DE1175731B - Demodulation circuit for signals of a color television system - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Boarding school KL: H 04 η

German class: 21 al - 34/31

Number: 1175 731

File number: T 23919 VIII a / 21 a

Filing date: April 27, 1963

Opened on: August 13, 1964

The invention relates to a simplified demodulation circuit for signals of an NTSC system line-frequency switching of a modulation axis (NTSC-PAL).

For an NTSC system with line-frequency switching of a modulation axis, a so-called transit time demodulator has been proposed (earlier application T 23282VIIIa ^ Ia ^{1 of} the inventor), in which the color carrier-frequency signals of two consecutive lines are constantly added in an adder stage and constantly subtracted in a second adder stage . Because of the switchover, the color carrier-frequency signals separated according to the modulation axes and without carriers are produced at the outputs of these adding stages. The reference carrier restored at the receiver is added to these color carrier-frequency signals at the output of both adder stages, after which demodulation is possible with simple amplitude rectifiers.

The invention is concerned with the task of simplifying this carrier addition.

The invention is based on a demodulation circuit for signals of a color television system which a color subcarrier is quadrature-modulated with two color signals with respect to two vertical axes and a modulation axis is switched line frequency by 180 °, the received complex conjugate color subcarrier-frequency signals of two consecutive lines after a delay of the one signal can be added or subtracted in two adding stages and the demodulation of the thereby generated, after the axes separated color carrier frequency signals after the addition of a transmitted color sync pulses restored reference carrier takes place (NTSC-PAL). she consists in that the reference carrier precedes the received complex conjugate color carrier frequency signals the delay, constantly offset by 45 ° with respect to the two modulation axes, is added, where the phase of the added reference carrier according to the line-frequency switchover at the transmitter a modulation axis is switched line frequency by 90 °.

The invention has the advantage that the carrier additive only once, namely in the common path of the complex color carrier frequencies received Signal before the delay device is required. The carrier is thereby in such a Phase position added so that amplitude-modulated color carriers appear at the outputs of both adding stages, which can be demodulated with simple amplitude rectifiers.

Demodulation circuit for signals of a color television system

Applicant:

Telefunken patent collecting company

m. b. H., Ulm / Danube, Elisabethenstr. 3

Named as inventor:

Dipl.-Ing. Werner Scholz, Hanover-Linden

The invention is based on the following mathematical knowledge: The complex color subcarrier-frequency signal has the form in line 1

F ₁ = (Q '+ in

and in the following line 2 the form

In all of the following considerations, it is assumed that the color signals Q ' and /' are practically the same in successive lines, an assumption which is largely correct. The added reference carrier, which is at 45 ° to the modulation axes according to the invention, has the form in line 1

T ₁ = T (1 + j)
and in line 2 the form

T ₂ = T (I - '/).

In the adder, which is before the delay device, are now color carrier frequencies complex color signal and reference carrier composed. This results in:

T ₁ + F ₁ = A ₁ = T + Q '+ J (T + Γ)

and in line 2.

T ₂ + F ₂ = A ₂ = T + Q! ■■ - J (T + / ').

A ₁ and A ₂ are the phase- and amplitude-modulated color carriers obtained by combining the reference carrier and the fairbträgerfrequentem signal. In the so-called transit time demodulator, these color carriers are now timed

: · .Ι. -.C ■ ;; r. - .., ':; «!! .. 409 640/166

consecutive lines added or subtracted. This results in

Qy and // are therefore color carriers that are each amplitude-modulated with only one color difference signal Q ' or /'. Qj and // can be demodulated in simple amplitude rectifiers. The video frequency signals Q ~ ' and T.

The invention is explained in more detail below with reference to the drawings.

In FIG. 1, the color carrier-frequency complex signal without a carrier passes from a terminal 1 to an adder stage 2, to which the reference carrier generated in the reference carrier oscillator 3 is also fed via a switchable 90 ° phase rotating element 4. The reference carrier oscillator 3 is phase-synchronized by the color synchronizing pulses to the frequency of the color carrier via a line S. The phase-modulated and amplitude-modulated color carriers A ₁ and A ₂ at the output of the adder 2 reach the adding stages 8 and 9 via a 180 ° phase rotation element 6 and a line 7 and also via a delay device 10 with a delay of a line duration (total phase rotation of the color carrier 0 _tot = η 360 ° + 180 °; η = 283) also to both adding stages. The 90 ° phase rotating member 4 is switched by line sync pulses 15. At the output of the adder stages 8, 9, which together with the 180 ° phase rotation element 6 and the delay device 10 serve as a so-called transit time demodulator, the color carrier-frequency signals A _t + A ₂ and ± (A ₁ ~ A ₂ ) arise in normal amplitude modulation, which are demodulated in simple amplitude rectifiers 11, 12. The demodulated color signals Q 'and T are then available at terminals 13, 14.

2 shows the vector image for the vectors F ₁ , F ₂ , T ₁ and T _{2 given} in the equations. The index 1 always means the line 1 and the index 2, the temporally subsequent line 2. The drawn changeover of 90 ° from T ₁ to T ₂ is effected by the phase change-over switch 4 in Fig. 1. The vector F ₁ has an arbitrary phase position that depends on the color transferred in each case.

In Fig. 3 shows the addition of the color carrier frequency complex signal F ₁ and the complex reference carrier T ₁ in the adding stage 2 on the basis of vector images. Fig. 4 shows the same addition in the adding stage 2 for the temporally following line.

In FIG. 5, the addition of the phase and amplitude modulated at the output of the adder 2 is shown Color carriers of chronologically successive lines are shown in adder 8. F i g. 6 shows analogous to the subtraction of these signals in the adder 9.

The amplitude rectifiers 11, 12 supply the envelope of the amplitude-modulated carrier A ₁ + A. _z or ± (A ₁ -A ₂ ). The polarity of the resulting demodulated signals T and Q ' is therefore only dependent on the polarity of the diodes in the amplitude rectifiers 11 and 12 and not on the phase of the carrier-frequency signal. Therefore, the changing sign in the output signal of the adder 9 (FIG. 1) is also irrelevant. A particular advantage of the invention arises when the reference carrier is generated in a start-stop oscillator and its phase is changed by 90 ° from line to line by a color sync pulse that is different from the NTSC signal. This can be achieved, for example, by changing the phase of the oscillations contained in the color sync pulses at line frequencies, which then cause the start-stop oscillator to start oscillating in successive lines with different phases at the beginning of a line. This results in a particularly simple demodulation circuit for signals of the NTSC-PAL system, since then no switching meander and no 90 ° phase rotating element are required. The additional burst that ensures the correct polarity of the switching meander can also be omitted. The phase shifter 4 switched by the line sync pulses 15 in FIG. 1 would then be omitted, and the phase position of the reference carrier supplied by the oscillator would already be in each line via the line 5 through the color sync pulses corresponding to F i g. 2 set to the correct value.

Claims (2)

Patent claims: 1. Demodulation circuit for signals of a color television system ^, in which a color carrier is quadrature modulated with respect to two vertical axes with two color signals and a modulation axis is switched line frequency by 180 °, the received conjugate complex color carrier frequency signals of two temporally successive lines after delaying the one signal in two Adding stages are added or subtracted and the demodulation of the color carrier frequency signals generated thereby, separated by the axes, takes place after the addition of a reference carrier restored by transmitted color sync pulses (NTSC-PAL), characterized in that the reference carrier (T ₁ , T ₂ ) conjugates the received one complex color carrier frequency signals (F ₁ , F ₂ ) before the delay (10), constantly offset by 45 ° with respect to the two modulation axes (/ ', Q') , the phase of the added reference carrier (T ₁ , T ₂ ) according to the line f on the transmitter side frequency switching of a modulation axis (/ ') line frequency is switched by 90 ° (4). 2. Color television system according to claim 1, characterized in that the reference carrier in a start ^ stop oscillator is generated and that the phase of the reference carrier in each line through a phase of the phase contained in the color sync pulses, which differs from line to line Color carrier frequency vibrations is set. 1 sheet of drawings 409 640/166 p. 64 <$ Bundesdruckerei Berlin