562 Chapter 17
to any video, audio, or fi lm time code and operate synchronized to a time code reference.
As the heads are wider than the tracks, a useful proportion of the data can be read even
when the tape is being shuttled. Subcode data are repeated many times so that they can be
read at any speed. In this way an RDAT machine can chase any other machine and remain
synchronized to it.
While there is nothing wrong with the performance of RDAT, it ran into serious political
problems because its ability to copy without loss of quality was seen as a threat by
copyright organizations. The launch of RDAT as a consumer product was effectively
blocked until a system called serial copying management system was incorporated. This
allows a single generation of RDAT copying of copyright material. If an attempt is made
to copy a copy, a special fl ag on the copy tape defeats recording on the second machine.
In the meantime, RDAT found favor in the professional audio community where it
offered exceptional sound quality at a fraction of the price of professional equipment.
Between them, the rapid access of hard disc-based recorders and the low cost of RDAT
have effectively rendered %inch analogue recorders and stereo open reel digital recorders
obsolete.
17.8 Digital Compact Cassette ......................................................................................
DCC is a consumer stationary head digital audio recorder using data reduction. Although
the convertors at either end of the machine work with PCM data, these data are not
directly recorded, but are reduced to one-quarter of their normal rate by processing. This
allows a reasonable tape consumption similar to that achieved by a rotary head recorder. In
a sense, the complexity of the rotary head transport has been exchanged for the electronic
complexity of the data reduction and subsequent expansion circuitry.
Figure 17.29 shows that DCC uses stationary heads in a conventional tape transport that
can also play analogue cassettes. Data are distributed over nine parallel tracks, which
occupy half the width of the tape. At the end of the tape the head rotates about an axis
perpendicular to the tape and plays the other nine tracks in reverse. The advantage of the
conventional approach with linear tracks is that tape duplication can be carried out at high
speed. This makes DCC attractive to record companies.
However, reducing the data rate to one-quarter and then distributing it over nine tracks
means that the frequency recorded on each track is only about 1/32 that of a PCM