Handbook for Sound Engineers

Magnetic Recording and Playback 1079

recorder performance to a level that rivals digital
recorders.
DBX is also a “compander” system, but instead of
dividing the spectrum into bands and acting on each
band differently, DBX acted on the true RMS value of
the signal. This resulted in SNR improvements that
were significantly better than Dolby A, but many users
felt that the DBX system was more prone to audible
artifacts of the process.

28.4.5 Sync Operation

Multitrack recording requires that artists be able to
listen to the previously recorded tracks while simultane-
ously adding their new performance in synchronism
with the prior tracks. Analog recorders accomplish this
by using some tracks of the record head as playback
sources while simultaneously recording on other tracks
of the same head.

28.5 Digital Magnetic Recording

The information in this section on digital magnetic
recording is presented as an attempt to document a bit
of audio history. It is very likely that by the time the
next edition is published, all recording will be
performed in RAM or other solid state memory, and
magnetic recording both analog and digital will be of
historical interest only. As of this publication there is a
clear trend toward digital audio workstations as being
the standard recording devices, with the audio stored on
hard drives. Tape-based systems are virtually gone.
Currently, computers exist which utilize flash memory,
and as such have no moving parts at all. At the moment
these computers are expensive and have relatively low
capacity compared to conventional hard-disk-based
computers, but they are clearly the wave of the future.

28.5.1 Longitudinal Digital Tape Transports

Longitudinal digital tape recorders came in many vari-
eties, the most successful were the DASH and ProDigi
formats. DASH is an acronym for Digital Audio
Stationary Head. The tape transports for these recorders
were very similar to any high-quality analog mastering
machine, but the very high density of very narrow
tracks required extremely accurate tape guiding and
head placement. For example, the DASH format with
52 tracks across a ½ inch tape specified head height to
0.6 mils (0.015 mm) and a guide placement to 0.2 mils
(0.005 mm).

Tape speeds for multitrack DASH and ProDigi

machines was 30 in/s for normal 48 kHz sampling of

16 bit data, but the DASH high resolution (HR) upgrade

boosted the speed to 45 in/s for recording 24 bit data.

The tape speed was servo controlled by the capstan to

exactly match the sampling rate of the data recorded on

the tape. The sampling rate could be varied from

nominal by up to ±7% for varispeed operation.

Both formats included tape cleaning devices to

remove loose debris from the surface of the tape. Loose

particles were wiped from the oxide surface by passing

the tape across a post covered with lintless fabric tape.

A clock motor mechanism slowly advanced the fabric to

refresh the wiping surface.

The DASH and ProDigi formats used conventional

reels of tape that were mounted on the reeling spindle

and threaded through the machine by hand. Aside from

a few types of reel-to-reel digital instrumentation

recorders, all other modern longitudinal digital tape

formats utilize self-threading tapes that are perma-

nently enclosed in a cartridge or cassette.

28.5.1.1 Signal Flow

A 48 channel DASH digital recorder contains more

electronic circuits in one channel than all the audio elec-

tronics in an entire 24 track analog recorder, Fig. 28-45.

Some DASH recorders offered analog to digital

conversion as extra cost options. Generally, digital data

was fed through the Digital In port. This data was imme-

diately routed to the output for monitoring via the Input

Select selector. The data were also fed to a Crossfader

that smoothly switches between the input source and

tape playback for punch-ins. The data to be recorded was

first spread out by the Interleave circuit to minimize the

impact of a burst error. A powerful Reed-Solomon error

correction encode process was applied by the RSC

Coder. The data was then delayed by a variable amount

Figure 28-45. Block diagram of the audio system of a typi-

cal digital recorder.

Decode Modify Encode Delay

Update

Delay

interval

Tape direction

Read Write