TECHNICAL BULLETIN #3

BIAS
Terence O’KeIIy
The use of bias in tape recording has become much
more widely known as the popularity of cassettes hasgrown. Bias switches first appeared on cassette decks
to provide a bias setting for chromium dioxide tapes
when they first appeared in the late 60’s.
Developments in tape formulation have forced
hardware manufacturers to include settings for normal,
chrome, ferrochrome, and metal tapes and even, inthe most advanced decks, variable bias controls foreach type. While bias is widely recognized asimportant, what it is and what it does are not generally
understood.
The benefits provided by the use of a bias signal werediscovered by accident in 1940. H .-J. v. Braunmehl
and Walter Webe two engineers at AEG, the company
that built the first tape recorders in conjunction withBASF, discovered that tapes recorded on a particularmachine sounded louder and clearer than ever before.
Their examination revealed that the record amplifier inthis machine had accidentally gone into oscillation,and this oscillation was aiding the recording process.
So they purposely built a high frequency oscillator into
the deck to provide a high frequency signal that, whenmixed with an audio signal during recording,eliminated a great deal of distortion. This highfrequency signal was called the ‘bias” signal.
In order to understand how bias helps recording, onemust understand the basic recording process. Whensound waves strike a microphone, the mic converts
the sound waves into electronic signals. Theseelectronic signals are routed through an amplifier,which electronically boosts the signals, to speakers,
whose elements vibrate and push air into the same
sound waves that struck the microphone. Usually,
however, the electronic signals are stored in anintermediate step between mic and amplifier: either asphysical grooves in plastic (disc records) or as
magnetic prints on tape. The tape is a flexible plastic
strip coated with a binder (“glue”) that contains trillionsof microscopic permanent magnets (about 20 billion
per inch; FIG.1). These magnets are needle-shaped

particles of iron oxide (rust) or chromium dioxide or
tiny strands of pure metal. These needles or strands
are suspended in the binder and aligned so that theiraxes are parallel to the edges of the tape. The positive
and negative poles of these tiny magnets have nospecial arrangement, however, in blank tape. The
positiveness and negativeness of the poles iscompletely random. This randomness means that
there is no pattern — and no recorded signal. The
poles are arranged into patterns by the signals fromthe record head that coerce or force the particles tochange their magnetic polarity according to themagnetic pattern to be printed. The particles are glued
into position and do not physically move; they onlychange their positive and negative poles.

The tape recorder is a magnetic printer and reader.
The electronic signals sent to it for storage on tape arerouted to the record head, which is an electromagnet
divided in halves separated by a gap. The signals try
to pass from one half of the head to the other; when
the tape bridges the gap (FIG.2), the signals cause thehead to”print” a magnetic pattern on the tape bymagnetically arranging the poles of the particles in afashion analogous (“analogue recording”*) to thefrequency of the electronic audio signal: low or bass
frequencies are printed as an arrangement of positivepoles together and, a little later on the tape, a group ofnegative poles together.

As frequencies of the electronic signal go higher into
the treble region, the distance between the group of
positive poles and negative poles becomes closer.
Loudness depends on how deeply into the magnetic
coating the arrangement penetrates and how manymagnets it affects: the deeper it goes and the moremagnets arranged, the louder the signal (FIG.3).

Playback of recorded signals works in reverse. The
magnetic prints from the tape moving past theplayback head create an electronic signal in the head,
and this signal is amplified and sent to speakers toreturn as audible sound waves.

The entire record/playback system works amazingly
well in that recorded sounds do sound very much likethe original sound waves that struck the microphone.
There are imperfections in the system, however. The
signals sent to the tape head can be represented
pictorially as combinations of sine waves. When thecycle passes from a strong positive excursion to astrong negative excursion (or negative to positive)(FIG.4A), it crosses a zero point. When the head ismagnetically aligning the poles into patterns, this zero
point produces distortion in the head and on the tapebecause both the head and the tape particles must be
driven hard enough magnetically to avoid this
distortion. (FIG.4B) The process is somewhat liketrying to ride a bicycle at 1/4 m.p.h. — the bike will
wobble or “distort.” Unless the bike, the head, and thetape are driven hard enough to work properly, they will
distort.

The addition of a bias signal significantly reduces
distortion. Bias is merely a high frequency signal(generally five times higher in frequency than thehighest signal to be recorded and much higher thananyone could hear; if the deck can record out to 1
5kHz, the bias frequency should be at least 75kHz)
mixed with the audio signal (FIG.5) so that, when theaudio signal reaches the zero point, the high frequency
bias signal is still driving the head and the tape. The
bias signal is a pure sine wave; but since it has many
more cycles per second, there is always a strong
element of bias present to magnetically influence thetape particles when the audio waveform is at a very
weak point or even zero. (FIG.4C) The high frequency
bias signal is used only during recording to get theprints on the tape properly. Once they are on the tape,bias is not needed. Cassette decks for playback only,
such as those used in car stereos, do not have a bias
oscillator.
Different tapes need different amounts of bias signal
depending on the coercivity of the tape. Coercivity is ameasure of the magnetic force needed to coerce orforce a particle to switch its magnetic poles for theThe application of bias to a particular tape can be
critical because bias affects not only distortion butoutput as well. As the bias is increased, the output at afrequency of 315 Hz gradually increases and thedistortion decreases. (FIG.6).

prints. The more coercive a tape, the harder it is tomagnetize and the more bias it needs to get theparticles to react without distortion. (Increasing bias
does not raise its frequency — only its amplitude orstrength or ‘loudness” if it could be heard; thefrequency stays the same.)
The more difficult it is to magnetize a tape, the moredifficult it is to demagnetize it. This difficulty becomesan advantage for high frequency response. Figure 1
shows the different print patterns for different audio
frequencies. As the audio signals become higher infrequency, the prints of positive and negative patterns
become narrower and closer together and more indanger of erasing each other as the positive prints
cancel negative prints. The two ways to preserve theprints would be to separate them farther by speedingup the tape (the open-reel solution) or making the tapemore coercive and more difficult for the prints to erase
themselves. The chromium dioxide and metal tapes
are harder to magnetize and, therefore, have betterhigh frequency response than normal tapes. It is also
for this reason that they need more bias than normal
tapes in order to be properly magnetized and tominimize distortion.

The output at a high frequency of 10kHz, for example,
will be much more radically affected by changes inbias. Too little bias will increase overall distortion andexaggerate high frequency output because the bias
current is not deeply penetrating the tape coating andis favoring the particles nearest the surface of the tapecoating — those particles that are responsible for thehigh frequencies (see FIG.2). Increasing bias beyond
the “optimum” point will both reduce overall distortion
to a degree and reduce high frequency output because
the increased bias moves the high frequency prints too
deeply into the magnetic coating to be retrieved on
playback. Too much bias will begin to increase overall
distortion.
FIG 6A below shows how bias level has such an effecton high frequency response. The reduction in highfrequency output with excessive bias levels is due to a
combination of particle self-erasure and spacing loss.

Spacing loss is a reduction of output due to thethickness of the tape. Magnetic output from shortwavelengths is more effective closer to the surface ofthe tape because the short wavelength patterns
deeper in the coating change the direction of theirmagnetic patterns as they leave the recording area ofthe record head. Although this is also true for longwavelengths, there is a much greater area of “correctlyaligned” flux with the wider and deeper magnetic
patterns that long wavelengths produce than thenarrow bands that short wavelengths produce. Thechanged flux patterns deep in the coating tend tocancel each other out until they get closer to thesurface. Decreasing the bias during recording brings
more short wavelength magnetic energy closer to thesurface and reduces short wavelength loss due to self-erasure, but the bias reduction also reduces thepenetration into the coating and results in lower
output/higher distortion from the long wavelengths
because less pigment is properly magnetized.
Increasing bias magnetizes more pigment, up to apoint, but also drives the short wavelength prints
deeper into the coating and increases self-erasure witha reduction in high frequency output.
The “optimum” bias point is a compromise between
low and high frequency output and minimum distortion.
Since high frequency output can be controlledsomewhat by the recording equalization, setting bias
for a reduction of distortion is generally favored oversetting bias for flat frequency response. Thosecassette decks with adjustable bias and test tones of400Hz and 8 or 10 or 15 kHz, however, recommend
adjusting bias for equal output at the low and the highfrequencies. This procedure sets the bias level for flat
frequency response rather than for lowering distortion.
When an erase head removes recorded signals, it isbeing fed the bias signal alone. The erase head istrying to print positive and negative patterns of an
extremely high frequency signal, and the positive
negative changes are too rapid for the particles to
accept in a definite pattern. The erase head, in effect,“randomizes” the particles and removes almost all
magnetic print patterns and the signal. There is aminute amount of bias signal left on misaligned ormagnetically “maverick” particles. This signal, known
as “bias noise,” is audible as an extra amount of hissadded to virgin blank tape when it is recorded orerased with an erase head. A bulk demagnetizer willremove bias noise and reduce any hiss to that of virginblank tape.
A significant advantage of chromium dioxide as amagnetic particle is that its synthetic nature can assurea more uniform size and more precise particle
orientation and alignment in the binder. The bias
noise, therefore, is usually at least 2.5dB less thanferric oxide tapes or ferric-cobalt “chrome equivalents”
at the 70-microsecond equalization setting.
Bias noise of different tapes can be compared byputting a tape deck in the record mode and recording
no signal on the tape. When the tapes are monitored,
only the hiss will be heard. The tape hiss of chromium
dioxide or ferrochrome tapes will be audibly less thanthat of any other kind of tape.

* “Analogue” comes from two Greek words loosely meaning “word for word,” as in a translation. The adjective is a way ofdescribing information in one understandable way analogous to or similar to the actual way. The description is often applied
to the use of a “picture for picture” instead of a “word for word” translation. For example, an analogue clock has hands thatmake a complete circuit in a minute or in an hour or in half a day, depending on which hand it is. The hands continually goaround just as the earth turns completely around on its axis in a day. Analogue recordings “draw” an impression of soundwaves in the squiggly groove in vinyl records or as variations of magnetic energy in cassette tape. The vinyl records and tapestore these “pictures” of the sound patterns and allow them to be played back. The problem with this system is that theinformation gets mixed up with the flaws of the medium. A clock hand that does not keep up with the other hands gives
inaccurate information. Dust in a record groove causes sounds and noises not meant to be heard. Tape imperfections cause
hiss that was not part of the original sound.
Digital recording is a method that avoids these flaws. Digital recording does not try to draw or imitate the information that isbeing saved. Instead, it converts the information into a mathematical code that ignores the flaws of whatever medium isstoring the data. To use an analogy, a canvas painting of a landscape records the landscape with all the “flaws” of canvass
and paint texture (those “flaws” that make a painting an inaccurate but artistic impression). If oil is spilled on the painting, it isdifficult to restore what was there because the oil becomes part of the record. If, however, someone recorded the landscapewith a “paint-by-number” scheme in great detail, the oil would not matter. The oil stain had no numbers assigned to it; so theartist could reproduce the landscape by following the number code exactly. The more numbers involved, the more accurate
and detailed the reproduction would be—and every copy would be almost identical to the original.
The word” digital” refers to digits or numbers. It comes from the Latin word digitus, or “finger,” because everyone learns tocount on his or her fingers. We have ten fingers, so our common numbering system is to the base 10 and uses ten digits—0to 9. The mathematical code used in digital recordings is very intricate and needs computer chips to encode and decode; butcomputers don’t have fingers. They have transistors that recognize only two states: on/off (or “0/1,” “change/no change,”“+/-,” etc.). Computer engineers use the binary numbering system for computers, a numbering system to the base 2 thatneeds only two numbers, 0 and 1, to construct any value.