- Correlative terms that may appear in TIGHAR resources
- radiotelephone, radiotelephony
- voice transmission
|A1||Continuous wave (CW)|
|A2||Modulated continuous wave (MCW)|
|A3||Voice modulation, radiotelephone|
To transmit voice over the radio, the audio wave pattern picked up by a microphone is superimposed on the transmitter's carrier wave; the audio wave modulates (controls the shape of) the radio wave.
To equip a transmitter to send voice, one needs at least a microphone and a modulator circuit. In the Radio equipment on NR16020, the audio waveform picked up by the microphone was amplified in the modulator:
- The Model 13C transmitter employed screen-grid modulation of the power amplifier stage. This is a form of low-level modulation in which audio voltage is coupled to the amplifier tube screen grids through a transformer, after being amplified in an audio power stage.
- The single audio amplifier stage in this transmitter was driven by audio from a carbon-type microphone, transformer-coupled to the audio tube control grid.
One of the breakdowns in communication between the Itasca and NR16020 was due to the fact that Itasca’s high-frequency transmitter (the one used to transmit on 7500 kcs) was not equipped for voice modulation. The Itasca could only transmit using Morse Code transmitted via CW on 7500 kcs. Tragically, this was the only frequency on which Earhart heard the Itasca’s transmissions; if the Itasca could have transmitted to her in voice on this frequency, the problems with the direction finding might have been sorted out.
The Itasca was only prepared to transmit voice on 3105 kcs:
- ALL COMMUNICATION FROM PLANE WILL BE ON 500, 3105 OR 6210 KILOCYCLES BY VOICE, POSITIONS BEING GIVEN AT FIFTEEN AND FORTY FIVE MINUTES PAST THE HOUR. ITASCA ADJUST TRANSMITTER FOR POSSIBLE USE 3105 KILOCYCLES FOR VOICE.
- Mike Everette, "A Technical Analysis of the Western Electric Radio Communications Equipment Installed on Board Lockheed Electra NR16020."
- 6/28/37 from Itasca: "ITASCA TRANSMITTERS CALIBRATED 7500 6210 3105 500 AND 425 KCS CW AND LAST THREE EITHER CW OR MCW."
- "Log Jam."
- "Radio Traffic," 6/25/37.