Cobra Electronics Cobra 148 GTL - Manual (Page 8)

Section IV Operation (Continued)
Thus when a voice is used in place of a whistle or tone, in the proper listening
mode the voice will be received correctly whereas in the incorrect mode, the
voice will be translated backwards and cannot be made intelligible by the voice
lock control. When listening to an AM transmission, a correct sideband is heard
in either mode since both upper and lower sideband are received.
Once the desired SSB mode has been selected, frequency adjustment may be
necessary in order to make the incoming signal intelligible, the VOICE LOCK con-
trol allows the operator to vary frequency above and below the exact-center fre-
quency of the received signal. If the sound of the incoming signal is high or low
pitched, adjust the operation of the VOICE LOCK. Consider it as performing the
same function as a phonograph speed control. When the speed is set too high,
voices will be high-pitched and if set too low, voices will be low-pitched. Also,
there is only
one correct speed that will make a particular record produce the
same sound that was recorded. If the record is played on a turntable that rotates
in the wrong direction (opposite sideband) no amount of speed control (VOICE
LOCK) will produce an intelligible sound.
An AM signal received while listening in one of the SSB modes will produce a
steady tone (carrier) in addition to the intelligence, unless the SSB receiver is
tuned to exactly the same frequency by the VOICE LOCK control. For simplicity it
is recommended that the AM modes be used to listen to AM signals.
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Section IV Operation (Continued)
OPERATING PROCEDURE TO TRANSMIT
1. Select the desired channel of transmission.
2. Set the DYNAMIKE control fully clockwise.
3. If the channel is clear, depress the Push-To-Talk switch on the microphone
and speak in a normal voice.
RECEIVING SSB SIGNALS
There are three types of signals presently used for communications in the Citizens
Band: AM, USB, and LSB. When the MODE switch on your unit is placed in the
AM position, only standard double-sideband, full carrier signals will be detected.
An SSB signal may be recognized while in the AM mode by its characteristic
“Donald Duck” sound and the inability of the AM detector to produce an intelligi-
ble output. The USB and LSB modes will detect upper sideband and lower side-
band respectively, and standard AM signals.
SSB reception differs from standard AM reception in that SSB receiver does not
require a carrier or opposite sideband to produce an intelligible signal. A single-
sideband transmitted signal consists only of the upper or the lower sideband and
no carrier is transmitted. The elimination of the carrier from the AM signal helps to
eliminate the biggest cause of whistles and tones heard on channels which make
even moderately strong AM signals unreadable. Also, SSB takes only half of an AM
channel, therefore two SSB conversations will fit into each channel expanding the
40 AM channels to 80 SSB channels. The reduction in channel space required also
helps in the receiver because only
half of the noise and interference can be
received with 100% of the SSB signal.
An SSB signal may be received only when the listening receiver is functioning in
the same mode. In other words, an upper sideband signal (USB) may be made
intelligible
only if the receiver is functioning in the USB position.
If a lower sideband (LSB) signal is heard when the receiver is in the USB mode,
no amount of tuning will make the signal intelligible. The reason for this may be
understood if you consider that when the modulation is applied to the transmit-
ter’s microphone in the USB mode, the transmitter’s output frequency is increased
whereas in the LSB mode the transmitter’s output frequency is decreased. The
result in listening to the receiver is that when the MODE switch is in the proper
position (either USB or LSB), a true reproduction of single tone of modulation will
result, and if the tone is increased in frequency (such as a low-pitched whistle or a
high-pitched whistle) you will hear the increase in the output tone of the receiver.
If the incorrect mode is selected, an increase in tone of a whistle applied to the
transmitter will cause a decrease in the resultant tone from the receiver.
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