Patent GB-679417-A (GB)
Title: Amplitude modulation of radio frequency carrier waves
Inventor: KOROS LESLIE LASZLO
IPC: H04N7/20
Filed: 1950-08-11
Abstract: 679,417. Television. RADIO CORPORATION OF AMERICA. Aug. 11, 1950, No. 20053/50. Class 40 (iii). [Also in Group XL (c)] An amplitude-modulated carrier wave is produced by first amplitude-modulating the signal on a sub-carrier wave, using the output to frequency-modulate the carrier wave, and filtering the resultant to select and transmit one of the resultant side-band groups. The signal may be a television signal, and an associated sound signal may be transmitted on an adjacent carrier as a F.M. signal by a similar process except that the sub-carrier is frequencymodulated by the sound signal. In a modified form of the invention, one carrier source is frequency-modulated by the signal and a particular side-band group is selected for radiation, the carrier itself being supplied by a second unmodulated source. The invention is designed for use with very high carrier frequencies such as are produced by magnetrons, but is not limited thereto. Fig. 1 shows a combined picture and sound transmitter, in which the video signal amplitude-modulates at 23 a 20 m.c. sub-carrier 25 derived from a 5 m.c. source 24. The modulated output, which may be filtered at 26 to suppress one side-band, passes to a frequency modulator 28 associated with a 3975 m.c. source 27, preferably in the manner described in Specification 646,009, [Group XL (a)]. A first order side-band group comprising a 3995 m.c. carrier with an A.M. side-band is selected by a filter 29 and passed to a combining-unit 30 for radiation. The audio signal is applied to a frequency modulator 35 fed from the 5 m.c. source 24, and its output, quadrupled in frequency at 36, is fed to a frequency-modulator 38 associated with a 4020 m.c. source 37. From the resultant a side-band group, comprising a 4000 m.c. carrier with F.M. side-bands, is selected by a filter 39 and passed through the combiner 30 for radiation. The combined video and audio outgoing signal is represented at Fig. 2e. The centre-frequency of the magnetron 27 is synchronized with a stable oscillator 31 by the normal method of frequency and phase comparison at 32 giving an error voltage which is used to adjust the magnetron frequency at 28. The second magnetron 37 is similarly stabilized at 4020 m.c. by comparing its mean output frequency with a resultant frequency derived by mixing the output of the oscillator 31 (3975 m.c.) with that of a frequency multiplier 41 which provides a harmonic (45 m.c.) of the 5 m.c. source 24. In an alternative form of the system, the same carrier frequency is used for both video and audio signals. In a modified form of the invention, Fig. 9, a video signal fi is fed through an amplifier 12 to a balanced amplitude modulator 13 associated with a stable 6 m.c. source 11. The side-band output without carrier passes to a frequency modulator 2 associated with a 994 m.c. source 1, one side-band being substantially suppressed, if desired, by a filter 22. The output is taken through a combining unit 5 and filter 6 to an aerial, and the filter 6 is designed to select for radiation the first order upper side-band group which comprises one or both of the video sidebands centred on a missing 1000 m.s. carrier. This carrier is supplied to the coupling unit 5 by a 1000 m.c. source 3 in the correct phase to constitute an amplitude-modulated carrier at the aerials 7, 8. The sources 1, 3 have their mean frequencies stabilized as in Fig. 1. Modulation distortion is reduced by detecting the outgoing signals and feeding them back negatively at 20 to the video amplifier 12. In order to control the brightness of the received picture, the amplitude of the outgoing 1000 m.c. carrier is varied in accordance with the brightness of the transmitted picture. The latter is measured at the output of the video amplifier 12 and the resulting voltage is used to vary the gain of an amplifier 21 interposed between the 6 m.c. or other source 11 and the frequency modulator 4 associated with the 1000 m.c. source 3. As the gain at 21 is increased, the modulation index at 3, 4 rises, causing the amplitude of the 1000 m.c. carrier to fall off. The Specification also describes an audio signal transmitter similar to Fig. 9, but without the brightness control, Figs. 5 and 7 (not shown). Figs. 10a, 10b, 10c (not shown) indicate desirable frequency response characteristics for the video amplifier 12 in order to strengthen the high-frequency components relatively to the low so as to compensate for the reduced modulation index associated with the high-frequency components. Compensation may also be provided for the enhanced effect of the low-frequency components due to the presence of a vestigial sideband. Fig. 11 shows an alternative system for producing the radiated side-bands with carrier suppressed. The output 47 from the balanced modulator 13 of Fig. 9 is applied in push-pull to frequency or phase modulators 45, 46, associated with magnetron generators 48, 49, the latter operating at the same mean frequency and in opposed phase. The outputs are fed through a combining unit 5 and filter 6 to the aerial 20 as to radiate a selected side-band group, preferably a first order group. The system cancels out the carrier frequency and the even order side-bands, and thereby simplifies the design of the filter 6. A carrier 60 of suitable frequency and phase to form with the side-bands an amplitudemodulated wave, is also radiated. This carrier may be supplied by the system shown in Fig. 13, which employs two magnetrons 50, 51 of the same frequency and phase associated with frequency modulators 53, 54. A sub-carrier of lower frequency is applied through an amplifier 52 to modulate 53, 54 in push-pull, the gain of the amplifier 52 being varied in accordance with the picture brightness. The output of the two magnetrons is passed through a combining unit 5 and filter 6 to an aerial, which is also fed with the necessary side-bands 61 which may be produced by the circuit of Fig. 11. The effect of the Fig. 13 system is to suppress first-order side-bands, thereby facilitating the selection of the pure carrier by the filter 6.