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 & Matsushita|
|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 was developed in cooperation with Matsushita, and the first DCC recorders were introduced at the Firato consumer electronics show in Amsterdam in 1992. At that time, not only Philips and Panasonic (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.
In November 1995 at the "HCC dagen" computer fair in Utrecht, The Netherlands, 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), however this claim has yet to be verified.[by whom?]
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.
Magneto-Resistive stationary heads
DCC used a Magneto-Resistive (MR) head, which 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. The advantages of a stationary head are clear: DCC players were extremely insensitive to shock and vibration, and compared to a mechanism based on rotary head, the DCC mechanisms were cheaper to produce. In fact, 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.
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 or similar device is used on such heads, the heads can be damaged or destroyed.
In stationary DCC-recorders (i.e. recorders meant for use in stereo systems), the head-assembly was usually a combination of a 9-track DCC-head with a 2-track analog stereo head, mounted on a mechanism that would also have the erase-head on it, and would flip the entire head assembly around by 180 degrees when the B-side was played or recorded. In portable DCC recorders the head assembly consisted of two 9-track DCC heads (and two erase heads if the device was capable of recording), which were in a fixed position, i.e. didn't flip around for the B-side. When playing analog cassettes, portable players would simply amplify the signal from two of the nine heads of the "other side": analog cassette recorders used the "bottom half" of the tape, while digital recordings used the "top half". Presumably[speculation?] the fixed assembly for portable recorders was more difficult to make and gave lesser sound quality (especially dynamic range) when playing analog cassettes (which was acceptable for use with headphones), but took up less space and was less sensitive to mechanical problems which were more important for the portable application.[original research?]
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.
Because of the low tape speed, the achievable bit rate was limited. 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 timing and text information and for markers to indicate the start of a song or the end of a recording.
The (theoretical) 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 switch to side B, there would always be an interruption in the sound at the end of side A, so the maximum theoretical continuous recording time was 60 minutes. DCC recorders could record from digital sources that used the S/PDIF standard, at 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, Philips used an audio compression codec based upon MPEG-1 Audio Layer I (MP1) and termed PASC (Precision Adaptive Sub-band Coding). PASC lowered the typical bitrate of a CD recording of approximately 1.4 megabits per second to the much lower bitrate of 384 kilobits per second, a compression ratio of around 4:1.
The PASC compression scheme was believed to give better quality audio than the 5:1 compression used by ATRAC in the original MiniDisc, but not as good as the uncompressed DAT, although both Philips and Sony stated the difference was imperceptible to listeners' ears (like MP3 or AAC files today). Besides storing the data in compressed format, PASC also provided redundancy by adding CIRC (Cross Interleaved Reed-Solomon Code) bits for error detection and correction, and by scattering the data across the tracks in what Philips called a "checkerboard 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 data 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, called "super user tapes", by entering title information for each track. However, contrary to prerecorded tapes, the title information was stored only once, at the start of the track, right after the track marker, so unlike prerecorded tapes it wasn't possible to see what the name of the track was at any position within the track (the user would need to rewind to the beginning of the track), and there was no way to enter album information. 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 were missing.
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 (which might indicate that Philips never had a clear internal standard for how text recording should work); for example:
- Stationary recorders that had simple fourteen-segment displays, all track information was converted to upper case. They could display symbols that were impossible to enter with their own track info editors (such as the apostrophe).
- The Philips DCC-822 car stereo 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
- Portable recorders were able to display text from prerecorded tapes, but not from super-user tapes. Interestingly, this was impossible even on the DCC-175 which was capable of recording the text information (via the computer) unlike the other portables which didn't have the text recording capability at all.
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 (which was hated because it was fragile). 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 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-130 (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 printer 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 cable contained a couple of custom chips that were made especially for this purpose, which made it impossible for people who owned a DCC-175 but no PC-link cable to make their own. Also, even though the outside of the DCC-170 was similar to the DCC-175, they were radically different on the inside so it wasn't possible to make a 175 out of a 170.
The PC-link cable package included software to use the cable on an IBM-compatible PC, running under Windows 3.1. The software consisted 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. Also, because the tape ran at its usual speed, it took 90 minutes to record approximately 250 Megabytes of uncompressed data. These properties made the backup programs 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, edit track titles on the PC keyboard (so all lower-case and upper-case characters and symbols were available) and write them to tape all at once, edit audio by cutting and pasting fragments, and automatically record mix-tapes. The program was capable of writing the track information to the tape while it was recording the audio, which was a big advantage over the two-step process (record audio first, then edit each track title) which was involved in regular CD-to-DCC recording. Also, because a regular keyboard was used to enter track information, it was possible to enter lower-case characters and symbols that weren't available on stationary recorders.
The DCC-Studio program used the recorder as playback and recording device, it didn't use the PC's sound card (something that not even every PC had in those days). Working with the PASC data without the need to compress and decompress it also saved a lot of hard disk space, and most computers in that time would have a hard time encoding and decoding PASC data in real-time anyway. However, many users complained that they would have liked to have the possibility of using WAV files, and Philips sent registered users a floppy disk in the mail, containing programs to convert a WAV file to PASC and vice versa. Unfortunately this conversion was extremely slow but it was later discovered that the PASC files were simply MPEG-1 Audio Layer I files that used a hardly used, hardly documented padding feature from the MPEG standard to make all frames the same length, so it was easy to convert PASC to PCM and vice versa.
Because of the possibility to create new tapes with DCC-Studio, regardless of the source they were recorded from, the DCC-175/PC-Link cable/DCC-Studio combination effectively circumvented the SCMS copy protection scheme that all digital audio recording devices for consumers are supposed to have. It is possible that this raised some eyebrows in Philips' legal department, and that this is the reason why DCC-175 and PC-link were never sold outside the Netherlands.
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.
- Newspaper article on the demise of DCC
- Philips DCC page preserved as part of the DCC FAQ page
- 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.
|Wikimedia Commons has media related to Digital Compact Cassette.|