Audio Engineering

756 Chapter 26

26.8 Recording Formats—Analogue Machines ............................................................

Early tape recorders recorded a single channel of audio across the whole tape width. Pressure
to decrease expensive tape usage led to the development of the concept of using “ both sides ”
of a tape by recording the signal across half the tape width and subsequently fl ipping over the
tape to record the remaining unrecorded half in the opposite direction. The advent of stereo
increased the total number of audio tracks to four: two in one direction, two in the other.
This format is standard in the familiar analogue cassette. From stereo it is a small conceptual
step (albeit a very large practical one) to 4, 8, 16, or more tracks being recorded across the
width of a single tape. Such a development demanded various technological innovations, the
fi rst was the development of composite multiple head assemblies. Figure 26.6 illustrates the
general principle. Given the dimensions, the construction of high-quality head assemblies
was no mean achievement. The second was the combination of record and replay heads.
Without this development, the signal “ coming off ” tape would be later than the signal
recorded onto the tape, a limitation that would make multitrack recording impossible. In early
machines, the record head was often made to do temporary duty as playback head during
the recording stages of a multitrack session, with its less than perfect response characteristic
being adequate as a cue track. The optimized playback head was reserved for mix down only.

Despite this, the number of tracks that it is practical to record across a given width of tape is
not governed by head construction limitations only, but by considerations of the signal-to-
noise ratio. As shown earlier, the signal recorded onto tape is left as a physical arrangement
of magnetic domains. Without an audio signal, these domains remain unmagnetized and
persist in a random state. These cause noise when the tape is replayed. Similarly, at some
point, when a strong signal is recorded, all the domains are “ used up ” and the tape saturates.
A simple rule applies in audio applications: the more domains onto which the signal is
imprinted, the better, up to the point just below saturation. This may be achieved in various
ways: by running the tape faster and by using a greater tape width for a given number of
tracks. Figure 26.7 illustrates this by depicting the saturation levels of a commercial tape at

Individual

tracks

21

43

65

87

Tape

head

Figure 26.6 : Multiple tape tracks across width of the tape.