SU121146A1 - The method of frequency modulation of video signals with a small modulation index and device for its implementation - Google Patents

Description

The known methods of frequency modulation during phototelegrafting use the frequency spectrum of the communication channel with an average carrier frequency located symmetrically with respect to the boundary frequencies. With such a carrier arrangement, the present frequency band is not fully utilized.

In the described method, the average carrier frequency is located in the spectrum of the communication channel asymmetrically, which allows more efficient use of the expensive communication channel. To implement the described method, a device is proposed which differs significantly from that known by using a low-pass filter with a gentle characteristic as a discriminator and a class C amplifier for restoring the compressed dynamic range.

FIG. 1 shows the frequency response of the discriminator in the form of a low-frequency filter; in fig. A 2-skeleton diagram of a device implementing the described method.

FIG. 1 shows the dependence of the transmission coefficient

yy from frequency. "

In order to more efficiently use the frequency band of the communication channel, the average modulation frequency c is located asymmetrically with respect to the channel boundary frequencies: and the low frequency and the g - upper channel frequency.

The modulus of the transmission coefficient (- remains constant and

is equal to В „aks in the frequency range from„ to “o - 2 Ash, where Ds is the maximum frequency deviation of the modulated phototelegraphic signal. For the average modulation frequency, the gain is B

 one /

JSo 121146-2 -

In the range from «Jg - 2Aoj to the upper cutoff frequency, the dependence of the transmission coefficient is linearly falling. At the same time, the modulus of the transmission coefficient at the frequency в is not equal to zero. The latter is necessary to reduce the non-linear distortion arising from the non-linearity of the phase response of the low-pass filter. With the discriminator feature described and the asymmetrical arrangement of the center frequency, the speed of wiring over the communication channel is increased by 2K times. Here / (- index cha% -%

C70T modulation is significantly less than one.

A device for carrying out the described method comprises an amplitude detector, a frequency modulator, a linear amplifier, a limiter, a discriminator and a dynamic range restorer.

The signal from the phototelegraph transmitter is fed through the input / to the amplitude detector 2, where the video signal of the image is extracted. This signal acts on the frequency modulator 3, which has an average frequency SI 0, symmetrically located in the frequency band of the communication channel. The amplitude-frequency-modulated signal through the linear amplifier 4 and the output circuit 5 enters the communication channel. From the communication channel, the signal enters through input 6 to limiter 7. After restriction, the signal is transmitted to discriminator 8, which is identified as a low-pass filter with the characteristic shown in FIG. one.

The coefficient of amplitude modulation of the amplitude-frequency-modulated signal allocated by the discriminator 8 is less than the coefficient of amplitude modulation of the signal in the transmitting phototelegraph apparatus. To restore the amplitude modulation coefficient, a push-pull amplifier 9 is used, operating in class "C" mode, the input of which is connected to the output of the discriminator. The dynamic range phototelegraphic signal through output 10 is transmitted to the receiving phototelegraphic apparatus.

Subject invention

Claims (2)

1. The method of frequency modulation of video signals with a small modulation index, characterized in that in order to increase the speed of phototelegraphy and more efficient use of the communication channel, the average modulation frequency is set asymmetrically relative to the boundary frequencies of the communication channel in exact accordance with the frequency response of the coefficient transmission frequency discriminator at the reception, for which this characteristic is chosen to be constant from the lower boundary frequency of the communication channel to a frequency lagging behind the upper limiting channel frequency by twice the frequency deviation value, and the linearly falling frequency in the remaining segment. 2. A device for carrying out the method according to claim 1, characterized in that, in order to reduce the non-linear distortion of the signal and restore its compressed dynamic range, a low-pass filter with a flat frequency characteristic is used as discriminator, the output of which is connected to a push-pull amplifier in the class mode "WITH.