TECHNICAL BULLETIN #4

EQ
Terence O’KeIIy
During a lecture in 1819, Hans Christian Oersted
discovered to his surprise that electrical current
running through a wire made the wire act like amagnet. In October, 1831, Michael Faraday
discovered the reverse effect: that moving a magnet in
and out of a coil of wire produced electric current in thecoil; and the faster the magnet moved, the morecurrent was produced. These two discoveries led tothe development of electricity as a medium of energy.
Today we generate electricity by using water or steampressure to turn shafts with huge magnets attached tothem. The shafts and magnets are surrounded by coilsof wire so that, as the shaft turns, the moving magnets
produce electric current in the coils. If an electric motoris plugged into the current, the electricity flows into themotor’s coils and turns the magnets on the motor’s
shaft: the drill drills, the turntable turns.
Faraday’s discovery also applies to tape recording andplayback. A tape head is actually an electromagnet
with coils of wire inside. When music is translated into
electrical current, the current is sent to the head to act
as a magnet putting magnetic print patterns on thetape.
When recorded tape runs past the playback head, themagnetic prints on the tape create a current in thehead that is amplified in the chain of audio
components and is finally fed to the speakers. The
current induced in the head (its output), however, does
not depend directly on the magnetic signal on the tape;the current depends on the rate of change in the flux
level, or, in other words, on how fast the magnetic
prints move past the head, just as in Faraday’s coil.
Recorded tape not moving past the head generates nosignal in the head at all. Since high frequencies havemore prints than low frequencies (cf. FIG. 3,“Inventor’s Notebook” #3), they have a greater rate ofchange (they seem to be “moving faster”) and,
consequently, they have more output. In terms ofvoltage, the output increases at a rate of 6dB per
octave—twice as much output every time thefrequency is doubled. To compensate for thisFIG 3 from Inventor’s Notebook #3, “Bias”

increasing output (FIG. 1A), the playback section of atape recorder has a decreasing slope built into its
circuitry (FIG. 1B) so that the increase plus thedecrease is “equalized” for a flat response (FIG. 1C).
Equalization makes up for the differences in output by
making them equal. However, as frequencies increase
and the distances between prints (wavelengths) growshorter, high frequency losses due to such factors as
spacing losses in addition to slow tape speed, wide
playback gaps, and limitations in the tape cause the6dB/octave output increase to level off and begin todrop. To solve this problem, the effect of thedecreasing slope (B) is removed at a particularfrequency point so that the natural high frequency
output increase makes up for the treble losses.
The first ferric oxide tapes made for cassette recorders
began to lose their 6dB/octave increase at a relatively
low frequency because the cassette tape moved much
more slowly than open-reel recorders. The output
increase dropped 3 dB at a transition point of 1,326.3
Hz, so the “equalization” decrease (B) was removed at
this frequency. In electronic engineering terms, thistransition point was achieved with “a time constant of120 microseconds” (μs, one millionth of a second).

The chief reason for the decrease in output seen in
FIG. 1A is spacing loss, which is a combination oflosses due to surface irregularities and to thethickness of the magnetic coating. Although shortwavelengths occur with increasing frequency and,
therefore, produce more current in the playback head,
short wavelengths begin to lose that output when thehead begins to have difficulty reading them because
the roughness of the tape and because the amount ofmagnetic energy they hold can no longer reach thehead gap. Magnetic strength falls off exponentially as
distance from the magnetic field increases, so evenslight movements of the tape away from the head due
to surface roughness will cause losses in output, andthese losses are most apparent at short wavelengths.
This effect is significant enough that tapemanufacturers have been able to increase highfrequency output of some tape formulations bypolishing the tapes through hard calendering during
the production process without having to resort tochanges in the coercivity of the oxide.

wavelength loss due to self-erasure, but the bias
reduction also reduces the penetration into the coating
and results in lower output/higher distortion from thelong wavelengths because less pigment is properly
magnetized. Increasing bias magnetizes more
pigment, up to a point, but also drives the shortwavelength prints deeper into the coating andincreases self-erasure with a reduction in highfrequency output). Thinner coatings improve highfrequency output, but at the loss of low frequency
output. The standard coating thickness typical of
cassette tape means that the playback head loses its
ability to resolve short wavelengths due to spacing
losses by 3 dB at 1,326.3 Hz, and that is why the
downward equalization in FIG. 1B is removed at thatpoint.

Loss in output due to the thickness of the tape is theother form of spacing loss. Magnetic output from shortwavelengths is more effective closer to the surface ofthe tape because the short wavelength patterns
deeper in the coating changed the direction of theirmagnetic patterns as they left the recording area of therecord head. Although this is also true for longwavelengths, there is a much greater area of “correctlyaligned” flux with the bigger magnetic patterns thatlong wavelengths produce than short wavelengths canproduce (FIG 2A). The changed flux patterns deep inthe coating tend to cancel each other out until they getcloser to the surface. (Decreasing the bias during
recording brings more short wavelength magnetic
energy closer to the surface and reduces short

The invention of chromium dioxide tapes significantly
improved cassette high frequency response with a
totally new kind of tape oxide that was more efficient at
short wavelengths than ferric oxide formulas. The
uniformity of the oxide particles allowed smoother tapesurfaces, and its higher coercivity allowed it to accept
higher levels of high frequency energy. At the time ofits introduction, however, the biggest complaint of thecassette as a music medium was its tape hiss.
Tape engineers decided to sacrifice the higher output
or “SOL” (saturated output level) of chrome in order toreduce tape hiss. They decided to move the transition
point of CrO2 to a higher frequency of 2,273.6 Hz, or70-μs EQ*. The time constant of 70-μs is
advantageous because it allows the 6dB/octave slope(B) to continue longer and, in effect, “boosts” fewer
high frequencies (FIG. 2); and less high frequency
boost means less tape hiss gets boosted at the sametime. The major audible difference between cassettetapes is not in high or normal bias, but in 70- or 120-μsequalization: the 70-μs EQ will always provide about
4.5 dB less tape hiss. Chromium dioxide, chromeequivalent, ferrochrome, and metal particle tapes alluse 70-μs playback EQ.

that all decks follow (FIG. 3). All decks use someamount of record EQ to give the high frequencies aboost before recording because the high frequency
losses cannot be compensated enough simply by
removing the decreasing slope with playback EQ.
Boosting the high frequencies before recording also
avoids increasing the tape hiss that accompanies
playback EQ, but the record EQ must not be so greatthat it boosts the highs to noticeable distortion or
saturation caused by overloading the tape. So, while
playback EQ is standard on all machines, eachmachine has an adjustable record EQ that is used toinsure flat frequency response at the standard
playback equalizations.
There is another cassette playback time constant inaddition to 120 μs and 70 μs that is not as widely
known: 3,180 μs (50-Hz transition point). Instead ofboosting the lowest bass frequencies so much that
there may be problems with hum or low frequency
noise, those frequencies below 50 Hz are rolled off
slightly on playback. To make up for this roll-off, therecord EQ gives those frequencies a boost during
recording equal to the playback roll-off. Somemanufacturers, however, are abandoning the 3,180-μs
time constant standard because they feel that newhead designs for better low frequency response arerestricted by this time constant. A tape recorded on amachine with the 3,180-μs EQ and played back on adeck without it will have too much bass unless a tone
control can reduce it to a degree.
Playback equalization is standardized so that any taperecorded on any one machine will sound the same on
all machines when it is played back because allmachines should have exactly the same playback
EQ. Since the types of heads, circuitry, and tapeformulations do differ, any adjustments are made torecording equalization or ‘pre-emphasis” in particular
recorder using a particular tape. The adjustments are
made to record EQ so that the frequency response on
playback is flat according to the standard playback EQOpen reel tapes also use equalization, and the fasterspeeds available mean less drastic frequencyalteration. Complications arise, however, because of avariety of speeds and because of different standards—
NAB, DIN, IEC, and CCIR. Most American andJapanese open-reel decks conform to NAB standards
somewhat similar to those used in cassette decks. The
chief difference between the NAB and the other,European, standards is that the NAB allows the6dB/octave slope in playback to continue to a higher
transition frequency to reduce more tape hiss, and thebass does not receive as much playback boost inorder to avoid hum problems and low frequency noise.
NAB standards call for a 3,180-μs EQ roll-off for thebass for 3 ¾ , 7 ½ , and 15 inches per second (ips);
the high frequency time constant is 90 μs (1,768 Hz)
for 3 ¾ and 50 μs (3,183 Hz) for both 7½ and l5 ips.
At 30 ips there is no bass attenuation; 17.5 μs (9,095Hz) is the high frequency EQ.
European standards, known as DIN (German), IEC(international), or COlA (French) use different time
constants (FIG. 4). The 3 ¾ ips speed uses the sameas those of the NAB, but other speeds have no bassroll-off at all. High frequency time constants are 70 μs(2,274 Hz) for 7½ ips and 35 μs (4,547 Hz) for
15 ips.

Some professional machines are designed so that achoice of NAB or IEC playback EQ is available. An
open-reel recorder using NAB EQ uses a bass boost
on record equalization to make up for the 3,180-μs
roll-off on playback. NAB time constants also call for
more record pre-emphasis of the high frequencies
than the lEC time constants because the highfrequency turnover or transition point occurs at higher
frequencies to avoid more hiss (compare the 4.5 dB
less noise of a cassette time constant of 70 μs
compared to 120 μs). The drawback is that the extra

energy gets an extra 16-20 dB boost, it can easily
saturate the tape.
The change in the 6dB/octave slope to a less steep
curve is determined by an electronic resistor- capacitor
network built into a tape deck. The turnover frequency
is determined by the formula:where T is the time constant in microseconds, and theTime Constant Value is an integral number such as 70
or 120. The time constant itself is determined by thevalues of the resistance multiplied by the capacitancein the R-C network. For example, a 10,000-ohm
resistor X a 0.007-microfarad capacitor will provide atime constant of:
10,000 ohms x (7 x 10 farads) = 70 x 106 seconds =70 μs
The seconds in microseconds refers to time.
Resistance multiplied by capacitance results in units oftime when all the other values cancel each other out:

pre-emphasis pushes the high frequencies closer tothe point of distortion or saturation because of theboost. A 14,000-Hz tone will be boosted about 6 to 8dB higher than a tone of 333 Hz at 7 ½ ips; 10-12 dB
at 3 ¾ ips; and as much as 16-20 dB at cassette
speeds. What musical content exists at such a highfrequency generally has about 40 dB less energy thanmusical content in the midrange, but live music andhigh quality discs can contain much greater highfrequency energy. When that greater high frequency*When it became clear in the early 1970s that Dolby B noise reduction would become standard on all high fidelity cassetterecorders, BASF engineers regretted the move to 70-μs equalization because Dolby NR reduced tape hiss by 10 decibels. Ifchromium dioxide tapes had stayed at 120-μs equalization and let Dolby NR take care of the hiss, chrome tape would gainback the 4.5 dB of SOL it sacrificed to reduce hiss.
That opportunity presented itself when chromium dioxide tape was adopted for high-speed tape duplication. BASF convinced
A&M Records to release the Police’s “Synchronicity” album on cassette for playback at 120-μs EQ. The unmatched low noiseof BASF chromium dioxide tape with additional Dolby B noise reduction and increased SOL levels gave the tape a dynamic
range that was very close to Type IV metal tapes using Dolby NR.