DE2221887C3 - SECAM television decoder - Google Patents

Description

2. SECAM television decoder according to claim 1, characterized in that the frequency discriminator (21) an amplitude modulator (24) is connected downstream, which the at the discriminator output (22) existing demodulated signals of a higher frequency supplied by an oscillator (25) Oscillator oscillation modulated.

3. SECAM television decoder according to claim 1, characterized in that a bucket-chain delay circuit (10a) for the demodulated color difference signals (R- V) and (B-Y) is connected to the discriminator output (22).

4. SECAM television decoder according to claim 1, 2 or 3, characterized in that the frequency discriminator (21) a single limiter (5) is connected upstream, which has a saturation control circuit for both color channels (17) contains.

The invention relates to a SECAM television decoder in which to demodulate for each color in alternating lines of color difference signals frequency-modulated around different carriers The only discriminator that can be switched line by line in its center frequency is used and in which the Color information of successive lines made available simultaneously with the help of a delay memory and a SECAM switch operated at half the line frequency.

Such a decoder has already been proposed in DT-OS 13,937. From DT-OS 15 37 516 is on the other hand, a SECAM color television decoder is known, in which the color information of successive lines can also be made available at the same time with the help of a delay memory and one with the half line frequency operated SECAM switch is provided, and the delay memory and the SECAM switch to the discriminator to separate the sequential and alternating at its output occurring demodulated color signals are connected downstream.

In the SECAM system, only the R- K information or the B- K information occurs individually in successive lines and this information appears in the form of a frequency modulation on the color carriers, which differ slightly in their frequency. The color subcarrier frequency is 4.406 MHz for the R- K signal and 4.25 MHz for the B- K signal. With the aid of a delay line, the delay duration of which corresponds to a line length, the R-Y information of the previous line is made visible on the screen in each line which does not contain any R-V information. The same applies to the SK information. The two color signals are derived by any of the known frequency discriminators, which convert frequency deviations from a center frequency into corresponding amplitude fluctuations. All of these discriminators use frequency-sensitive time constant circuits.

The object of the invention is a structurally more favorable embodiment of the older principle according to DT-OS 21 13 937 in the sense of a more economical use of assemblies and circuit parts, a reduction in the number and difficulty of adjustments and operating settings and in a reduction in the risk of mutual interference two color signals R-Y and B-Y. Furthermore, the input signal fed to the SECAM receiver should not be changed.

This object is achieved by the features specified in claim 1. The art the tuning of the frequency discriminator between its two center frequencies by switching of a time constant component represents a structurally easy to implement and functionally reliable Solution, which also does not lead to undesired signal corruption.

Developments of the invention are set out in the subclaims. The invention is as follows Hand of the representations of some embodiments explained in more detail. It shows

Fig. 1 shows a typical SECAM decoder of known type,

F i g. 2 and 3 different embodiments of the invention,

F i g. 4 shows a delay memory circuit, FIG. 5 and 6 different frequency discriminators with a switched tuning time constant reactance.

According to FIG. 1 leads a color input line 3 sequentially the frequency-modulated R-Y and B-Y color carriers of successive lines to the usual "bell filter" 4 to compensate for the amplitude modulation of the frequency modulation signal, and then via a limiter 5 to the connection point 6, where the signal is fed to two input connections 7 and 8 of a two-pole changeover switch 9 is distributed. The branch leading to terminal 7 contains a delay element 10 with a delay duration of 64 μ5, that is to say one line duration, compared to the branch leading to terminal 8. The two output connections 11 and 12 feed an R- K discriminator 13 and a B- K discriminator 14 via an R- K limiter 15 and a B- K limiter 16, respectively. The common saturation adjustment potentiometer 17 determines the limiting effect of the limiters 15 and 16; however, separate adjusters can preferably also be provided for the possibility of adjusting the color balance.

The switch 9 is for each line with the help of the identification square wave 18 of 7.8 kHz (half European line frequency) switched over, so that each one discriminator receives the undelayed signal from the terminal 8 and the other the signal delayed by the delay line IO at the same time receives. The switch 9 is the usual SECAM switch, which the /? - K signals and the ß-K signals

to their assigned correct discriminators. The SECAM switch processes carrier frequency signals, which go up to 4 MHz.

In contrast to this, the circuits according to FIGS. 2 and 3 no switching. for the carrier frequency range, so that two limiters and a discriminator are saved. Furthermore, the DC voltage level adjustments between d_-n two demodulated color signals simplified.

According to FIG. 2, the common saturation setting potentiometer 17 acts on the limiter 5, and its output signal is fed directly to a common discriminator 21 via the signal line 20, which is initially set to 4.406 MHz center frequency for one line for demodulating the R- V signal and is then switched to 4.25 MHz for the next line in order to make the β-V signal available on the output line 22, etc., ίο that the two color signals appear sequentially. The square wave 18 is now used to change the tuning of the common discriminator 21 between the individual lines, and for this purpose a capacitance or another time constant element (which is not shown in FIG. 2) in the discriminator is connected to ground every two lines, and via a control line 23 and a transistor 19, which is driven by the square wave 18 at its base so that it is saturated.

There still remains the problem of providing a delay memory with a delay time of one line in order to transfer the blue color information of the previous line into each line without receiving any B- V information; the same applies to the red information. However, a delay of 64 μβ for a demodulated color signal that occupies a wide band of low frequencies (for example from direct current to!, 5 MHz) cannot easily be realized. The in F i g. The embodiment shown in FIG. 2 illustrates a delay technique, FIG. 3 another.

The output 22 of the discriminator in FIG. The output signal appearing in FIG. 2 is modulated with the aid of a modulator 24 as amplitude modulation, oscillator oscillations of approximately 3 to 5 MHz, which originate from an oscillator 25. The broadband nature of the color signals is less critical in the higher frequency range, which in itself permits the use of normal SECAM or PAL delay lines. It is true that the oscillator and the amplitude modulator represent additional components compared to FIG. 1, but the frequency can be relatively imprecise or unstable because there are no special constancy requirements. The two re-modulated color signals are again divided into a delayed and an undelayed signal path at the switching point 6, these signal paths leading to the SECAM switch 9 and the separate β, Y and RV signals again being demodulated by simple diode demodulators 26, 27. There are fewer crosstalk problems between these two signals than with the circuit of FIG. 1. Crosstalk occurs because only one of the carriers is used at a time and the low frequency beating between the standard SECAM color carriers is avoided. Is a delimiter of the delay line and the switch nachgeschaliet (Fig. 1), then ^fl 5, the relationship between the desired and the undesired carrier to reduce the inclination for each color, and therefore, these unwanted beatings are increased.

If only a single discriminator is used, a phase shift of the zero point has a shift of the DC voltage level of the two differential signals by the same amount. There are so no color errors and only a slight change in brightness can be observed on the screen. Also the Saturation setting, which is often the DC voltage level of the differential signals at the output of the discriminator shifts, can only result in a change in brightness.

The in F i g. The alternative embodiment illustrated in FIG. 3 uses the same assemblies up to the discriminator output 22, which branches directly to the delayed and the non-delayed line in the circuit point C *. Since it is very inconvenient to delay low frequencies with passive components by exact amounts, an active delay line, known per se, is used here, which is described as a bucket chain in the literature (see FIG. 4), in which two out-of-phase clock pulse trains shifts a charge level across a series of cascaded transistors. Analog samples are taken from the signal to be delayed at the frequency of the clock pulses. The frequency of the clock pulses should of course be higher than the highest frequency component of the signal on line 22 by an amount which depends on the bandwidth required. The clock frequency can also be supplied externally and controls the step-by-step advancement: the total delay is determined by the number of steps. The lower frequencies no longer offer difficulties (actually even less with regard to the bandwidth) for such a delay circuit which, because the delay is dependent on the clock frequency, is also independent of internal changes. Such circuits can also be easily produced in integrated form.

In the circuit according to FIG. 3, the output signals of the undelayed and the delayed conduction path with regard to the colors are again separated by the SECAM switch 9. Compared to the circuits according to FIGS. 1 and 2 there are fewer crosstalk problems since the color signals are available at the same time as a result of the effect of the delay circuit. The reason for this is that the only crosstalk can occur between the low-frequency den.odulated signals. The modulated color signals no longer exist at the same time, which leads to problems, especially with printed circuit cards. If the execution is not extremely good, then the frequency difference between the SECAM color carriers for the RV and B- V color difference signals is clearly visible on the screen. In the circuit according to F> g. 2, instead of three limiters, only one is provided, and instead of two frequency discriminators, only one is provided.

Another advantage of the circuit according to FIG. 3 compared to that in FIG. The prior art illustrated in Fig. 1 is that the saturation control changes the amplitudes of the signals RV and BY by the same amount (because the two signals pass through the same limiter). Adjusting the saturation does not lead to different DC voltage level shifts for the signals R- Y and B- V, and therefore no color error is visible on the screen (only one frequency discriminator).

The F i g. 5 and 6 show circuits in which a

frequency or time constant dependent circuit element of a frequency discriminator, about 21 in the F i g. 2 and 3, inserted or detached in circuits and in this way the center frequency of the discriminator is changed cyclically.

In the case of the FIG. 5, a coincidence detector 28 subtracts a phase-sensitive discriminator demodulated form of an oscillation arriving on path 29 to line 20 (FIG. 2 or 3) of a phase-sensitive demodulated form of the same oscillation, which along the path 30 through the 180 ° phase shifter 31 is guided. The phase shift is more or less than 180 °, as long as the frequency of the oscillation is not exactly on the Center frequency is, and the subtracted input signal, which is from the coincidence detector 28 on the line 22 results in an output signal illustrated by reference numerals 32 and 33 Shapes when there are deviations from the center frequency. The phase shift by the phase shifter 31 is only at a frequency of 180 °, which is determined by the reactance 34, and this is for every second Line under the control of the identification oscillation 18, which saturates the transistor 19 on Ground placed so that the desired milten frequency is set. Such discriminators are known

S and the time constant circuit 31 are used to change the center frequency.

Fig. 6 shows another known form of frequency demodulator; the components can have the specified values for use as a convertible demodulator. The capacitor 35 is connected or disconnected from the secondary winding of the transformer during alternating rows and the signals fl-V and R-Y appear alternately on the output line 22. The reactances shown influence the tuning, and the output oscillations are the same as those on the Output line 2 \ des in FIG. 5 shown discriminator. The source of the square wave 18 is normally the identification flip-flop, but any other control source operating at half the line frequency can also be used.

For this purpose 2 sheets of drawings

Claims (1)

Patent claims: 1. SECAM television decoder used for demodulation from occurring in alternating lines for each color to different carriers frequency-modulated A single color difference signal, its center frequency can be switched line by line Discriminator is used and in which the color information of successive lines with the help a delay memory made available at the same time and one with half the line frequency operated SECAM switch are supplied, characterized in that one with the half line frequency operated component (19) for tuning the frequency discriminator (2S) is provided via a time constant component (34 or 35) contained in it.