Patent GB-1385953-A (GB)
Title: Signal modulating and demodulating system
IPC: H04N9/82
Filed: 1972-04-24
Abstract: 1385953 Television recording NIPPON VICTOR KK 24 April 1972 [26 April 1971(2)] 18958/72 Heading H4F A signal for transmission is amplitude modulated, one of the side bands is attenuated relative to the other and after reception the signal is angle demodulated to produce a signal corresponding to the difference between the two side bands. As described, the method is applied to a television video signal which is recorded on a magnetic medium, the electromagnetic conversion characteristic of the recording and reproducing system producing the required asymmetry in the side bands. The video signal is divided into low (0 to 1À4 MHz) and high frequency bands by means of low pass filter 24 and differential amplifier 28. The filter has a linear falling characteristic of 6 dB/oct. which passes - 6 dB at 1 MHz. The two components are then provided with inclination to the frequencyamplitude characteristic to improve the signal/ noise ratio by pre-emphasis circuits 26, 29. The signals are supplied to a combined AM-FM modulator 30 in which the low frequency component is used to frequency modulate an 11À5 MHz carrier which is then amplitude modulated by the high frequency component. This may be achieved by passing the signals to separate AM and FM modulators to modulate carriers differing in frequency by 11À5 MHz, the modulated carriers then being supplied to a frequency converter whose output is the difference between the two carriers. The resulting signal is passed through higher pass filter 31, for removing harmonics, and variable gain amplifier 32, whose gain is varied according to the radial position of the recording heads 12 on a magnetic disc recorder. It is not required with a tape recorder. The signals pass alternately through gates 33, equalizers 34 and amplifiers 35 to writing heads 12. Magnetic recording and reproduction has the effect of producing a differentiating action so that the modulated signal having side bands Va<SP>1</SP> and Vb<SP>1</SP> (Fig. 10A) is reproduced with side bands Va and Vb (Fig. 10B) which are asymmetrical. The signal is effectively converted from an amplitude modulated wave to a frequency modulated wave. During reproduction the outputs from heads 12 are pre-amplified at 37, equalized in their frequency characteristics at 38 and supplied to gates 39 which enable the outputs from the channels to be reformed into a continuous signal. This signal is corrected by a radius equalizer 40, passed to a dropout compensator 41 and amplitude limiter 43 before being frequency demodulated at 44. A demodulated output 100 (Fig. 10B) is produced which is equivalent to the difference 100<SP>1</SP> between the side bands. Since low frequency modulation components are not effectively reproduced by such conversion of the amplitude modulated wave to a frequency modulated wave these frequencies (0 to 1À4 MHz) are frequency modulated and demodulated in the normal manner. Alternatively the low frequency component may be frequency converted and used for amplitude modulation. After demodulation the signal is passed through a low pass filter 45 and a deemphasis circuit 46, to cancel the effect of circuits 26, 29, and finally to an amplifier 48 whose gain is corrected according to the radial position of the heads. If recording is not required switches 42, 49 may be switched to E-E in which case the output of modulator 30 is passed to demodulator 44 which demodulates only the frequency modulation produced by the low frequency component since the side bands produced by amplitude modulation remain symmetrical. The high frequency component is transmitted directly to mixer 47 where it is added to the low frequency component. With a video tape recorder, in which the relative velocity between the head and and the tape is low, the carrier frequency is selected close to the recordable upper limit frequency so that the upper side band is attenuated. Conversion of amplitude modulation to frequency modulation may be effected by integration of the signal instead of differentiation, or by means of high pass or low pass filters. Instead of the amplitude modulated wave being conbined with a frequency modulated wave it may be combined with PPM, PAM, PWM or PCM waves. A direct wave and an amplitude modulated wave may be transmitted by superposing one upon the other without forming a combined modulated wave; or a signal may be transmitted by an amplitude modulated wave alone. It is stated that this method of transmission enables a large modulation index to be attained, with a low signal/noise ratio. With a colour video signal inter-modulation produced when the whole video band is frequency modulated is avoided. Since the information signal energy is contained in the two side bands a smaller bandwidth is needed than if a wholly frequency modulated signal is used.