Handbook for Sound Engineers

Magnetic Recording and Playback 1043

the tape past the record/play heads will almost always
be noticeable and, unless you are trying to sound like a
small rodent, undesirable! The mechanism responsible
for moving the tape past the heads in a constant and
repeatable manner is called the transport.

28.2 Tape Transports

The beginnings of modern-day tape transports can be
traced to Vlademar Poulsen, the Danish inventor of the
magnetic wire recorder. Poulsen’s experiments in 1898
consisted of moving an electromagnet along a piece of
steel wire to record and reproduce sound. He soon
learned, just as every tape recorder operator today
learns, that the relative motion between the transducer
(the electromagnet) and the storage medium (the wire)
must be uniform and repeatable.
Many of Poulson’s solutions to this problem, such as
sliding the electromagnet down a long, sloping wire
worked reasonably well but were hardly practical! The
functions of his transport device, however, were the
same as modern tape recorders, specifically:

1. To drive the tape (or wire) at a repeatable, and pref-
   erably constant speed over the surface of the trans-
   ducer heads.


2. To maintain a fixed mechanical alignment of the
   tape as it crosses the heads.


3. To provide contact pressure between the tape and
   head by either tensioning the tape or pushing the
   tape against the head.


4. To provide the necessary auxiliary motions of the
   tape required for functions such as rewind, search,
   and editing.

The early German Magnetophon developed by I.G.
Faben in the 1930s satisfied all of these requirements
with a simple mechanical layout. Over 70 years later,
today’s recorders have essentially the same layout,
shown in Fig. 28-3. The reels of tape are mounted on the
shafts of two motors that provide the high-speed spooling
and the play-mode tape tensioning. The tape moves from
the supply reel on the left to the takeup reel on the right.
As the tape leaves the supply reel, it is steered by guides
to pass over the erase, record, and playback heads.
Following the heads is a constant-speed tape drive
consisting of a rotating shaft called a capstan and a pinch
roller to press the tape against the surface of the capstan.
The tape then passes to the takeup reel, Fig. 28-3.
This layout was used on virtually every tape recorder
ever built except for the infamous Ampex 400 built
sometime in the early 1950s that placed the
capstan/pinch roller assembly to the left of the heads.

The typical degree of precision that is available

today in a professional recorder includes a tape speed

variation of a few hundredths of a percent, mechanical

alignments of less than one-thousandth of an inch

(0.001 inch) and three-thousandths of a degree (0.003°),

and tension variations of a few percent. Even these

seemingly small variations create readily observable

errors in recordings, leaving opportunity for future

improvements.

28.2.1 Tape Metering

Ever since the early introduction of tape recorders to

radio broadcasting, it was desired to have world stan-

dards that would permit tapes to be freely exchanged

between facilities around the world. Furthermore, it was

necessary to be able to freely exchange segments within

a reel by editing. This requires absolute speed accuracy

throughout the reel.

Broadcasters were concerned about the running time

of a radio show. If the show was timed at exactly 30 min

when it was recorded, it should also play in exactly 30

min on the air. A common timing accuracy specification

of 0.2% means that the tape could play up to 0.2% of

30 min fast or slow or 3.6 s of error in either direction.

This could result in either 3.6 s of overlap with the

subsequent program, or 3.6 s of dead air silence while

waiting for the next show to start.

A more demanding speed specification is the abso-

lute speed error throughout the reel. If the tape machine

runs 1% fast at the beginning of the reel, and 1% slow at

the end of the reel, the overall timing might come out

just fine. But when you cut a segment of music from the

head of the reel into a song at the end of the reel, you

now have a 2% speed jump at the splice, with a very

noticeable pitch change.

A simple speed control technique is to clamp the

tape to a surface that is moving at the desired tape

speed, such as the outer periphery of a rotating drum.

The tape is thus forced to move at exactly the same

Figure 28-3. Classic tape transport layout.

Supply reel Takeup reel

Erase Record Playback

Capstan

Pinch roller