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)|
|Write mechanism||multi-track stationary head|
|Developed by||Philips & Panasonic|
|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 cheaper alternative to DAT. DCC shared a similar form factor to analog cassettes, and DCC recorders could play back either type of cassette. This backward compatibility allowed users to adopt digital recording without rendering their existing tape collections obsolete.
DCC signalled the parting of ways of Philips and Sony, who had worked together successfully on the 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), was developed in cooperation with Matsushita, and the first DCC recorders were introduced at the CES in Chicago in May, 1992 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-receiver combinations for automotive use.
At the "HCC dagen" computer fair in Utrecht, The Netherlands, on November 24, 25 and 26, 1995, Philips presented the DCC-175 portable recorder that could be connected to an IBM-compatible PC using the "PC-link" cable. This was the first (and only) DCC recorder that could 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).
DCC was quietly discontinued in October 1996 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 hadn't 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.
Magneto-Resistive stationary heads
DCC used a 9-track Magneto-Resistive (MR) head for playback. The head was fixed to the mechanism of the player/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 players were less sensitive to shock and vibration. And because of the dimensions that were so similar to analog cassettes, existing auto-reverse audio cassette recorder mechanisms could easily be adapted for use in DCC-recorders simply by mounting a DCC head instead of an analog stereo head. In fact, Philips did this during development.
Magneto-resistive heads don't use iron so they don't 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 used a head assembly that had 9 (MR) playback heads and 9 (coil) recording heads for DCC, and two (MR) heads for playing analog 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 were used. The head assembly was fixed to the mechanism and didn't need to rotate for side B.
- Portable recorders such as the DCC-170 and DCC-175 used 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 was fixed to the mechanism and didn't need to rotate for side B.
Tape specifications and PASC audio compression
The tape speed of DCC was the same as for analog cassettes: 1 7⁄8 inches (4.8 cm) per second, and DCC cassettes used tape that was the same width as analog cassettes: 1/8 of an inch (3.175 mm). The tape that was used in production cassettes was chromium dioxide- or cobalt-doped ferric-oxide, 3-4 µm thick in a total tape thickness of 12 µm, identical to the tape that was widely in use for video tapes.
Nine heads were used to read/write half the width of the tape; the other half of the width was used for the B-side. Eight of these tracks contained audio data, the ninth track was 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 was always a short interruption in the audio between the two sides of the tape. DCC recorders could record from digital sources that used the S/PDIF standard, at sample rates of 32 kHz, 44.1 kHz or 48 kHz, or they could record from analog sources at 44.1 kHz.
Because of the low tape speed, the achievable bit rate was limited. To compensate, DCC used an audio compression codec based upon 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 sub-bands, and then use adaptive allocation and scaling to decide how many bits should be assigned to represent each frequency band. When decoding, the sub-band 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 kbps data stream, followed by 8b/10b encoding, the resulting bit rate is 768 kbps, which is recorded onto the eight data tracks at 96 kbps per track in a checkered pattern. According to the Philips webpage, it was possible for a DCC recorder to recover all missing data off a tape even if one of the 8 audio tracks was completely unreadable, or if all tracks were unreadable for 1.45 mm (about 0.03 seconds).
On pre-recorded tapes, the information about album artist, album title and track titles and lengths was recorded in the auxiliary ninth track continuously for the length of the entire tape. This made it possible for players to recognize immediately what the tape position was and how to get to any of the other tracks (including which side of the tape to turn to), as soon as a tape was inserted and playback was started, regardless of whether the tape was rewound before inserting or not.
On user tapes, a track marker was recorded at the beginning of every track, so that it was possible to skip and repeat tracks automatically. The markers would be automatically recorded when a silence was detected during an analog recording, or when a track marker was received in the S/PDIF signal of a digital input source (this track marker would automatically be generated by CD players). It was possible to remove these markers (to "merge tracks"), or add extra markers (to "split tracks") without re-recording the audio. Furthermore, it was possible to add markers afterwards that would signal the end of the tape or the end of the tape side, so that during playback, the player would stop the mechanism or fast-forward to the end of the A-side or would switch from A-side to B-side immediately.
On later generations of recorders, it was possible to make a third tape type, referred to by service documentation as "super user tapes". The DCC-730 and DCC-951 made it possible to enter title information for each track, which was recorded on the auxiliary track after the start-of-track marker. Because the title information was only stored in one place, so unlike prerecorded tapes where users could see the names of all tracks on a tape, it wasn't possible to see tracks names of any other track than the one that was currently playing. Entering track information was a slow process (although easier with a remote control); only upper-case characters were supported and some commonly used symbols such as the apostrophe couldn't be entered.
The three tape types (prerecorded, user and super-user) are compatible with all recorders and it's impossible (and unnecessary) to recognize the difference between a user tape and a super-user tape without playing it. There were some interesting minor compatibility problems with text on super-user tapes; for example:
- On stationary recorders that had simple fourteen-segment displays, all track information was converted to upper case. They were capable of displaying symbols that were impossible to enter with their own track info editors (such as the apostrophe), but they were unable to show lower case characters.
- The Philips DCC-822 / DCC-824 car stereo with DCC player had a full dot-matrix text display which could display upper case and lower case titles from prerecorded tapes as well as super-user tapes
- Later generation portable recorders DCC-134, DCC-170 and DCC-175 were capable of displaying text information from prerecorded tapes, but not from super-user tapes. The DCC-175 was capable of writing and reading the text information to/from a super-user tape via the PC, but didn't show the text information on the display.
All DCC-recorders used 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 e.g. an audio CD or a prerecorded DCC) was allowed, but the recorder would change the "original" bit to the "copy" state on the tape to prevent further copying of the copy.
- Recording digitally from a source marked "unprotected" was also allowed; the "original/copy" marker was ignored.
- Recording digitally from a source marked "protected" and "copy" was not allowed: the record button would not work and any ongoing recordings would stop, and an error message would be shown on the display.
Analog recording was not restricted: tapes recorded from analog source were marked "unprotected". The only limitation to analog recording compared to DAT recorders was that the A/D converter was fixed to a sample frequency of 44.1 kHz. On the DCC-175 portable recorder it was 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 cassettes were almost identical to analog cassettes, except there were no "bulges" where the tape-access holes were located. The top side of a DCC cassette was flat and there were no access holes for the hubs on the top side (they were not required because auto-reverse was a standard feature on all DCC-cassette players and recorders), so this side could be used for a label. A spring-loaded metal slider similar to the sliders on 3.5 inch floppy disks and MiniDiscs covered the tape access holes and locked the hubs while the cassette wasn't in use. Cassettes provided several extra holes and indentations so that DCC recorders could tell a DCC cassette apart from an analog cassette, and so they could tell what the length of a DCC tape was. Also, there was a slider on the DCC to enable and disable recording. Unlike the break-away notches on analog cassettes and VHS tapes, the slider made it easier to make a tape recordable again, and unlike analog cassettes, the slider would protect the entire tape and not just one side.
The cases that DCC cassettes came in generally didn't have the characteristic "folding" mechanism of the old analog cassette. Instead, DCC cassette cases tended to be simply plastic boxes that were open on one of the short sides. The front side had a hole that was almost the size of the cassette, so that any label on the cassette would be exposed even when the cassette would be in its case. This allowed the user to slide the cassette in and out of the case with one hand, and it reduced production costs, especially for prerecorded cassettes because a label was needed only for the cassette, not for the case. Format partner Matsushita did however produce blank cassettes (under their Panasonic brand) with a clam-shell style case. Because DCC cassettes had no "bulges" near the tape access holes, there was more space in the case behind the cassette to insert e.g. 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 were exactly identical to the cases of analog cassettes, so they could be used in existing storage systems. The Matsushita design clam-shell case was slightly thinner than an analog cassette case
There was only one DCC-recorder that had the capability of being connected to, and controlled by a computer: the DCC-175. It was 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 looked very similar to the other portables available from Philips and Marantz at the time: the DCC-134 (player) 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 could 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 shows that in the recorder, the cable was connected to the I²S bus that carried the PASC bitstream, and it was 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 contained 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 was impossible for people who owned 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 worked 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 didn't support long file names which had been introduced by Windows 95 just a few months before the release. Also, because the tape ran at its usual speed and data rate, it took 90 minutes to record approximately 250 Megabytes of uncompressed data. Other backup media common in those days were faster, had more capacity and supported long file names, so the DCC backup programs were relatively uninteresting for users.
The DCC-Studio application however was a useful application that made it possible to copy audio from tape to hard disk and vice versa, regardless of the SCMS status of the tape. This made it possible to circumvent SCMS with DCC-Studio. The program also allowed users to manipulate the PASC audio files that were recorded to hard disk in various ways: they could change equalization settings, cut/copy and paste track fragments, place and move audio markers and name those audio markers from the PC keyboard. It was 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 could 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 wasn't 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 was possible to use characters in song titles that were not available when using a stationary machine's remote control.
The DCC-Studio program used the recorder as playback and recording device. Most PC's in those days didn't have a sound card and none was needed either. Working with the PASC data directly without the need to compress and decompress, it also saved 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 was extremely slow (it took 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 were simply MPEG-1 Audio Layer I files that used 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.
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 use the GMR variant, whereas DCC used the earlier AMR.
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 players.
- "As new digital audio tape formats shape up, the analog cassette keeps its lead", Al Fasoldt, The Syracuse Newspapers, 1991. Retrieved 2017-01-16.
- Consumer Electronics Show Carries A World`s Fair Air, Chicago Tribune, 1992-05-22. Retrieved 2017-01-16
- Newspaper article on the demise of DCC
- "Sony Revives MiniDisc in Package Deal", LA Times 1996-08-27. Retrieved 2017-1-16.
- Philips Early DCC Deck Prototype, Dutch Audio Classics.
- Philips DCC page preserved as part of the DCC FAQ page
- U.S. Patent 4,620,311Method of transmitting information, encoding device for use in the method, and decoding device for use in the method, June 1986.
- Digital Compact Cassette (DCC) - Matsushita and Philips Develop New Standard. Retrieved 2017-01-16.
- Magnetic Multilayers and Giant Magnetoresistance - Uwe Hartmann, R. Coehoorn et al. Retrieved 2007-10-9.
- Hi-fi failure helps to brighten beer - New Scientist Retrieved 2007-4-2.
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