Evening star., 1925-05-31
No. 1S-From Radio to Audio.

Smith is one of those rare persons
who always manage to take adVan-
tage of disadvantage. There are some
nights when reception Is not so gooo,
or times when he doesn't like what he
can get over the air, but instead OF
grumbling he recognizes the chance 10
learn something more about no" the
set works.
1t was at one of those eventful occa-
sions that ! happened to find him try
ing to trace the radio circuits from
imput to output The various basic
units of the set were becoming as IA
miliar to him as the arrangement OF
engine clutch, transmission and OIL
ferential in his car. He had formed a
definite mental picture OF the order
from aerial to tuner, to radio frequen-
cy amplifier tubes. to detector, TO
audio amplitier tubes and to LOUD
speaker. but it was that changeover
from radio to audio that still puzzled
him.
Il don't blame you for being puz-
zIed," I cheered him bit. There is
probably no part of radio theory that
is more carelessly explained than the
change from radio TO audfo. l think
you have got it pretty well fixed in
your mind that the original sound
electromagnetic waves oscillating
anywhere from $4,000 to 100000
cycles per second. and you recognize
this as alternating current. but from
the talk you hear ii is certainly diff
cult to know whether the current is
alternating or direct after the de-
tector tube has rectified it."
Smith Has Same Problem.
That's just my problem he ad-
mitted. II can visualize the radio
frequency-those high speed oscilla-
tions-reaching the grid of the de-
tector tube. ! can see the negative
electrons being attracted across the
gap to the plate of the tube which is
positively charged with the current
from the "B" battery. l can clearly
understand that this arrangement
means that the current from the
plate 10 the earphones can go only in
one direction because II has been recti-
fed by the tube. But then why do
they talk about this rectified current
as 'audio frequency? ] thought fire.
quency' was the term used to define
the number of cycles of any oscillation.
And after the current is converted into
the audio type it's supposed to go in
just one direction
Here he pointed to contradictory
statements which he had picked from
various sources. which certainly left
doubt as TO just what did happen after
the detector got through with the
radio frequency form of the current.
I've been puzzled over just the
same point l told Smith. "but I've
finally got il straight from practical
electricians. 1t is a pretty big story,
and / guess there is much that has
to be taken for granted, but it will
ease you considerably TO know that the
current is no longer alternating after
ii has passed through the detector.
whether you use a crystal for this
purpose or a detector tube
Fine. Smith exclaimed. But then
why speak of audio frequency when
the current in audio form is not alter
| mating? How can there be oscillations
in direct CURRENT"
Term Found Confusing.
That's just where they confuse us
i explained. You see. all electrical
circuits are oscillations. 1t is the
basis upon which radio stands. But
the term is rather vaguely used
First they are using it to mean alter
| nations, then to mean pulsations.
| This is confusing, and unnecessary.
Just picture the current coming in
| over the antenna as alternating cur
| rent. Picture its changing its direc.
| titon thousands of times a second
And regard it as a series of electrical
| oscillations. Now you have the tern
oscillations .in its most logical sense
and you can figure that. used in other
connections, ii is often somewhat loose
ly employed. l will give you an in
| stance:
| ION these oscillations of the current
| being sent out by the broadcastine e
station are impressed the audio fire.
| quencies or sound wave variations
After this current is picked up, am.
| plified and changed by the detector
| tube to direct current the original
oscillations disappear and the only
| characteristics that remain to influ
ence the direct current are the original
| sound waves. This causes the direct
| current to pulsate. Instead of being
constant it is really intermittent be
cause it is being controlled by the
| radio frequency current. These pulsa
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