NOTICE:

Digital Compact Cassette

DCC is a medium on which audio information is digitally encoded and which reproduces CD quality sound. Since it uses the standardised format of analogue cassettes, it is completely compatible with analogue cassette decks.

Text mode allows a DCC deck to display support information (eg track title, artist name) in several different languages, about the recordings on the tape. The styling has been improved: the DCC tape has a slide off cover which makes access easier, plus the front is completely smooth and suitable for album art. The DCC cassette is protected from dirt and wear by a sliding door, so that the cassette will not easily become jammed or tangled.

DCC introduces a real breakthrough: PASC. PASC is precise and efficient. It compresses the data so that it can be accommodated on standard length audio tape. PASC ensures only sounds within the hearing threshold and takes into account the fact that loud sounds mask soft ones.

DCC tape frames contain PASC information in a checkerboard pattern, which stops drop-outs impairing the quality of the sound performance. DCC has all of the error correction possibilities of compact disc.

Azimuth is the position and angle of the tape in relation to the head. In conjunction with the Fixed Azimuth Tape Guidance (FATG) mechanism fitted to the DCC head assembly, the Azimuth Locking Pins (ALPs) ensure not only improved wrap-around tape-to-head contact, but also consistent azimuth alignment.

To play back information in the DCC miniature track dimensions, DCC uses magneto-resistive (MR) technology. MR technology is a major advance in the thin-film head for digital playback. MR's high-read sensitivity allows narrower tracks to be used for digital coding, so that overall tape data density is increased.

DCC takes the best of years of analogue cassette development, and adds the perfect sound of digital technology.

Introduction

In the Eighties, a Philips invention captured the limelight. Compact Disc opened a new era of digital, perfect sound. Digital audio in the CD format offers high dynamic range and very low noise, as well as low distortion, wide channel separation and total absence of wow and flutter: in a word, natural sound. Digital audio also offers extra user convenience with fast track access and programming. The error correction process in the CD player corrects any mistakes from slightly soiled or damaged CDs, so that the recordings retain their original purity. Consumers recognise this and the CD has become overwhelmingly accepted.

Yet in the Nineties another Philips invention has centre-stage: Digital Compact Cassette, (DCC). DCC is the marriage of compact cassette to Digital Audio, forming a union that combines perfect sound and high convenience with even greater versatility.

With the latest advances in digital audio technology, it has become possible to record digital-quality sound on a new type of audio cassette, which runs at normal compact cassette speed. With its revolutionary and extremely efficient PASC coding (see below), DCC achieves up to 18-bit resolution, producing superb digital sound of Compact Disc quality. DCC uses digital technology to produce digital quality on tape. And DCC will playback standard analogue tapes. In this chapter, we will look at several aspects of DCC and we will examine PASC coding. We will also look into the new head assembly which is a key to the DCC design. Finally, we will discuss the mechanical parts of the DCC cassette.

Important aspects of DCC

DCC operating convenience is well up to CD standards, especially with pre-recorded cassettes.

A number of features has been incorporated in DCC tapes and decks.

Track and time codes are on the tape. These codes, combined with autoreverse, make track access effortless and fast. DCC decks can locate a chosen track on either side of the tape.

A brand new feature of pre-recorded DCC is text mode. Text mode allows cassette decks to display support information about the recordings on the tape, such as album title, a complete list of track titles, names of the artists on each track, and lyrics (displayed in sync with the music). Television screens or remote control units can be connected to the cassette deck to display more extensive information. Text can be written on the tape in up to seven languages.

The well-known durability of cassettes is enhanced in DCC by digital error correction, improved mechanical design and built-in tape protection. As for styling, the new DCC design, which is smoother and slimmer, features an integral cover design, which has more visual appeal, and is easier to handle, carry and store.

In addition, DCC decks have a unique and practical advantage: they are compatible with their analogue predecessor. Customers can play their current analogue cassette collection on their DCC deck. DCC is available to the customer as a total system package, including decks from a range of manufacturers, and blank cassettes as well as cassettes pre-recorded on leading music labels. Numerous digital first generation copies on to DCC blank tape can be made from an original, pre-recorded DCC. But, any further copies (ie 2nd, 3rd etc generation) made from the first generation copy will not be digital.

Figure 2 Only one generation of digital to digital copying is permitted

Autoreverse is a standard feature of DCC decks, which allows continuous listening to both sides of an analogue or a DCC tape.

All these factors make DCC the logical, digital successor to Compact Cassette. It meets higher demands for sound quality, durability and style. It is designed for the new generation of music lovers in a new digital age.

DCC Cassette

A number of design features of the DCC cassette improve upon its analogue predecessor.

The cassette is smooth on the top side, which can now be used for artwork or information. This is because the drive hub openings are only needed on one side of the cassette (as autoreverse is standard on DCC).

The tape and tape drive wheels, which are exposed in the analogue version, are concealed in DCC cassettes by a metal sliding panel called a slider. This slider, which is pushed aside automatically when the cassette is loaded, also locks the tape hubs. This means that:

• the tape is protected against soiling and scratches
• the tape does not tangle, unwind or jam
• cassettes can be carried around safely without their cases.

DCC cassettes are provided with cases, which provide additional protection for the cassette and space for extra information such as a booklet. The case is in the form of a slide-out sleeve which allows the smooth side of the tape to be visible (eg to display artwork) and facilitates easy access to the cassette.

The DCC cassette is made of new materials which are specified for use over a wider temperature range than those of the analogue cassette. The length of a blank DCC cassette can be indicated by holes in the rear of the housing. These enable DCC decks to calculate and to display the time on the cassette. Accidentally writing over a recording can be prevented by a record protection switch.

The tape is a standard videochrome tape: chromium dioxide- or cobalt- doped ferric-oxide, 3-4 µm thick in a total tape thickness of 12 µm. As in analogue cassettes, the tape is 3.78 mm wide, and is bi-directional. This format reduces access time, since less tape needs to be wound. It also allows continuous repeat playback.

DCC coding

DCC uses PASC (Precision Adaptive Sub-band Coding), a newly developed system which compresses the audio information so that it will fit on an audiotape and produce CD sound quality.

How does PASC do this? PASC concentrates on maximising the efficiency of the digital coding, by taking into account two factors not previously considered in digital audio:

1. The ear hears only sounds above a certain loudness (dB) level, called the hearing threshold. The threshold of hearing depends on the frequency of the sound (since the ear is more sensitive to mid- range frequencies) and on the individual. Consequently, it is only necessary to record sound above the hearing threshold, provided that the threshold is taken as the reference for both recording and playback.

   

   Figure 4 The ear hears only sounds above a certain level, called hearing threshold.

2. Louder sounds hide (mask) softer sounds. A whisper, perfectly audible in a quiet room, will not be heard in a busy street. In fact, louder sounds dynamically adjust the threshold of hearing. With computer techniques, it is possible to track this threshold adjustment, making it necessary for only the sounds above this dynamic threshold to be recorded. Of course, this applies to both recording and playback.

   

   Figure 5 Louder sounds mask softer sounds.

PASC achieves very efficient sound recording indeed. It needs only one quarter of the bit rate of PCM (of CD). This level of efficiency creates adequate room for precise recording of what the ear actually hears. The sound quality of DCC is in every way comparable with Compact Disc.

More information on how PASC coding works can be found in the Appendix.

Tracks and tape frames

DCC signals are recorded on nine parallel tracks on the cassette tape. Eight "Main Data" tracks contain all the PASC data, error correction data and system information. The ninth, "Auxiliary Data" track holds mainly track and time information, similar to compact disc, with extra tape markers for easier operation. Start markers, for example, make track access easy, while reverse markers are used to initiate auto reverse. The auxiliary data can be scanned during high-speed search, making operation faster and more straightforward.

All the DCC data on tape is grouped into self-contained tape frames, separated by InterFrame Gaps (IFGs). To accommodate small deviations in the sampling frequency during recording, IFGs can vary slightly in length. They also help to locate the start points of the tape frames.

Each DCC tape frame contains 12,288 bytes of information (not including synchronisation). This is composed of: 8,192 bytes of PASC data, 128 bytes of system information (data for text-mode displays and information such as copyright and tape type), and 3968 bytes of error detection and correction information.

The PASC data is spread across the tape frame in a checkerboard pattern which stops drop- outs (missing signal on the tape due to damage of the magnetic layer), influencing the quality of the audio performance. Even large drop-outs will not impair sound quality (see figure 7.6). This can be compared to interleaving used in CD players, which compensates for any interruptions to the signal caused, for example, by dirt or grease.

Figure 6 The PASC data is spread across the tape frame in a checkerboard pattern.

A Cross Interleaved Reed Solomon Code (CIRC) protects the main data against random and burst errors. The two layers of CIRC data are spread across the eight main data tracks. This powerful error correction code allows for correction of drop-outs even up to 1.45mm in diameter. It can even compensate for a drop-out bigger than a completely missing data track.

In support of the revolutionary PASC, all the techniques which have made compact disc synonymous with audio excellence are applied to DCC. All are closely integrated, and optimised for the tape medium. They are fundamental to the extreme reliability and quality of this new digital audio system.

Azimuth

In audio terminology, azimuth is the position and angle of the recording or playback head in relation to the tape. Azimuth alignment is the position of the head gap in relation to the position and direction of the tape. Azimuth difference is a slight discrepancy between the position of the recording head gap and the position of the playback head gap. Azimuth error refers to problems in playback that arise because of azimuth differences. If the azimuth is not adjusted well, the head will not be in the best position to read the information on the tape and the sound will be negatively affected.

DCC has incorporated an important advance to ensure azimuth alignment and prevent azimuth differences and errors: Azimuth Locking Pins (ALPs). In conjunction with the Fixed Azimuth Tape Guidance (FATG) mechanism fitted to the head assembly, the ALPs ensure not only improved wrap-around tape- to-head contact (see left inset of figure 7), but also consistent azimuth alignment (see right inset of figure 7). The ALPs improve the wrap-around angle of the tape against the head. This extends the tape-head contact area and optimises the physical conditions for signal recording and reading. The exclusion of gaps in the head mechanism means less friction and so less wear on the tape (which will therefore last longer). The tape is also stiffened in this crucial tape guidance area, and this contributes to the high accuracy of the FATG mechanism.

Figure 7 ALPs and FATG

In the FATG mechanism, special slots are mounted either side of the head assembly. The two top edges of the slots are reference surfaces to align the tape with the head. Meanwhile, the sloping profiles of the lower ports of the slots gently force the stiffened tape upwards against both reference surfaces. This simple device eliminates azimuth error.

The ALPs/FATG design requires no complicated mechanisms or close tolerances. Its very simplicity ensures permanently accurate tape-head alignment.

Magneto-resistive

The DCC sound signal is recorded on eight parallel tracks, each 185 µm wide. The track width required for playback, on the other hand, is only 70 µm wide. This width factor helps to reduce the sensitivity to azimuth error. An additional track carries control and display subcode information.

To achieve these miniature dimensions, the DCC record/playback head assembly calls on the advanced thin-film head technology already well proven in multichannel professional recording. In one single head element, three sets of head elements are combined:

• Nine Integrated Recording Heads (IRHs) for digital recording
• Nine Magneto-Resistive Heads (MRHs) for digital playback; and
• Two Magneto-Resistive Heads for analogue playback.

Figure 8 Magneto-resistive technology is a major advance in the thin-film head for digital playback.

Magneto-resistive (MR) technology is a major advance in the thin-film head for digital playback. MR's high-read sensitivity allows narrower tracks to be used for digital coding, so that overall tape data density is increased. In addition, MR player output is independent of tape speed. So the problem of varying output levels with varying tape speeds, inherent in previous magnetic head designs, is eliminated.

The digital heads occupy one half of the head surface, while the analogue heads occupy the other half. So both digital and analogue tapes can be handled by the autoreverse head assembly.

In an integrated recording head (one head assembly with recording and playback functions), the signal current conductor is surrounded by a flux guide which concentrates the magnetic field into the recording gap in conventional fashion. The Magneto Resistive Head (MRH) playback head, on the other hand, features an advanced magneto-resistive element whose resistance varies with the magnetic field impressed on it from the tape, via the flux guide. A constant current is fed through the element, so that the voltage across it varies with the magnetic field on the tape. Magneto-resistive heads are excellent for reading DCC bit transition.

For analogue playback, the high stability and absence of noise of magneto-resistive heads also ensure top quality.

Basic structure

Figure 9 shows the basic structure of a digital compact cassette deck.
Figure 9 Blockdiagram: DCC.

The recording process begins when the L and R channels from the microphone enter the A-D converter (from left to right in figure 9). The signal to be recorded then passes through PASC coding, error correction and channel modulation/demodulation, before reaching the head amplifiers. After the head amplifiers, the signal goes to the head itself, where it is recorded on the tape.

Playback runs the opposite way, from right to left in figure 9. After PASC coding and each of the steps between it and the head, the signal to be played back goes to the DAC and then to the L and R channels for amplification and reproduction. Digital in/out is used for input from and output to other digital sources, such as a CD player.

Most frequently asked questions about DCC

Q.   What tapes will DCC decks play and record?

A.   The decks will record and play back DCC tapes and play back analogue

     cassettes.



Q.   What is the purpose of the metal "slider"?

A.   The slider protects the tape and locks the hubs.



Q.   What is the formulation of the tape?

A.   The tape is either chromium dioxide or cobalt-doped ferric oxide,

     similar to video tape.



Q.   What sampling rates can DCC use?

A.   32 kHz, 44.1 kHz or 48 kHz.



Q.   Why do the DCC decks have Dolby B and C noise reduction?

A.   Dolby B and C noise reduction is for playback of analogue cassettes.

     It is not used for either record or playback of DCC cassettes.



Q.   Will all DCC decks feature autoreverse?

A.   Yes, autoreverse is part of the DCC standard that all manufacturers

     of DCC hardware must follow.