Acetate Tape refers to the material used as the base for tapes manufactured
from the early 50’s through the 1970’s. During the 1970’s, polyester became the
preferred base material for analog tape. It’s worth noting that each material has
exhibited problems over time.
The acetate base becomes brittle over time. The tape can deform, causing a
condition known as “cupping.” Too much moisture during storage can cause a
condition known as “vinegar syndrome” in which the acetate base releases acetic
acid and the tape will have a vinegar odor.
In the world of magnetic tape recorders, azimuth refers
to the alignment of the gap in the record and playback
heads relative to the direction of travel of the tape itself.
The head gap represents the centerline of each head
and should be precisely 90 degrees to the tape. Any
variation will result in a loss of high frequency
information. Azimuth is one of the adjustable parameters
of each head and there are specific methods to check
and adjust for the proper alignment.
FIG. 1, to the right, is an illustration of a typical tape
recorder head. The vertical line in the center represents
the head gap.
Many brands of professional polyester audio tape had a
coating on the backside (opposite the oxide) that was
electrically conductive and performed some important functions:
1) The foremost purpose was to help prevent static buildup, which could cause
arcing and subsequent audible snaps in the recording.
2) While helping to eliminate static, the same back coating allowed for more even
packing of the tape on the reel.
3) The back coating also served as an insulating layer that helped prevent
, a condition in which the recorded magnetic field migrates from one layer
of the tape to another.
In a tape recorder, bias is a high frequency Alternating Current (AC) signal that is
applied to the erase and record heads during recording. The frequency of the
bias signal is supposed to be at least 10 times the highest audio frequency
capable of being recorded by a given recorder (but rarely is). A typical bias
frequency for consumer recorders is anywhere between 40 kilohertz (forty
thousand cycles per second) and 100 kilohertz (kHz). Professional equipment
can have a bias frequency approaching 250 kHz. The reason for the bias signal
is to magnetize the ferrous iron particles in the oxide in a manner which provides
the best linearity for recording. The fact is, anything that can be magnetized does
not magnetize in a linear fashion. Early attempts to record without a bias signal
on an oxide containing iron particles resulted in poor frequency response.
Applying a DC (Direct Current) bias proved to be of little benefit. The first patent
for AC bias for use in electronics was filed by W. L. Carlson and Glenn L.
Carpenter in 1921. However, it was Walter Weber who applied the technique of
applying AC Bias to magnetic tape recording.
This is a condition where the chemical binder in the oxide has absorbed moisture
over time to a point that causes a chemical change in the composition of the
oxide itself. This often results in
Sticky Shed syndrome
. Another variation in the
decomposition of the oxide results in a milky-colored stain on the surface of the
oxide that dramatically increases the friction during playback, often causing the
tape to come to a stop.
In the world of magnetic tape, Coercivity refers to the strength of a given
magnetic field (erasure) required to reduce the recorded magnetic field (signal)
on the tape to zero after the oxide has been driven into
. Tapes using
an oxide with greater coercivity, generally require a stronger Bias signal.
On a tape recorder, the Capstan is the rotating vertical shaft that pulls the tape
past the heads. The tape is usually squeezed between the Capstan and a
rotating wheel known as the
A device that emits a strong enough AC magnetic field to completely erase
magnetic media or magnetized metal parts.
Simply stated: “The process of removing magnetism from any material that can
be magnetized.” In the world of tape recording, tapes are demagnetized by being
exposed to an AC field strong enough to overcome the coercivity of the tape.
This process is accomplished by the erase head during recording. On tape
decks, the metal parts that come in constant contact with the tape (particularly,
the tape heads themselves) can become magnetized over time and have to be
routinely demagnetized using a portable degausser.
The process of demagnetizing the heads and various parts of a tape deck has its
own learning curve. If done improperly, one can actually magnetize the intended
parts and cause harm to tapes that are subsequently played on that machine.
A dropout is a brief loss or sudden decrease of signal level (volume) most often
caused by a defect in the oxide. Dropouts can also be caused by damage to the
tape itself, a temporary clogging of the record head during recording, or, likewise,
a clogging of the play head during playback. Tape defects can cause frequency-
selective dropouts such as a brief loss of high frequency information. Tape speed
can be a factor in the effect of tape-defect related dropouts as the condition is
less noticeable at higher tape speeds.
In the world of audio, Dynamic Range is defined simply as the range of volume
from the loudest to the softest of sounds. Dynamic Range is expressed in
(dB) which is a logarithmic scale.
We Audio Engineers often refer to Dynamic Range in terms of the difference
between the loudest undistorted signal that be recorded down to the noise level
(floor) of a given medium. Analog tape is capable of a dynamic range of roughly
70dB. A Compact Disk (Audio CD) has a theoretical dynamic range of 96dB. The
average human can hear a dynamic range of approximately 140dB.
The erase head on an analog tape recorder is somewhat self explanatory…It
erases tape! That’s its job. It does so by being energized by the bias oscillator
with enough voltage and current to saturate the tape with the bias frequency. The
erase head is only energized during recording. Otherwise, it would be erasing
tape all the time! The erase head is also the first to come on contact with the
tape during playback or recording. If you view the heads of a tape deck straight
on, the erase head will be on the left.
Tape recorders are designed to pass the tape across the heads at a very
constant speed. Any changes in the tape speed of a pre-recorded signal are
perceived as changes in pitch. If these speed changes occur very rapidly, the
effect is a “fluttering” sound; hence the term. Flutter can be caused by something
as simple as a piece of tape that becomes inadvertently wrapped around the
or a very worn
A type of tape
Sticky Shed Syndrome
can cause flutter as
the tape itself tends to chatter across the heads.
Tape guides are generally stationary posts that are placed very near the heads to
keep the tape in proper vertical alignment. Worn guides can not only loose their
ability to keep the tape properly aligned but can actually damage tape as it is
passed across them.
Tape that has been wound onto the supply reel is “Heads out.” In order to
indicate the state of the tape, the loose end is usually folded and fastened to the
reel with adhesive paper tape in such a manner that the folded and taped end
will face the operator when the reel is placed on the machine (or a table, for that
matter). This makes it easy to identify which way to mount a given reel of tape
onto the tape deck.
IPS (Inches Per Second)
The linear speed of the audio tape as it is played on the tape deck. Tape speeds
vary from as slow as 15/16 IPS to as much as 30 IPS. Generally speaking, the
faster the tape speed the better the sound quality.
Tape speeds of 3-3/4 & 7.5 IPS were common for consumer tape recorders and
professional machines typically operated at 15 & 30 IPS.
Leader is used to separate specific segments of audio tape such as each song
on music masters. It is a paper or plastic product cut the same width as the tape.
Leader is typically added to the beginning of a reel and sometimes to the end.
Some manufactured tape comes with leader already attached to the tape. Some
plastic leader is printed with markers for every second of time and is called:
This is a process whereby the tape is wound from one reel to the other at
reduced tension and at a speed that is much slower than full rewind speed,
typically around 45 IPS. This results in a very even wind on the take-up reel, and
configuration is desirable for long term storage.
The tape lifters are vertical posts that usually reside near the heads and are
energized during fast-forward or rewind. During this process, the lifters move the
tape away from the heads to prevent wear during the high speed shuttling of the
tape from one reel to the other.
Numerous articles and books have been written on this subject, and it is difficult
to summarize Noise Reduction in a few sentences. When it comes to tape
recording, there are two basic approaches:
systems generally deal with tape hiss and other anomalies after
the fact or after the recording is made. Today’s digital editing software is often
designed to be able to improve upon recorded anomalies after the fact and is a
good example of a single ended approach. A number of hardware devices were
made during the analog era for single-ended noise reduction by companies like
Burwen, DBX, Phase Linear and SAE and can still be found on the vintage
hardware market. These hardware devices work in “real time,” meaning that the
recorded audio has to be played through them for processing.
noise reduction involves processing the audio signal before and
after recording. Ray Dolby was one who first applied this process in 1966 with
the introduction of Dolby “A” type noise reduction intended for use with
professional audio tape recording. Dolby Labratories subsequently introduced
noise reduction processes for the consumer market and became a household
word. Besides Dolby Laboratories, DBX and Telefunken produced very effective
dual-ended systems that were used in both professional and consumer
This is the composition that has been applied to the side of magnetic tape that
contains the magnetic recording. Oxide consists of a slurry containing the metal
particles, a binder solution which helps adhere the oxide to the backing,
lubricants to make the dried solution both flexible and less physically noisy when
passing across the heads, and several other chemical compounds. The actual
chemical composition of oxide remains a closely guarded secret by the various
manufactures of magnetic tape.
The Pinch Roller (sometimes referred to as a ‘puck’) on a tape deck is that round
wheel that presses the tape against the
to move the tape across the
heads. Pinch rollers are free-wheeling and are usually made of rubber or a semi-
soft composite material.
If you were able to view the construction of a play head, it would look somewhat
like a horse shoe with several turns of fine wire wrapped around it. The tape is
pulled across the gap of the horse shoe shape.
The actual gap is tiny (microns across, in fact) in order to concentrate the
magnetism that has been imparted onto the tape by the record head.
for an illustration of a tape recorder head.
This is how the tape winds up on the take-up reel after being played. A “Play
Wind” is considered to be desirable for storage. The reason is that if
occurs, the print-through information will be after the recorded audio
and be masked or sound like echo. If the tape is stored heads-out, the print-
through information can precede the recorded audio on the tape and is known as
This refers to the type of base material used for audio and video tape since the
late 1960’s. Polyester, sometimes referred to as PET, is a substance known
scientifically as Polyethylene Terephthalate. (I can’t pronounce it well either)
Polyester tapes have shown to have adhesion problems as the oxide absorbs
moisture over time, and the unfortunate result is that the oxide separates from
the base, literally falling off in some instances or causing an unfortunate
condition called “
Sticky Shed Syndrome.
When audio tapes are tightly wound on a reel and/or stored for extended periods
of time, the adjacent layers can sometimes influence each other. That is, one
layer can partially magnetize an adjacent layer. This condition can be more
predominant if the recorded levels are extremely loud or “hot” or with thinner
varieties of tape. The use of conductive back coatings on many brands of
professional tape helped alleviate this condition. If the tape is wound
the partial magnetization will sound like pre-echo. This is one of the reasons for
storing audio tapes
. If print through occurs in a tails-out wind condition, it
is either masked or sounds more like natural echo. It has been recommended by
some that stored audio tapes be periodically rewound and stored again using a
play wind or, preferably, a
, a very labor intensive process which is, in
practice, rarely done.
The main difference between a record head and a playback head is the size of
the gap between the poles of the head itself. We’re talking microns here.
However, the gap on a record head is typically wider than that of a playback
for an illustration of a tape recorder head.
This is the state reached with magnetic tape when the oxide has been
magnetized to the point where it cannot be magnetized any further. Exceeding
this threshold with an alternating current magnetic field (via the record head) will
cause the magnetic particles in the oxide to become disoriented, and audible
Speaking strictly in engineering or scientific terms: “signal-to-noise ratio refers to
the strength of a given signal in relation to the background noise associated with
that signal”. Signal-to-Noise Ratio can be applied to everything from astronomy
to analog & digital recording. It is often abbreviated as SNR or S/N.
Sticky Shed Syndrome
This condition is particularly damaging to the tape being played. It’s a condition
whereby the binders and lubricants in the oxide have absorbed enough moisture
over time to cause the oxide to become soft. When played on a tape recorder,
the characteristic symptom is a squealing or squeaking sound which modulates
the audio being played. The tape will often shed oxide on all of the stationary
parts of the tape deck it comes in contact with. The accepted temporary fix is to
carefully bake the tape under controlled conditions.
The Digital Audio Tape formats (DAT, DTRS, etc.) have recently shown signs of
Sticky-Shed so if you own any of these, now is the time to inspect them.
Unfortunately, these formats will typically not show a problem until they are
played and, then it can be too late for your prized digital audio recorder.
(Particularly the rotary head formats)
An effect (sometimes a mistake) whereby a signal is recorded onto an existing
recording. When played back, both signals can be heard but not separated. This
process is made possible by disabling the erase head when applying the second
signal to an existing recording.
Tape that has been wound onto the take-up reel of a tape recorder is “Tails-out.”
In order to indicate the state of the tape, the loose end is usually folded and
fastened to the reel with adhesive paper tape in such a manner that the folded
and taped end will face the operator when the reel is placed on the machine (or a
table for that matter). This makes it easy to identify which way to mount a given
reel of tape onto the tape deck.
The process of baking analog tape involves raising the ambient temperature of
the tape to 120 - 130 degrees Fahrenheit (48.8 – 54.4 degrees Celsius) in a very
low humidity environment for specific amounts of time based on the width of the
tape, the thickness of the tape, and the size of the reel. The process was first
proposed (even patented) by Ampex Corporation in the early 1990's. This
procedure can only be applied to polyester base tape. Acetate base tape can be
damaged by this process.
The sudden change of pitch of a recorded signal caused by physically altering
the tape speed for a very short duration of time. The reasons for the occurrence
can be everything from mechanical failure to the tape itself being stretched. Wow
can also be created purposely by applying enough pressure to the supply reel
during playback to momentarily alter the tape speed.
On an audio tape recorder, Zenith refers to the relationship of the vertical
alignment of the heads and tape guides to the deck plate or mainframe in order
to provide even contact over the entire surface of the tape. Zenith, like
is usually an adjustable parameter of each tape head.
Howard M. Tremane, “Audio Cyclopeia” Second Edition, Howard W. Sams, 1973
Rudolph F. Graf, “Dictionary of Electronics” Howard W. Sams, 1974
Glenn D. White, “The Audio Dictionary” University of Washington Press, 1987
Dr. John W. C. Van Bogart, “Magnetic Tape Storage and Handling” National
Media Lab 1995
© Corey Bailey Audio Engineering
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inside of a Scotch 111