The Washington times., 1924-01-21
WHISTL|NG
CON NTR0L. .
POSSIBLE
John v. L. Hogan Tells How
Interference May Be
COPIED With.

ident of the Institute of Radio
Engineers and author of "Outline
of Radio" delivered the ninth of
his series of talks on radio,
through WEAF and WCAP Satur-
day night. In his talk on "Whis-
Ting Interference-Its Causes
and Cures" He said:
May Spoil Music.
There are two kinds of whistling
interference. Both are caused in
the same general way One kind
sounds, in the telephones or loud
speaker, like a high note of practi-
cally constant pitch and strength. 1t
may Quaver a little In pitch and
may gradually increase or decrease
in intensity or in pitch, but the
sound is generally an almost unI-
form high note.
IF you listen to distant stations
on the 833 kilocycle 1850 meter)
wave you have often heard sucn
a uniform whistle in the back
ground. Sometimes it is not strong
enough to prevent fairly good re-
ception, but whenever you can hear
it you may expect the quality Of
music or speech to be partly or
wholly spoiled by It.
This kind of whistle is almost
always caused by interference from
a distant broadcasting transmitter
that has accidentally changed its
wave frequency. For example, let
us suppose that you are listening
to WEAF, whose normal wave fre.
quency is 610 kilocycles. II voc,
in Davenport, Iowa, is sending at
its proper wave frequency of $40
kilocycles 1t will not interfere. But
II las has happened once or twice.
woes wave frequency should drop
to say 613 kilocycles, there would
be a whistle in the background of
each stations program. People liv.
ing about half way between the two
stations might hear cross-talk in
addition to- a very loud whistle; peo.
pie who were too far from one Of
the stations to hear it's program.
but who could hear the other, would
be troubled by the whistle.
The Whistles Pitch.
I II the waves Of the two inter.
fering stations are, as we have
assumed, 6l0 and 613 kilocycles, the
whistle that is heard whenever
both waves are received has a pitch
OF three kilocycles or 5000 cycles,
Ih1s corresponds to the highest G
on the piano keyboard. The whistle
Is always a pitch equal to the dif.
ference in the frequencies of the
two interfering waves, as in this
case where 613 kilocycles minus $40
kllocycles equals three kilocycles.
As another example we NAV
consider whistling interference be-
tween WIZ in New York and
WJAZ In Chicago. These stations
normally use waves of $50 and $40
kilocycles, respectively, and the diff
ference of ten kilocycles is enough
to prevent a troublesome whistle,
But on some occasions either or
both waves have slipped away from
the normal frequency; If they be.
came, for example, 662 and 664
kilocycles the difference would be
only two kilocycles or 1000 cycles
and the whistle would have the
pitch of the third c above middle
c on the piano.
Interference of this kind, where
two inaudibly high frequency waves
Csuch as radio waves of 610 and 613
k1locycles) interact to produce an
audible frequency equal to their
frequency-difference, is called beat
or heterodyne interference. A quite
similar effect is had in music. for
II two sounds of almost equal pitch
or frequency are played together.
their waves will interact to produce
pulsations or beats at a rate equal
to the difference in their freuuen.
cies.
| Constant Whistling Scarce.
"Constant-pitch whistling inter.
Terence caused by interfering broad
casting transmitters is becoming
more and more scarce, for most Of
he stations are doing bett-r in
holding to their assigned wave fire.
quencies. The only cure for it Is
TO keep the stations adjusted to
radiate their correct wave fire.
quencieS and there is nothing that
either you or I can do to stop such
cases as do occur except to report
them to the broadcasting station that
Is lnterferred with. If 1t were not for
the fact that this inter.station
Whistling Is 80 much like the sec.
ono Kind OF whistling interference,
ana SO convenient for explaining IL,
would not have been lust { I'd in
giving 1t so much time tonight.
Il hope that the foregoing has
made clear to you that whenever
your receiver picks up two con.
tinuous radio waves whose fre-
quencies are quite nearly alike, you
will hear a whistling note whose
pitch is equal to the difference in
the radio wave frequencies. This
brings us to the second type of
whistles, which are usually not uni-
form in pitch or intensity and which
are not caused by interference be.
tween broadcast transmitters.
) These whistles change in pitch,
either uniformly or in Jumps, and
sometimes slowly and sometimes so
rapd'v that they sound ike ch,rps
Sometimes they are faint in the
background; sometimes they are so
loud that they completely spoil re.
ception from powerful nearby broad
cast stations.
Effects NeighbOrs Too.
/. "If you have a simple rcgenera-
tive receiver of any type, you have
heard just such whistles as you
turned the tuning knobs. When
you have had your tickler coupling
or your plate variometer too far up
the scale. you have heard loud
1 taNl In.