Voice Encoder. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer
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Effective protection of radio and telephone conversations from outside listening is provided by the "frequency inversion" method. By the way, it has long been very successfully used by the American police services.
The frequency inverted signal is extracted from the lower sideband of the spectrum of the balanced conversion of the audio signal from the supersonic carrier. Two successive inversions restore the original signal, i.e. the device is used as a codec (encoder and decoder). Synchronization between transmitter and receiver is not required.
The device, the circuit of which is shown in Fig. 1, consists of a generator (DA3), which generates a signal with a frequency of 14 kHz, a 3,5 kHz carrier divider-shaper (DD1), an analog switch of a balanced modulator (DA2), an input bandpass filter with a band of 300 - 3000 Hz (DA 1.1) and a balanced modulator adder with a low-pass filter (DA1.2). Multi-turn tuning resistor R1 carry out individual adjustment of the conversion frequency.
(click to enlarge)
For normal operation of the device, it is necessary to effectively limit the spectrum of the speech signal in the range of 300 - 3000 Hz. Otherwise, the part of the spectrum above the carrier is bent and superimposed on the low frequency region, creating strong additional interference. Within the specified frequency band, speech intelligibility after two conversions is at least 65%. Figure 2 shows the spectra of the input and converted signals.
The encoder/decoder carrier pairs are tuned to the same frequency. When tuning, the encoded speech signal can be recorded on a tape recorder and the decoder conversion frequency can be selected according to the highest speech intelligibility during decoding.
When working with different receivers and signal sources, additional amplification may be required. In duplex communication channels, to increase the confidentiality of negotiations, it is advisable to use
different conversion frequencies. The location of the device elements on the board and the image of its double-sided "printing" is shown in Fig. 3 и Fig. 4 respectively.
In a portable version, a pair of encoder / decoder, a power source ("Krona") and additional microphone amplifiers are built into the body of the handset. Acoustic communication with the telephone line is carried out with the help of an additional microphone and a small-sized speaker mounted in the back of the case opposite the telephone and microphone capsules of the handset of the telephone set. A diagram of such a pairing of a device with a telephone line is shown in Fig. 5.
(click to enlarge)
However, those who will use a scrambler in confidential communication should be warned that, with a certain patience, one can learn to parse the content of "frequency inverted speech." Therefore, do not prolong negotiations or periodically change the frequency of the conversion carrier.
Literature
- Radio amateur №2, 1993
Publication: N. Bolshakov, rf.atnn.ru
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