DE1963464A1 - Television receiver circuitry to compensate for phase errors - Google Patents

Description

Patent attorney

Aeasfder; Philips PatenWerwalt-mg Gmba

Act « PHD- 1439
Registration voiai , I7, December I.969

Philips Patentverwaltung GmbH, 2 Hamburg 1, Mönckebergstr.

"Television receiver circuit arrangement for compensating for phase errors ¹¹

Addendum to patent 1 904 528 and 1 940 563

The invention "relates to color television receiver circuitry to compensate for phase errors of a color television transmission with a periodic in phase Switched modulation component of a color subcarrier (PAL), where the switched and the not switched Components that are taken from the synchronous demodulator, the portions above the switching frequency are filtered out by filters and combined with the remaining parts of the lower frequency! are preferably multiplied, and form a rule variable proportional to the error angle through which the phase of the color subcarrier reference oscillator controlling the demodulation is readjusted according to this product to a minimum © phase deviation.

According to the exemplary embodiments for the patents mentioned the signals taken from the demodulator are passed through filters

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exempted from the components lying above the switching frequency; this will be used for the following combination Average values formed over at least one line duration.

However, if the mean value of the color signals over a. Row practical Is zero, the output variable of the filter is practically zero and no controlled variable is formed. Then you can Phase errors have the effect as color distortions.

In a circuit arrangement of the type mentioned at the beginning these disadvantages are avoided if according to the invention the signals only during a, preferably small, partial interval of the line period are fed to the filters. Then only this signal part is evaluated, and in the Practice shows that it is far less likely that the mean value is in a certain signal part the color signals become zero. Not even with one

at

standing image, ζ \ B. a test image, for a longer period of time Ratios may exist, is preferably the position of the partial interval changed periodically; suitable for this a frequency that is small compared to the scanning frequency and is preferably about 0.5 to 5 Hz.

The invention is explained in more detail below with reference to a drawing that provides a schematic representation explained.

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A chrominance signal voltage u ™ transmitted in accordance with the PAL system is fed to a synchronous demodulator 1 at a carrier frequency of, for example, 4.43 MHz. - ^U (RY} ^o f "fc * ^{re s} · I ^m I ¹ SLIE a phase error of chrominance signal u ~ let the distorted signal voltages represented as follows?

^U 1 ^{= U} (BY) ^COS * - ^ (RY) ^sin * (Ό

^cos Φ ""

U / τ, _v n and Uz _{15 Ύ} \ are the color difference signal voltages caused by the transmitter, while ψ denotes the phase error.

The signal U ₂ is fed to a periodically operated pole changer switching stage 3 in such a way that the component TJ / -O y \ cos φ appears at the output with a constant sign.

The color sync signal is fed to terminal 4 of a stage 5, which on the one hand produces a constant reference carrier oscillation to the synchronous demodulator 1 and also supplies a control signal for the switching stage 3 in the correct phase.

The components supplied by the synchronous demodulator 1 (cf. formulas 1 and 2) are passed through low-pass filters, which consist of longitudinal

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resistors 10 and 11 and cross capacitors 12 and exist and whose cutoff frequency is slightly below the Umsehaltfrequenz, preferably half the line frequency, fed to a combination stage 2. As is known, it can be assumed that the signal content of two image lines transmitted one after the other largely coincide. A mean value of the supplied signals is formed by the low-pass filters, whereby the components with alternating signs in the cycle of the switchover drop out, so that signals U ₁ and U ₂ remain:

U ₁ = U ( _B _ _Y ) cos ♦ (3)

U ₂ - - ^Ü (BY) ^sin

Both of these signals contain the blue color difference voltage U / _B _ _r \ ♦ as an amplitude factor by multiplying these signals to obtain a quantity U ^ that is suitable for phase correction:

U _k BU ₁ U ₂ = (U _(B _ _Y)) ² * sin 2 ^Λ ♦ (5)

This product U _k is therefore independent of the sign of the blue color difference _{voltage U / B} _y \, but is proportional to the product of the sine and cosine of the phase error angle ψ and thus to the sine of twice the phase error angle 2 Ψ; consequently its sign changes with the

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Sign of the phase error. This term thus results in a controlled variable with which a between the color subcarrier oscillator switched on in stage 5 and demodulator stage 1 Phase shifter 8 is controlled such that the added subcarrier is readjusted in accordance with the phase error Ψ contained in the chrominance signal, see above that the demodulation takes place in the direction determined by the transmitter and the error in the output signals is practically eliminated.

The sign of the amplitude factor U / -g γ \ can also be can be made ineffective in another way, e.g. by forming the quotient TJg / U ^ or by switching XIg accordingly the sign of U ^.

The response time constant of the control should be small and preferably in the order of magnitude of a few line periods, so that even short chrominance signals require the correction set; but the decay time constant of the regulation should be large enough compared to a picture perlode and e.g. be more than 10 to 100 ms.

Practically the same controlled variable is obtained if the incorrect color difference signals i

^Ü (BY) ^cos * ± ^ (RY) ^Sin

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^U 2 ⁼ i ^U (RY) ^cos * " ^U (BY) ^Sin ♦ (2)

in time with the switching frequency changes sign and then supplies it in the "described manner, the multiplication stage. Since the signal is u ₂ subjected already at the stage 3 a periodic · polarity change is to only one additional pole change-step in the supply of the signal U ₁ required for multiplication stage 2; the other signal could be fed from the output of stage 3 to multiplier stage 2. This additional change in sign means that the components that are proportional to U / ü_y) then have a sign that is independent of the switchover, while the components with U ( _B _y \ change in sign periodically and are therefore filtered out by the low-pass filter. After multiplication, one then obtains a controlled variable

2 - sin 2 *. (6)

If you put both ways of generating the controlled variable together is also used if one of the color difference components Becomes zero, still a phase control; a control option is only omitted if both color components

to
nents become zero at the same time.

In the Sohaltung after DBP 1,904,528 the signals are

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U ₁ and U ₂ are fed directly to resistors 10 and 11. According to the invention, these supply lines are routed via a switching stage 15 'which allows the signals to pass through only during a part of, for example, 5 to 20 # of the period of a line scan. The averaging by means of the filters 10, 12 and 11, 13 thus only takes place during part of each line. Since the controlled variable in stage 2 is stored for a certain time by RC elements or the like, this does not have a detrimental effect on the readjustment of the oscillation from oscillator 5.

The gating pulses for the switching stages 15 are from a generator 16 supplied by the terminal 17 by sync pulses or return pulses of line frequency is synchronized.

In order to prevent the signals passed through to form the controlled variable having the mean value zero even in the case of a stationary image, the scanning interval can be shifted periodically by means of an oscillator 18, for example sinusoidally or sawtooth-shaped. The oscillator 18 can effect this shift with a frequency of 1 Hz in such a way that the scanning takes place temporarily in the left part of the picture, temporarily in the middle and temporarily in the right part of the picture.

Patent claims:

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Claims (3)

Patent Approach ■ \ 1 i Farlafe? ^ Sehßriip | ängex - Schal1; ungsaiii3rd.nuiig zum Ausgleiph νοϊΐ phase error ?! A television transmission with a periodically switched over in the ptiase Mpglulatipns-Kpmppnente of an i'arfetrM-gers (PAIj), in which from the unswitched-and 'the not switched component, which are taken from the Synphrqndgmpjiulatpr. about the? ümschaltfrecjüenz lying m portions of aurph filter are affixed and leave Δβτ the. Any remaining portions of lower ffreg.ue, nz miteinan, f | er kpmbiniert, preferably multiplied, and, form a control variable proportional to the error angle, through which; ' the phase of the cjie pemod; ulatipn controlling color carrier reference oscillator (5.) is readjusted according to this prqdukt to minimal phase deviation, according to patent ....... (patent application P 19 OA 528.4-31) and patent ...... . (Patent application P 19 40 563.1), characterized in that the signals only k .during one, preferably small, partial interval of the line period fed to the filters 2. Circuit arrangement according to claim 1, characterized in that that the phase position of the sub-interval, preferably is sinusoidal or sawtooth, periodically changed. 3. Circuit arrangement according to claim 2, characterized in that the change frequency is small compared to the grid, frequency and preferably about 0.5 to 5 Hz. 109829/07 0 1