having
a
phase detector, we're able
to
detect
the L - R
part
of the
signal.
An AM
detector
is
absolutely
insensitive
to
phase modulation.
An
FM
detector,
when
properly
constructed,
is
insensitive
to
amplitude
modulation,
so the
phase detector picks
out the L- R
because it's phase modulated
and
ignores
the
L+R
component.
The
output
of
each detector then
is
composed
of an
L+R
signal
and an
L- R
signal.
Those
two
signals
are
combined
in an
appropriate matrix
to
produce
a
left
and a
right output.
In
addition
to
that there
is a
pilot tone that
is
broadcast
at 25 Hz at
4%
modulation
to
turn
the
stereo decoder
on or off and
light
up an
indicator
light
to let you
know that
you are
listening
to
stereo.
The
features that this tuner
has of
receiving
AM
stereo
are
vast.
Our
circuitry, unique
to all
circuits,
has a 25 Hz
pilot cancelling
circuit.
Most
AM
receivers,
to
eliminate
the
25Hz from their output, will simply
put
in a
bandstop filter
and
nothing below
25 Hz
gets through.
Our
system
is
flat down
to
below
20 Hz by
using
a 25 Hz
pilot cancelling signal
so
the
pilot
will
be
cancelled
and
will
not
appear
at the
output
as a
low
frequency undesired rumble.
In the
absence
of a 25 Hz
pilot tone,
the
stereo decoder will switch off. This
is
desirable when
the
tuner
is
receiving
a
signal
in
mono, otherwise
it
would
be
somewhat noisier.
The
difference
in
noise
is
theoretically about 3dB,
in
practice
we
find
it's about
14dB.
However, it's
14dB
noisier
whether
the
signal
is
strong
or
weak.
It
actually works
out
better than
it
does with
FM.
Because
when
the
signal
is low in FM, you can go
from stereo
to
mono
or go
from
an
unlistenable condition
in
stereo
to a
listenable condition
in
mono. With
AM
that doesn't happen.
In AM, the
signal will remain
eminently listenable, even when switching
to
stereo
on a
weak
station.
This
is the
only
AM
tuner capable
of
15
kHz
response
and
bandwidth.
The
typical
AM
bandwidth
is
approximately
3
kHz; good enough
for
speech
and not
really good enough
for
high fidelity listening. High
fidelity requires
15
kHz or
more. This radio
has a
unique
IF
system
(intermediate frequency).
It has
been carefully designed
to be
perfectly
flat
15
kHz on
either side
of the IF
carrier. Most other radios
have
a
response that
is
flat maybe
to 2 kHz and
several
dB
down
at
3
kHz;
the
best
of
them
are
usually
10dB
down
at 6
kHz.
The
TX-11
b
system
is
perfectly
flat
and you
will immediately hear
the
difference.
When
you
listen
to an AM
radio,
you
will hear sparkle, life-like timbres
just
the way you
hear those
sparkly,
life-like
high
frequency tones
when
you
listen
to FM.
Why
are all
other tuners
are
only
3 kH
wide?
The
main reason
is
that
it is
very
difficult
to
design
a
wide band
AM
radio.
It
requires heroic
efforts.
It
requires that
the AM
antenna
be
wide band with high
selectivity;
it
requires
a
great many
IF
stages.
It
requires
an IF
tuning
mechanism that tunes
simultaneously:
the
oscillator,
the
antenna,
and
the IF
must
track;
almost
impossible
to do
with
modern
electronic
varactic tuned systems.
In the old
days,
the
tracking
was a
matter
of
capacitor geometry (some
of you
might remember tuning capacitors
that
had
serrated metal interleaved finger).
It was
relatively easy
to
make
those
track
well
up and
down
the
band
simple
because
it was a
matter
of
doing
the
geometry right. They
had an
interesting quasi-
eliptical
shape
to
them
as
they rotated
in and out of
mesh. Modern
varactic tuned systems
are
very difficult
to
make track properly,
so
difficult
in
fact
that
most
AM
radio
manufacturers
gave
up and
made
the
bandwidth very, very narrow.
By
doing
so,
front
end
varactic mistracking
was
masked
by the
narrow
bandwidth. Even oscillator mistracking, oscillator,
IF and RF
mistracking
was
almost totally masked. Even today with most radios
you
will find different frequency responses
on
different segments
of
the
dial.
The low end of the
dial
may
have
a
higher audio frequency
response when measured than
the
high
end of the
dial
that
is due to
mistracking. What
we had to do was to
develop
a
very accurate
tracking system
for our
varacters,
and
this included enclosing them
in
a
very powerful feedback loop
to
make sure that they track
and
were
in
fact, also wide band.
The
goal
was to
make
the
bandwidth
15
kHz:
same
as FM.
Now
an
immediate question comes
up; if the
tuner
is
capable
of
such
fine
performance, what about
the
broadcast transmitters?
A
survey
in
the
Seattle area immediately demonstrated that
the
majority
of the AM
transmitters
are
broadcasting signals
out to
15
kHz. Almost
all of
them
have
signals that
go out to at
least
12
kHz, many
to
beyond
15
kHz.
You
will notice
it
immediately.
15
kHz is an
accepted minimum
for
high fidelity,
and the FM
system
that
we use is
limted
to
15
kHz and it
stops instantly
at a
brick wall
at
15
kHz.
The AM
system
is not
quite that much
of a
brick wall
and the
response actually continues
a
little
bit
beyond
15
kHz.
We
can't make
it
20 kHz
becuse that would
put the
modulated carrier squarely
on top
of
an
adjacent channel. Channels
in
this country
are
spaced
20 kHz
apart.
This tuner
has
incredible specifications.
It has an AGC
range that
is
approximately
10
times greater than
a
standard radio. That means
that
on a
strong station,
the
output level from
a
strong
to
weak station
will
remain essentially constant;
it'll
behave
in
that regard like
an FM
system;
a
weak station won't drop
in
volume
and it
won't
be
susceptible
to
airplane flutter, won't
be
susceptible
to
fading
the way
a
regular
AM
radio would
be.
That's
No.
1.
No. 2 of
course,
is the
wide
bandwidth which results simply
in
high
fidelity
because
it
picks
up the
high frequency components
of the
music.
19
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