Coding of TV channels in cable networks of collective TV reception is fulfilled by coders that are inserted in a loop break in signal transmission from receiving to transmitting terminal of the head-end station (Figure 1).

Figure 1. Connecting coders to the equipment of the head-end station of collective TV reception network.

    After coder creates a modified video signal a radio-frequency modulator of the video signal creates modified radio signal of TV image with levels, which is than transmitted to the collective TV reception network.
    Oscillograms and levels of initial and modified radio signal of TV image are represented at the Figure 2 and in a table accordingly.

Table. Rating values of the levels of initial and modified radio signal of TV image.

Levels of radio signal of TV image	Initial radio signal according to the State Branch Standard 7845	Modified radio signal "CryptOn"
Line and field sync pulses, %	100	45
Black-out line and field pulses, %	75	45
White level, %	15	15
Minimum level, %	7	7

Figure 2. Oscillograms of the initial and modified radio signal of TV image.

    The renewal of the coded TV signals is fulfilled by subscriber decoders, series "Crypton R".
    Decoder is inserted between subscriber lead-out of the collective TV reception network and radio-frequency input of TV receiver.
    To receive control signals a decoder is connected to the video output of TV receiver.
    Power supply of a decoder is carried out from ac power supply with voltage 220V and frequency 50Hz.

Figure 3. Connecting subscriber decoder "Crypton R" to the TV receiver in collective TV reception network.

    The renewal of sync and black-out pulses is accomplished by means of changing the transmission coefficient of high-frequency section of a decoder.
    To renew black-out pulses the transmission coefficient is fixed as:

    To renew sync pulses the transmission factor is fixed as:

    The declinations of the levels of the modified radio signal at the input of the subscriber decoder from the rating values leads to the error in the renewal of the levels of sync and black-out line pulses and as a consequence to the distortion of the renewed image (Figure 4).



а) - decrease of image contrast because of radio signal limitation by main amplifier overload;
b) - decrease of image contrast because of small swing at the modulator input;
с) - absence of gamma gradation on light and dark parts of the image, sync disorder because of big swing of video signal of the modulator.

    Distortions of the levels of the modified radio signal of the image may be caused by nonlinear distortions of cable network repeater equipment (Figure 4a), by insufficient or by excessive level of video signal range at the input of a TV image radio-frequency modulator (Figure 4б, 4в).
Distortions can be also caused by particularities of modulator construction, which automatically support levels of the image radio signal when changing input video signal parameters in a standard form. To stabilize the levels of the black-out and sync pulses a scheme of the renewal of the constant component (RCC) is applied, to stabilize the white level a scheme of automatic regulation of the input video signal level (ARL) is applied.
    When working with the modified video signal the incorrect work of these schemes can cause distortions of the video signal, as the result of what a distorted modified image radio signal is formed at the output of the modulator.
    The work of the ARL scheme can also result in modulation of the video data levels depending on the structure of the image, and as a result in the loss of the signal identification of the coded channel by subscriber decoder with the following switching off the procedure of decoding.
To exclude influence of these schemes on the work of the modulator some changes disabling their work are made in the construction of the modulator.
    The exclusion of the influence of the RCC is usually reached by means of raising time constant of the input circuit, or by video signal injection direct to the input of the modulator, going by the repeater. The ARL scheme is usually disabled by the fixation of the voltage feedback, for example, for chips TDA5666-5 a divider fixes voltage at the pinout 11 at the level about 3V.
    In the case of exclusion from the design of the modulator of the input video amplifier circuit in coders series K-303 an electronic regulation of the swing and shift of the constant component of the video signal at the output of each of the 4 channels is provided, and also an automatic regulation of the amplifying of the input signal of the each of the 4 channel has been introduced.
    Control over the modified image radio signal levels is fulfilled by means of changing levels of radio signal, which is renewed with the help of the exemplary decoder with the accepted values of the ratio between Kb and Ks.



    Control of the levels is fulfilled while giving to the coder input television pattern signals:

• with the alternation of white and black lines;
• with the alternation of white and black fields;
• by the change of black field to the white one and backwards in a steady mode of work of the modulator.

    By regulating the level of video signal at the input of the modulator the equality of the black level and the renewed black-out level on the whole image frame should be reached.