 with the received intelligence signal to receover the voice-modulated waveform represented by the sec- ond factor. This receiver mixer appears to be similar in many respects to Marchand's bandpass correlator. To overcome some of the fundamental weaknesses of TR systems (see Chapter 1, Section 3), Hansen threw in an additional twist: The filtering The Historical Origins of Spread-Spectrum Communications A M or NOISE AMPLIFIER TRANSMIT TER SOURCE 45 MC. CARRIER A.M. or TRANSMITTER 30 MC. CARRIER RECEIVER MC. SWING MULTICHANNEL SELECTIVELY ADJUSTABLE B.P. AUDIO FILTER CHRONOMETER 70 MC. SUMMATION GENUINE CARRIER VOICE SIGNAL TRANSMITTER ADDITIVE HIGH PASS 40 MC. CARRIER MIXER FILTER 10 KC. SWING NOISE I MC. WIDE CENTER ON 70 MC. MULTICHANNEL A.M. or SELECTIVELY ADJUSTABLE RECEIVER CONTROL TRANSMITTER 45 MC. 30 MC. AUDIO FILTER CHRONOMETER RECEIVER LOW PASS SHARP TUNING FILTER MIXER 40 MC. Figure 2.4. This TR-FM-SS block diagram redrawn from Hansen's patent [65] illus- trates one method for denying an unauthorized listener direct access to the refer- ence signal used by the receiver to detect the information bearing transmission. Emerging Concepts of the reference channel signal, used to generate n(t), was made time dependent, with transmitter and receiver filters required to change struc- ture in virtual synchronism under the control of a chronometer. This struc- tural change could not be detected in any way by observing the reference channel. When presenting his design along the TR-FM-SS lines, Hansen notes that the intelligence signal cannot be heard by unauthorized narrow-band receivers because "such wide-swing modulations in effect tune the transit- ted wave outside the frequency band of the unauthorized listener's receiver for the greater portion of the time and thus make such a receiver inopera- tive." Concerned about the fact that a wide-band FM receiver might con- ceivably recover the signal An(t) + Bv(t), he also concludes that "if therefore the noise [n(t)] has important components throughout the range of