July 1958 Radio-Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published 1929 - 1948. All copyrights hereby acknowledged.
My first major high fidelity stereo system purchase came during
my senior year of high school (1976 - ugh!)
when I had saved enough money to buy a combination AM/FM receiver,
8-track tape deck, turn table, and two speakers with separate woofers,
midranges, and tweeters. At the time I thought the setup might impress
friends and relatives... until I learned quite quickly that 'serious'
stereo sound connoisseurs decidedly did NOT have equipment
with "Reader's Digest" logos on it. Oh well, the price
seemed like a really good bargain to me give the promised tonal
superiority. Compared to the clock radio I used previously for my
music listening sessions, the Reader's Digest stereo system produced
music hall quality sound. Ah, the deep bass notes were grand. Spending
most of my earned money on model airplanes, rockets, and my '69
Camaro left little disposable income for LPs, so the turntable did
not get much use. I did, however, read up on how to balance the
tone arm for just the perfect amount of pressure to assure top notch
music. What I was not aware of was that not all styluses used the
same amount of pressure, so I probably did more harm than good with
my efforts. I did convince myself, of course, that the music definitely
sounded better after the painstaking adjustments. This article brought
that experience back into my mind.
This proves you can find just about anything on eBay.
After writing this article, I did a quick search for that
Reader's Digest stereo system and found the receiver and
8-track tape unit. Speakers and turntable were not available.
How the Stereo Disc Works
The facts on the 45/45 system
By Norman H. Crowhurst
Fig. 1-The basis behind the wrong idea that each channel
is cut into one wall of the groove in the 45/45 system:
a - no modulation; b - modulation in channel 1 appears to
cut only the right wall; c - modulation in channel 2 appears
to cut only the left wall.
Fig. 2-A more careful analysis of how the cutter, groove
and playback stylus move for different intensity and phase
relations between channels at anyone frequency: a - in phase;
b - out of phase; c - intensity difference; d - 90°
phase difference; e - 30° phase difference; f - intensity
and phase (30°) difference.
A recent conversation about the stereo discs ran something like
this: "How on earth can a pickup stylus vibrate two ways at the
same time and give different inputs for two preamplifiers?"
"That's easy," the answer came.
"Every groove has two walls and one recording is cut into each
wall of the groove."
The puzzled expression on the questioner's face showed that this
did not satisfy him - and for good reason. It is impossible for
the stylus to follow different contours on both walls of the groove
at the same time. This may have been a simple explanation but it
was certainly not the correct one.
In literature issued by Westrex the difference between cutting
the two channels appears somewhat as in Fig. 1. It shows a record
cutter that has an angle between the cutting faces of precisely
90°. The two drive mechanisms, arranged at 45° to the vertical
so the total angle between is 90°, drive this cutter in directions
parallel to its faces.
Consequently the diagram may make it seem that the vibrations
due to one drive are recorded in one wall of the groove while vibrations
due to the other drive are recorded in the other wall.
Stylus and cutter motion
In practice, there is invariably some program in both channels
and both vibrations occur at once. So it is more important to consider
what happens to the point of the cutter than to the contour produced
by its two angular cutting edges. This is because the pick-up stylus
will (or should) ride in the bottom of the groove, cut by the point
of the cutter, and it is the movement of this point in the groove
that determines the program content of the two channels. To clarify
this, let us consider what happens with different program combinations.
If both channels carry the same program at the same intensity
and in phase, the movement of the cutter and the pickup stylus will
be in the form of a lateral vibration (Fig. 2-a). ("In phase" as
explained further down, refers to the system phase, not that of
the cutter drive mechanisms.)
But if the inputs are precisely out of phase, the corresponding
output from the loudspeakers on playback should also be out of phase,
with one loudspeaker pushing when the other pulls. Then the motion
of the cutter, groove and pickup stylus will be up and down in a
vertical direction (as shown in Fig. 2-b).
If both channels have the same frequency in phase, but the intensity
of one channel is stronger than the other, the motion will be predominantly
lateral, with a slight angle but less than 45° (as shown in
If both channels carry the same frequency, but the phase relationship
is neither in nor out of phase, other movements can occur. At a
phase relationship of precisely 90", the motion of the cutter and
the stylus, will be in a circle (Fig. 2-d). Other phase relations
with equal intensity will produce an elliptical movement, as shown
in Fig, 2-e.
Finally, if there is a difference in both intensity and phase
that does not correspond with either an in-phase or out-of-phase
condition, the motion of the cutter and ultimately that of the stylus
will be in a modified ellipse as shown in Fig. 2-f.
The ESL stereo pickup separates the movement right at
the stylus. This is the prototype model.
The Electro-Voice pickup uses two crystal elements
The Electro-Voice cartridge, right, is shown in the insert
on the cover with the lower part of the housing (shown as
white in this photo) removed and the copper clip normally
attached to it floating in air. The two ceramic elements
are clearly seen at the base of the red coupling element,
running between it and the grayish mounting block at their
The motions depicted by Fig. 2, of course, represent only what
can happen at one frequency at a time. As practical program material
contains a variety of frequencies, at various intensities and phase
relationships, the cutter, and hence ultimately the stylus, perform
an endless variety of patterns, weaving in and out and up and down.
This explains the nature of the groove and what it represents.
Now we come to the question of how this information is extracted
from the groove by the pickup. But before we proceed, let's clear
up the question of phase, because different writings and discussions
on the subject use different terms of reference and apparent contradictions
Some refer the phase relationship, between the motion in different
directions, to the cutter or pickup mechanism while others refer
it to the overall action of the system. The definitions we have
used so far, both in the previous article ("Single-Groove Stereo
Discs," Radio Electronics, January, 1958) and in this one, are related
to system phase. This is based on the desirability of the lateral
direction of cutter and stylus motion corresponding with the in-phase
combination of program in the two channels.
But, related to the simple cutter design shown in Fig. 3, an
in-phase movement of the two cutter mechanisms will drive the cutter
vertically because the outward thrust of both elements results in
a downward movement while the inward pull results in an upward movement.
To reverse the phase relationship to come out with the in-phase
input to the two microphones producing a lateral cut, all that is
necessary is to reverse the connections to one of the cutter coils.
Similarly, if we take careful account of the phasing at the pickup,
we can reverse one of the connections between the pickup and the
amplifier or at the speaker, so lateral motion of the stylus results
in in-phase motion of the loudspeaker diaphragms.
So, if we talk about cutter or pickup phasing, in-phase action
produces a vertical cut and out-of-phase a lateral one. But referred
to the system - the microphone input or loudspeaker output, it is
just the other way around. To avoid confusion, references to phase
in this article are related to the system rather than to the cutter
or pickup mechanism.
Now to the pickup
Basically a pickup for stereophonic discs can use the same kinds
of transducer elements, for converting the mechanical motion of
the stylus into electrical output, as nonstereo pickups. The more
common forms are magnetic, moving coil, crystal and ceramic. The
way these elements operate follows precisely the same principle
as their single-channel forebears. Now we come to the real question,
how do we separate the two forms of vibration?
There are two extremes in the way this can be done. One method
separates the two movements at the stylus point and couples them
by separate mechanisms to individual transducer elements. This is
the method used in reverse in the Westrex cutter head. The stylus
is carried on a fairly stiff rod, in line with the direction of
the groove. The two cutter motors are directly coupled to the stylus
point at 45° in two directions, as shown in Fig. 3.
Fig. 3 - Basic action of Westrex cutter couples movement
in two directions right at the stylus.
The same method of coupling, applied to a moving-coil pickup,
is exemplified in the ESL stereophonic pickup, shown in the photos.
It uses two d'Arsonval type moving-coil mechanisms, similar to their
well-known single-channel design, both coupled to the same stylus.
The stylus is the common point at which the drive separates the
two moving-coil transducers.
At the other extreme the stylus motion can be conveyed by a common
arm to a pivot or some kind of combination transducer where the
two outputs are obtained electrically by separation in the transducer
action itself. An example that excellently illustrates this method
is the Fairchild stereophonic pickup.
The photos are a magnified view of the central element of the
professional model of this pickup. As shown, the bobbins are empty
and the actual coils have not been wound. The transparent plastic
bobbins can clearly be seen at 45° to the square frame. The
thin wires passing through the centers of the side suspend the coil
assembly and provide a restoring force to center the coil in its
The stylus arm, shown coming down at approximately a 45° angle,
can move in almost any direction and produce a corresponding combination
movement in the coil. The magnetic field that produces the electrical
output when the coils move is produced by pole pieces in front and
behind the square. The magnetic poles are not shown in the photo
because they would obscure the mechanism.
Obviously, if the stylus moves at a 45° angle one way, one
coil will be rotating about its own axis and otherwise not moving
and will produce no output while the other coil will be rotating
like a d'Arsonval coil between the two pole pieces and produce an
electrical output (Fig. 4-a ). Motion in the opposite 45° direction
will make the first coil produce an output, while the second one
rotates about its own axis and does not produce any output (Fig.
This construction produces better than 20-db separation between
the two motions, over a frequency range extending from the low end
up to the region about 10,000 cycles. Above this, separation is
not so good, although the response of the pickup is maintained.
These may be regarded as two extremes in method. Practical pickups
may well bridge the gap and produce results by methods that would
be difficult to list under either one of these extreme headings,
perhaps coming close to achieving it by both of them.
Fig. 4 (a, above, and b, below) - The Fairchild cartridge,
a moving-coil type, uses the same stylus arm to transmit
the combined vibrations to a double-coil assembly mounted
in the same magnetic field.
The new Electro-Voice design is one such method. This pickup
is shown in the photos and its basic operation illustrated by Fig.
5. Like the Westrex cutter, the stylus is supported by a horizontal
rod in line with the direction of the groove. However, a much more
flexible rod is used because a stereo pickup requires a high compliance,
or flexibility, while the cutter requires quite a reasonable stiffness.
Underneath the clip toward the right of the photograph is a removable
mounting for the stylus and rod, so it can readily be changed without
replacing the pickup. The stylus arm drives the crystals through
a triangular piece of compliant or elastic material. Motion at one
45° angle drives one crystal, while not moving the other one
and vice versa.
As a crystal is basically a stiffness or compliant device (the
drive force is used to bend it rather than move it) and these elements
in this cartridge are rigidly mounted at their other end, the compliant
drive from the stylus arm to the crystal represents a mechanical
potentiometer, both elements of which are a compliance or stiffness.
This means that if the compliance of the coupling element is five
times that of the crystal then the motion at the tip of the crystal
will only be one-fifth of the stylus' motion in that particular
As crystals produce a high output from a microgroove recording,
this method still delivers enough output for any high-fidelity preamplifier
and it has the advantage, over the more usual method of coupling
through a mechanical lever system, that mechanical lever resonances
are completely avoided. The dominant mechanical impedance presented
to the stylus point is the compliance of the stylus arm itself and
its elastic coupling member.
If there is any mechanical resonance in the crystal element within
the audio range, it will modify the frequency response but will
not produce the damaging effect upon the record usually associated
with crystals. Also the use of this "potentiometer" arrangement
restricts the magnitude of movement required in the crystal and
lets it operate in a range where it is essentially linear. Driving
the crystal harder (as the normal lever system does) would run it
into nonlinear extremes.
With this pickup, separation of channels really takes place in
the triangular piece of compliant material. But as this is one piece
of material and not two separate links, it seems to "split the difference"
between the extremes we started out with. An advantage of this design
is the relative purity of the separating method, achieving a high
order of discrimination against cross-modulation.
Other variations in the method of splitting the output can be
seen by looking at possible ways of doing it in a magnetic construction
(see Fig. 6). Here, the slug of magnetic material that produces
the variation in magnetic reluctance is attached directly to the
stylus arm and varies the reluctance of both magnetic circuits,
one for each channel. Motion at one 45° angle will change the
gaps in that direction but not in the other direction, and there
will be an output from only one coil. Motion in the other 45°
angle will produce an output from the other coil. The complicated
motion of the stylus following a stereophonic groove produces an
output from both coils in the desired stereophonic relationship.
What's the delay?
From this discussion it is evident that there are a variety of
ways to make pickup for stereo discs. Also it is evident that there
are no serious problems in cutting the discs. A feedback cutter,
such as the Westrex, will do a good job of separating the two channels
and avoiding cross-talk, provided the two drives are mounted precisely
a 90° to each other. So what are we waiting for? Why have not
standards been laid down and records already in production, with
pickups to play them?
Fig. 5 - The Electro-Voice stereophonic pickup uses crystals
and transfers the vibrations through an elastic piece that
Fig. 6 - Another possibility is shown in this diagram
of a composite two-coil magnetic assembly.
Fig. 7 - How an unduly steep stylus-arm angle might cause
the vertical component of stylus motion to frequency-modulate
the recorded program.
Here we have a "which came first, the chicken or the egg?" type
of problem. The pickup manufacturers are looking for test records
to determine how good their pickups are, while the record manufacturers
need some way to measure precisely what they have cut on a recording.
The pickup manufacturers are looking for a perfect or at least adequate
test disc or series of test discs, while the recording manufacturers
are looking for a perfect pickup or one adequately within range,
to determine precisely what they have on their recording.
Then there has been the decision about the appropriate groove
contour - should the angle be exactly 90° as suggested by Westrex,
or would other angles, such as the system proposed by CBS, also
be suitable. Also the correct contour for both cutter and stylus
has not been selected. Will the contour now standard in the industry
for monogroove be used or some other contour? Then we need some
way to test for interchannel intermodulation, cross-modulation and
other effects that were completely unknown with mono-grooves, The
official approval of the Westrex system (which is interpreted to
include the CBS technique) may hasten action in standardizing these
Perhaps we should point out here that there is a difference at
the cutter end between the drive being at precisely 90°, or
two separate 45° angles, and the angle of the cutter wedge being
the same. If the stylus really runs along the bottom of the groove,
it is not important to have the angle of the groove a precise 45°
or 90°. Any angle, so long as it is suitably standardized, will
serve the purpose. The important thing is that the stylus must be
able to ride the bottom of the groove correctly.
In the design and testing of pickups there are some problems
to settle. For example, the Fairchild professional pickup has a
stylus-arm angle of about 45° sloping forward (quite apart from
the 45° angles of the coils themselves). If the stylus only
moves laterally, its motion will be similar to that of any lateral
recording. But if the stylus follows a vertical component, even
though this may be a partial component due to both the 45° elements,
the up-and-down movement of the stylus will also produce a component
(approximately equal to it) back and forth along the direction of
the groove (see Fig. 7).
This will amount to a Doppler effect along the groove or will
result in effective frequency modulation by the vertical component,
As the vertical component is not confined to one of the program
channels in 45/45, the combined vertical or out-of-phase component
of both channels will produce frequency modulation.
We could go on to theorize about this but the real question is,
does it really produce distortion? And if it does, is its effect
audible as a deterioration of program quality? This is one of many
things that still have to be proved. However, it is interesting
to note that a new Fairchild pickup has a stylus arm that comes
much nearer to being horizontal.
Another thing, this discussion is based on the motion of the
stylus remaining simple at all times - we assume it pivots about
the cross-bar junction in the center of the coil. But if the vibrations
runs into resonant conditions of the cross-wires or stylus arm at
any frequency instead of the stylus assembly vibrating as a whole,
the investigation becomes even more complex and it is possible that
some form of intermodulation or other distortion not yet considered
may take place.
Each kind of pickup, and each design approach, will have its
own problems. In general, the simplest mechanism is most likely
to avoid troubles. But the only real way to determine the effect
of these different possible forms of distortion is to measure them
- to get test discs with different frequencies recorded on the respective
channels, determine possible cross-modulation from one channel to
the other, or intermodulation of the audio on one particular channel
by other tones on either channel, etc., using discs yet to be made.
Finally, having discovered what degree of intermodulation or
cross-modulation exists with any particular pickup or record combination,
we still have to determine whether this is important in terms of
the end result - listening to the stereophonic program. It must
be conceded that all of the pickups recently demonstrated have produced
quite listenable stereophonic presentation, even though a rigid
theoretical investigation might suggest that some terrible distortion
From the work done so far and the technological improvement achieved
to date, it is evident that stereo discs have a great future. There
will be a little shakedown period - getting the bugs out and determining
what kinds of distortion really matter or whether they are really
present at all - and then it seems we shall have a golden opportunity
for getting stereophonic program at a cost no greater than regular
Posted August 3, 2014