Handbook for Sound Engineers

1042 Chapter 28

tape. If the tape recorder takes 15 seconds to rewind the
tape, the engineer would spend 500 15 second intervals
waiting for the tape recorder to rewind. That is over
2 hours spent in rewind mode!
Contrast this sluggish operation with a digital audio
system’s hard disk that can locate any position on the
disk in less than 10 ms. 500 rewinds might now take
less than 5 s!

28.1.1 The Family of Magnetic Recording Devices

All magnetic tape recorders are members of a larger
family of storage devices that utilize moving storage
media. Other members of this family include phono-
graph disk recorders, motion picture cameras and
projectors, optical laser disks for video and audio, and
magnetic disk devices for computer data storage. These
storage devices share one very important character-
istic—they all are complex electromechanical devices.
In addition to electronic circuits that amplify, process,
and control the basic signal that is to be recorded and
retrieved, each device also contains numerous mechan-
ical devices to move the media past the recording and
reproducing transducers and also position the trans-
ducers for optimum performance.
All magnetic recorders share key features:

1. The recording process is instantaneous, requiring no
   intermediate processing before the signal can be
   replayed.


2. The record and playback processes exhibit reci-
   procity, meaning a single transducer may be used for
   recording or playback.


3. The storage medium can be easily erased and
   reused.


4. The parameters of the system (speed, track width,
   encoding scheme, etc.) can be customized for a
   broad range of audio and video applications.

28.1.2 Tape Recorder as a Transformer

A magnetic tape recorder can be visualized as a special-
ized form of transformer. In a conventional transformer,
an electrical signal on the input or primary winding is
converted to magnetic energy in the magnetic core of
the transformer. This magnetic energy is then converted
to an electrical signal in the output or secondary
winding, proportional to the ratio of the windings.
Transformers can be quite efficient and the losses are
typically just a few percent of the total power passing
through the device. The best audio transformers intro-
duce only very small amounts of distortion to the ampli-

tude and frequency response of signals passing through

the transformer, Fig. 28-2.

For a tape recorder, the input and output windings

consist of the record head and reproduce head. The

magnetic core that couples these windings is a conveyor

belt covered with magnetic particles in the form of the

magnetic tape. A magnetic image is permanently

impressed on the conveyor at the record head. When

this image passes over the reproduce head, anywhere

from milliseconds to years later, the magnetic image

creates a signal in the head that is analogous to the orig-

inal signal.

Unlike the fixed transformer core, the recording tape

is fraught with numerous distortions, losses and imper-

fections that require attention. Virtually every compo-

nent in the record/reproduce chain, including the heads,

tape, signal electronics circuitry, and mechanical drive

system, contributes to these errors.

28.1.3 Changes with Time and Space

Those of us who work in audio and acoustics are used to

thinking of audio as a complex wave made up of

discrete frequencies. These frequencies move through

the medium (air) at a fairly constant speed. Distortions

due to changes in propagation time are not often

observed. In the equation

(28-1)

where,

C is the speed of sound in air and is essentially viewed

as a constant.

In the analog tape recorder, C, the speed of the tape

past the tape head, is a variable. Changes in the speed of

Figure 28-2. Transformer versus tape recording.

Secondary

playback

Secondary

playback

Primary

record

Primary

record

Tape motion

Magnetic

flux

A. Transformer. B. Tape recorder.

O C

f

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