Audio Engineering

Tape Recording 755

fl utter. However, these onomatopoeic terms relate to comparatively coarse effects. What
is often less appreciated is the action of speed stability upon the purity of audio signals—
a fact that is easier to appreciate if speed instability is regarded as a frequency modulation
effect. We know that FM modulation results in an infi nite set of sidebands around the
frequency-modulated carrier. The effect of speed instability in an analogue tape recorder
may be appreciated in these terms by looking at the output of a pure sine tone recorded
and played back analyzed on a spectrum analyzer, as shown in Figure 26.5. Note that the
tone is surrounded by a “ shoulder ” of sidebands around the original tone.

Happily, the widespread adoption of digital recording has rendered much of the
aforementioned obsolete, especially in relation to two-track masters. Where analogue
tape machines are still ubiquitous (e.g., in the case of multitrack recorders), engineering
excellence is a necessary byword, as is the inevitable high cost that this implies. In
addition, alignment and calibration to recognized standards must be performed regularly
(as well as regular cleaning) in order to ensure that multitrack tapes can be recorded and
mixed in different studios.

Table 26.1 Standard tape speeds in metric and imperial measures

Tape speed (ips) cm/s Application

30 76 Top professional quality

15 38 Top professional quality

7.5 19 Professional quality (with noise reduction)

3.75 9.5 Semiprofessional quality (with noise reduction)

1.875 4.75 Domestic quality (with noise reduction)

Good

transport

Poor

transport

Recorded tone

Wow

Flutter

Figure 26.5 : FM sidebands as a result of speed instability.