on MPEG-1 Audio Layer I (MP1) and termed PASC (precision adaptive sub-band coding). MPEG and PASC use digital filters to convert the audio into 32 frequency subbands, and then use adaptive allocation and scaling to decide how many bits should be assigned to represent each frequency band. When decoding, the subband bit stream is used to synthesize an uncompressed bit stream again. PASC lowers the typical bitrate of a CD recording of approximately 1.4 megabits per second to 384 kilobits per second, a compression ratio of around 4:1. The difference in quality between PASC and the 5:1 compression used by early versions of ATRAC in the original MiniDisc is largely a subjective matter.

After adding system information (such as emphasis settings, SCMS information, time code) and adding Reed-Solomon error correction bits to the 384 kbit/s data stream, followed by 8b/10b encoding,[7] the resulting bit rate is 768 kbit/s, which is recorded onto the eight data tracks at 96 kbit/s per track in a checkered pattern.[8] According to the Philips webpage,[6] it is possible for a DCC recorder to recover all missing data from a tape even if one of the 8 audio tracks is completely unreadable, or if all tracks are unreadable for 1.45 mm (about 0.03 seconds).

Auxiliary track

On prerecorded tapes, the information about album artist, album title, and track titles and lengths is recorded in the auxiliary ninth track continuously for the length of the entire tape. This makes it possible for players to recognize immediately what the tape position is and how to get to any of the other tracks (including which side of the tape to turn to), as soon as a tape is inserted and playback is started, regardless of whether the tape was rewound before inserting or not.

On user tapes, a track marker is recorded at the beginning of every track, so that it is possible to skip and repeat tracks automatically. The markers are automatically recorded when a silence is detected during an analog recording, or when a track marker is received in the S/PDIF signal of a digital input source (this track marker is automatically generated by CD players). It is possible to remove these markers (to "merge tracks"), or add extra markers (to "split tracks") without rerecording the audio. Furthermore, it is possible to add markers afterwards that will signal the end of the tape or the end of the tape side, so that during playback, the player will stop the mechanism or fast-forward to the end of the A-side or will switch from A-side to B-side immediately.

On later generations of recorders, it is possible to make a third tape type, referred to by service documentation as "super-user tapes". The DCC-730 and DCC-951 make it possible to enter title information for each track, which is recorded on the auxiliary track after the start-of-track marker. Because the title information is only stored in one place, so unlike prerecorded tapes where users can see the names of all tracks on a tape, it is not possible to see tracks names of any other track than the one that is currently playing.

The three tape types (prerecorded, standard-user, and super-user) are compatible with all recorders and it is impossible (and unnecessary) to recognize the difference between a standard-user tape and a super-user tape without playing it. There are some interesting minor compatibility problems with text on super-user tapes; for example:

  • On stationary recorders that have simple fourteen-segment displays, all track information is converted to upper case. They are capable of displaying symbols that are impossible to enter with their own track information editors (such as the apostrophe), but they are unable to show lower-case characters.
  • The Philips DCC-822/DCC-824 car stereo with DCC player has a full dot-matrix text display which can display upper-case and lower-case titles from prerecorded tapes as well as super-user tapes.
  • Later-generation portable recorders DCC-170 and DCC-175 are capable of displaying text information from prerecorded tapes, but not from super-user tapes. The DCC-175 is capable of writing and reading the text information to/from a super-user tape via the PC, but does not show the text information on the display.

Copy protection

All DCC recorders use the SCMS copy-protection system, which uses two bits in the S/PDIF digital audio stream and on tape to differentiate between protected vs. unprotected audio, and between original vs. copy:

  • Recording digitally from a source marked "protected" and "original" (produced by an audio CD or a prerecorded DCC, for example) was allowed, but the recorder will change the "original" bit to the "copy" state on the new tape to prevent copying of the copy.
  • Recording digitally from a source marked "unprotected" is also allowed; the "original/copy" marker is ignored.
  • Recording digitally from a source marked "protected" and "copy" is not allowed: the record button will not work and any ongoing recordings will stop, and an error message is shown on the display.

Analog recording is not restricted: tapes recorded from analog source are marked "unprotected". The only limitation to analog recording on DCC as compared to that on DAT recorders is that the A/D converter is fixed to a sample frequency of 44.1 kHz. On the DCC-175 portable recorder it is possible to circumvent the SCMS protection by copying audio to the hard disk and then back to another tape, using the DCC Studio program.

Cassettes and cases

DCC with the shutter manually opened

DCCs are similar to compact cassettes, except that there are no "bulges" where the tape-access holes are located. The top side of a DCC is flat and there are no access holes for the hubs on there (they are not required because auto-reverse is a standard feature on all DCC decks), so this side can be used for a bigger label than can be used on an analog compact cassette. A spring-loaded metal shutter similar to the shutters on 3.5 inch floppy disks and MiniDiscs covers the tape access holes and locks the hubs while the cassette is not in use. Cassettes provide several extra holes and indentations so that DCC recorders can tell a DCC apart from an analog compact cassette, and so they can tell what the length of a DCC tape is. Also, there is a sliding marker on the DCC to enable and disable recording. Unlike the break-away notches on analog compact cassettes and VHS tapes, this marker makes it easier to make a tape recordable again, and unlike on analog compact cassettes, the marker protects the entire tape rather than just one side.

The cases that DCCs come in generally do not have the characteristic folding mechanism of those for analog compact cassettes. Instead, DCC cases tend to be simply plastic boxes that are open on one of the short sides. The front side has a hole that is almost the size of the cassette, so that any label on the cassette is exposed even when the cassette is in its case. This allows the user to slide the cassette into and out of the case with one hand, and it reduced production costs, especially for prerecorded cassettes, because a label is needed only for the cassette rather than for the case. Format partner Matsushita does, however, produce blank cassettes (under their Panasonic brand) with a clam-shell-style case. Because DCCs have no "bulges" near the tape access holes, there is more space in the case behind the cassette to insert, for example, a booklet for a prerecorded tape, or a folded up card on which users could write the contents of the tape. In spite of the differences, the outside measurements of the standard DCC cases are exactly identical to the cases of analog compact cassettes, so they can be used in existing storage systems. The Matsushita-designed clam-shell case is slightly thinner than an analog compact cassette case is.

Data recording

There is only one DCC recorder that has the capability of being connected to and controlled by a computer: the DCC-175. It is a portable recorder that was developed by Marantz in Japan (unlike most of the other Philips recorders which were developed in the Netherlands and Belgium), and looks very similar to the other portables available from Philips and Marantz at the time: the DCC-134 and the DCC-170. The DCC-175 was sold only in the Netherlands, and was available separately or in a package with the "PC-link" data cable which can be used to connect the recorder to a parallel port of an IBM-compatible PC. Only small quantities of both recorder and cable were made, leaving many people searching for one or both at the time of the demise of DCC.

The DCC-175 Service Manual[9] shows that in the recorder, the cable was connected to the I²S bus that carries the PASC bitstream, and it is also connected to a dedicated serial port of the microcontroller, to allow the PC to control the mechanism and to read and write auxiliary information such as track markers and track titles. The parallel port connector of the cable contains a custom chip created especially for this purpose by Philips Key Modules, as well as a standard RAM chip. Philips made no detailed technical information available to the public about the custom chip and therefore it is impossible for people who own a DCC-175 but no PC-link cable to make their own version of the PC-link cable.

The PC-link cable package included software consisting of:

  • DCC Backup for Windows, a backup program
  • DCC Studio, a sound recorder and editor for Windows
  • A DCC tape database program that works together with DCC Studio

Philips also provided a DOS backup application via their BBS, and later on they provided an upgrade to the DCC Studio software to fix some bugs and provide better compatibility with Windows 95 which had come out just before the release of the DCC-175. The software also works with Windows 98 but not with any later versions of Windows.

The backup programs for DOS as well as Windows does not support long file names which had been introduced by Windows 95 just a few months before the release. Also, because the tape runs at its usual speed and data rate, it takes 90 minutes to record approximately 250 megabytes of uncompressed data. Other backup media common in those days are faster, have more capacity, and support long file names, so the DCC backup programs are relatively uninteresting for users.

The DCC Studio application, however, was a useful application that makes it possible to copy audio from tape to hard disk and vice versa, regardless of the SCMS status of the tape. This makes it possible to circumvent SCMS with DCC Studio. The program also allows users to manipulate the PASC audio files that were recorded to hard disk in various ways: they can change equalization settings, cut/copy and paste track fragments, and place and move audio markers and name those audio markers from the PC keyboard. It is possible to record a mix tape by selecting the desired tracks from a list, and moving the tracks around in a playlist. Then the user can click on the record button to copy the entire playlist back to DCC tape, while simultaneously recording markers (such as reverse and end-of-tape) and track titles. It is not necessary to record the track titles and tape markers separately (as you would do with a stationary recorder), and thanks to the use of a PC keyboard, it is possible to use characters in song titles that are not available when using a stationary machine's remote control.

The DCC Studio program uses the recorder as playback and recording device. Most PCs of that era do not have a sound card and none is needed either. Working with the PASC data directly without the need to compress and decompress, it also saves a lot of hard disk space, and most computers in that time would have had a hard time compressing and decompressing PASC data in real time anyway. However, many users complained that they would have liked to have the possibility of using uncompressed WAV audio files with the DCC Studio program, and Philips responded by mailing a floppy disk to registered users, containing programs to convert a WAV file to PASC and vice versa. Unfortunately this software is extremely slow (it takes several hours to compress a few minutes of PCM music in a WAV file to PASC) but it was soon discovered that the PASC files are simply MPEG-1 Audio Layer I files that use an under-documented padding feature of the MPEG standard to make all frames the same length, so then it became easy to use other MPEG decoding software to convert PASC to PCM and vice versa.

Derivatives

The technology of using stationary MR heads was later developed by OnStream for use as a data storage media for computers. MR heads are now also commonly used in hard disks, although hard disks now use the GMR variant, whereas DCCs use the earlier AMR.[10]

A derivative technology developed originally for DCC is now being used for filtering beer. Silicon wafers with micrometer-scale holes are ideal for separating yeast particles from beer. The beer flows through the silicon wafer leaving the yeast particles behind, which results in a very clear beer. The manufacturing process for the filters was originally developed for the read/write heads of DCC decks.[11]

See also

References

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