The Underestimated Cassette
We may take the humble cassette
system for granted and, because it's analog, dismiss it as rather primitive. But in fact
it's a remarkable and sophisticated audio medium, and one that performs far beyond any
expectation anybody had when it was invented. And even at a time when CD burners are
common and relatively inexpensive, good ol' tape is the medium most people still use to
preserve their music.
Here's how it works.
Any piece of wire with an electric current flowing through
it is surrounded by a magnetic field. Bending such a wire into a coil concentrates the
field inside the coil, and if an iron bar is placed within the field it becomes magnetized
for as long as the current is flowing. This is called an "electromagnet," and
its field varies in step with variations in the electrical current fed to the coil. If the
iron bar is bent into a crescent, so that the ends almost touch, the electromagnetic field
is concentrated in the gap thus formed.
This is what happens in a tape recorder. The electromagnet
is called a recording head, and it translates an audio signal fed to its coil into an
ever-changing magnetic field. If some material capable of being magnetized, such as a
cassette tape, is moved past this changing field, it will pick up magnetic patterns that
correspond to the original audio signal.
Tape recorders work because the process is reversible. If a
recorded tape is passed by the gap in a head, the magnetic patterns recorded there will
induce a changing magnetic field in the head, and the coil will translate this into a
varying electric current that corresponds to the original audio signal.
Early on in the development of tape recording, it became
clear that the recording process was not linear -- the tape responded differently at low
and high levels. But it was also known that there was a certain operating range for any
given tape where the recording was linear, and that if the whole signal -- even the
silences -- could be boosted into that range, flat recording could be accomplished. To do
this, an AC "bias" signal is added to the audio, high enough in frequency that
it is inaudible, and at exactly the right level to push the signal into the linear range.
Also, if a high-frequency signal is applied to the tape at
an extremely high level, it tends to scramble the magnetic patterns recorded there, and
the randomness that results translates into silence when the tape is played back -- the
signal is erased. Tape recorders contain special erase heads that sit in the tape path
before the record and playback heads, so that an old signal is automatically erased just
before a new one is recorded.
One of the most important things a tape recorder has to do
is to move the tape past the heads at exactly the same speed when recording and playing
back, and to do so as smoothly as possible (that is, with minimal wow and flutter). The
tape stored on a feed reel is pulled past the heads by being squeezed between the
"capstan," a finely-machined shaft that revolves at a constant speed, and the
pinch roller, a rubber "puck" that holds the tape firmly in contact with the
capstan. After passing the heads, the tape is wound onto the take-up reel.
A necessity for good high-frequency response, tape-to-head
contact is maintained either by stretching the tape slightly between two capstans placed
before and after the heads, or by pressure pads that hold the tape physically against the
heads.
One problem is that tape recording tends to inject noise or
hiss into the signal. The solution has been various types of electronic noise reduction.
The first of these was Dolby B, and it is virtually standard on today's cassette decks,
and in prerecorded tapes. It progressively compresses the high frequencies -- that is,
reduces their dynamic range -- in the recording mode, and expands them by an equal amount
on playback. This basic system has been joined in most newer machines by a more powerful
version, Dolby C, and some feature an even more advanced system called Dolby S.
One of the most difficult things to achieve in any tape
format is good high-frequency performance, which is primarily dependent on the number of
magnetic particles that pass the heads in a given period of time. To allow more
information to be packed onto the tape, tape manufacturers turned to the development of
higher-quality magnetic materials in the tape itself.
At first, this meant refinements to the material then being
used -- ferric oxide (in its natural form, that's rust). Finer milling of this oxide
allowed some improvement in response, and it is still used in less expensive tapes (often
called normal or Type I tape). There appeared to be a limit to what could be done with
normal oxides, so eventually an entirely new chemical, chromium dioxide, was introduced,
which had a very much smaller particle size.
While chrome did improve high-frequency response
considerably, it had its own limitations, and soon advanced ferric oxide formulas appeared
on the scene that offered the best of both worlds. These are variously known as
chrome-equivalent, high bias, or Type II tapes (true chromium dioxide is also Type II).
Eventually, an even finer formulation was introduced, using particles of pure magnetic
metal (Type IV), and practically all cassette recorders can accept it. (Type III,
incidentally, was a hybrid, dual-layer formulation that disappeared years ago).
The problem with these formulations is that they all place
different bias requirements on a recorder, and if recorder and tape do not match, sound
quality will be seriously compromised. All cassette decks now offer a range of bias
levels, the minimum being three fixed levels for the three common types of tape. More
advanced decks offer continuous bias trim controls that allow the machine to be matched to
any tape; the most elaborate are controlled by an internal microprocessor.
An important enhancement to the recording process is Dolby
HX Pro, a technique used only in recording. It counteracts the tendency of the high
frequencies in the music to act like erase signals, wiping themselves out to some extent.
This circuit varies AC bias in accordance with the program to maintain a constant level
and to improve high-frequency response.
All of it represents more than a century of technological
development, and when everything is working optimally, it can produce superb results.
...Ian G. Masters
ian@mastersonaudio.com
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