JP3411328B2 - Reconstruction and chrominance demodulation of color subcarrier frequency - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color subcarrier frequency recovery circuit having a burst phase detector for detecting the burst phase of a PAL signal. The invention also relates to a chrominance demodulation circuit in which the burst phase of the PAL signal is detected. The invention also relates to a receiver for PAL video signals.

[0002]

Such color subcarrier frequency recovery and chrominance demodulation is disclosed in US Pat. No. 3,877,066. With the PAL color television system,
Color information is modulated on alternating subcarriers. Therefore, a color subcarrier frequency reference signal burst is sent between each line of the television signal name during the color burst period. The phase of this burst alternates from line to line between 135 ° and 225 °. In the NTSC color television system,
The burst phase is fixed, it is 180 °.

In the color subcarrier frequency recovery circuit, this burst is used to recover the internal color frequency signal, which is phase locked to the burst. In the phase detector,
Voltage-controlled oscillator (VCO)>
The reference signal from is compared to the burst. A low pass loop filter connects the output of the phase detector to the phase control input of the VCO. The combination of the phase detector, low pass loop filter and VCO constitutes the phase locked loop.
The desired control behavior is the appropriate loop gain and low pass loop
It is obtained by selecting and. The behavior of the loop must be swift to allow the loop to adjust rapidly to the color burst, but from a noise standpoint, the behavior of the loop must be slow, resulting in a compromise. It must be. Generally, the damping factor is 0.5 and the resonance frequency is 200 Hz.
Is selected.

[0004]

By choosing the damping factor and the resonance frequency, a compromise is obtained between the desired control behavior and the desired noise appearance. In NTSC systems with a fixed burst phase, it is only necessary to suppress the dual color subcarrier frequency. This suppression is achieved by using a small capacitor in parallel with the resistor that forms the proportional part of the low pass loop filter.
However, in PAL systems, the effect of alternating bursts must also be suppressed. These effects are ±
Introduces a 45 ° line alternation phase error. The phase error has a relatively low frequency, i.e. half the line frequency. So in a low pass loop filter,
There is no choice but to increase the capacitance in parallel with the resistance.
As a result, the control action becomes worse. We have to find another compromise. This compromise is extremely difficult and can only be expected with low reliability. From the standpoint of conflicting requirements, the optimum values for the components of the loop filter are generally determined by experiment. Furthermore, in multi-standard receivers, the NTSC and PAL systems use the same loop filter, so the loop filter must be suitable for both standards. Furthermore, when the burst phase is detected at an angle of ± 45 ° rather than 90 °, the signal-to-noise ratio at the phase detector output is NT in the case of PAL signal reception.
It is 3dB lower than the SC signal reception.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of reproducing a color subcarrier frequency in which the deleterious effects of alternating burst phases are mitigated.
According to one aspect of the invention, there is provided a color subcarrier frequency recovery circuit according to claim 1. In another aspect, the invention provides a chrominance demodulation circuit according to claims 2 and 3. Further, the present invention provides claims 4, 5 and 6
The PAL video signal receiver described in 1. is provided.

According to the present invention, when the reference signal given to the burst phase detector also has a line alternation phase, the burst phase is detected at a fixed angle of 90 ° during each line period and the output of the burst detector is detected. Is based on the recognition that an error component of half the line frequency is not introduced. Suppressing this error component is no longer necessary, which greatly simplifies the design of low pass loop filters. Furthermore, the above-mentioned reduction of the signal-to-noise ratio of 3 dB at the output of the phase detector is avoided. With the exception of line alternation phase detection, the control and noise behavior of the color subcarrier frequency recovery circuit is now exactly the same for NTSC and PAL signals.

When multiplying the output of one of the demodulation branches in the chrominance demodulation circuit by the line alternation switching signal before combining it with the output of the other demodulation branch in the burst phase control loop, You can get the benefits of.

In all aspects of the invention, there is a switch at half the line frequency in the phase detection loop during the burst period.

[0009]

EXAMPLES The above and other aspects of the present invention will be clarified and explained in detail by examples.

In the color subcarrier frequency reproduction circuit shown in FIG. 1, the color burst signal is applied to the burst phase detector 1. According to the principles of the present invention, the burst phase detector 1
It has a multiplier 3 for multiplying the color burst signal by the ± sin phase reference signal and a multiplier 5 for multiplying the color burst signal by the cos phase reference signal. The outputs of the multipliers 3 and 5 are added by the adder 7, and the output is connected to the output of the burst phase detector 1. The output of this burst phase detector is coupled through low pass loop filter 9 to the input of voltage controlled oscillator 11. This low pass loop filter 9 will have a structure similar to the smoothing network referenced 15 in FIG. 1 of US Pat. No. 3,877,066. The voltage controlled oscillator 11 supplies the ± sin phase reference signal and the cos phase reference signal to the burst phase detector 1. The multipliers 3 and 5 could also be used to demodulate the chrominance signal.

FIG. 2 shows an alternative embodiment 1'of the burst phase detection circuit used in the color subcarrier frequency recovery circuit of FIG. In this alternative embodiment, the ± sin phase reference signal and the cos phase reference signal are added in adder 13 and the output reference signal is provided to multiplier 15, which also receives the color burst signal. To do. The output of the multiplier 15 is connected to the output of the burst phase detector 1.

The burst phase detection circuit of FIGS. 1 and 2 operates as follows. That is: The color burst signal, where ω is the color subcarrier frequency,

## EQU1 ## It can be written as ± cos (ωt) −sin (ωt). In addition, the phase detection criterion is that if θ is the phase error between the color burst signal and the reference signal,

## EQU2 ## It can be written as Asin (ωt + θ) + Bcos (ωt + θ). The output signal of the phase detector is formed by the product of the reference signal and the color burst signal. Color subcarrier frequency ω
After low pass filtering at low pass loop filter 9 to suppress twice the component of

[Equation 3] ± Asin (θ) −Acos (θ) ± Bcos (θ) + Bsin (θ) Becomes If in accordance with the principles of the present invention,

## EQU00004 ## -A. ± .B = 0 and B = 1 Because

## EQU5 ## A = ± B = ± 1 and ± A = 1 Therefore, the output of the phase detector is

## EQU6 ## (± A + B) sin (θ) = 2 sin (θ). From this, A alternates line by line with +1 and
It turns out that it must be -1. Therefore, the burst phase detection criterion according to this embodiment alternates line by line as seen in FIG.

As a result, color bursts are detected at an angle of 90 ° for each line period, so the output no longer contains alternating components, and hence the low-pass loop.
The range setting <dimensioning> of the filter 9 can be greatly simplified. The signal to noise ratio is improved by 3 dB. The control of the loop and the noise behavior <behavior> are exactly the same for the PAL method and the NTSC method, except for the line alternating phase detection.

Nevertheless, which line is received is +1 of the color burst signal with respect to the U chrominance signal.
It is necessary to determine whether the line is at 35 ° phase shift or the line at −135 ° phase shift (see Figure 3). If you detect the wrong line, it's no longer

[Formula 7] -A ± B = 0 Can't be,

[Equation 8] ± A + B = 0 And

Since -A ± B = ± 2, the output signal of the burst phase detection circuit has an alternating component and its amplitude depends on the phase error, but its time average value is always 0. The loop has no control.

The reference signal is formed by a sine wave signal or a rectangular wave signal. Low pass loop filter 9
Eliminates all harmonic signals introduced by using a square wave signal.

In a very simple embodiment, the burst phase detector 1 is time-discrete, either digital or using switched capacitor technology. In such a time-discrete embodiment, assume that the data clock signal is locked to the color burst and has a frequency that is four times the color subcarrier frequency. If the sine wave signal was sampled as 0 +1 0 -1, the cosine wave signal would be +1 0 -1 0. The sum of the cosine wave signal and the sine wave signal will be +1 +1 -1 -1 and the sum of the sine wave signal and the cosine wave signal will be -1 +1 +1 -1. A very simple multiplier to implement now is extremely accurate and can be easily switched line by line.

FIG. 3 relates to the phase of the U chrominance signal.
3 is a phase diagram of PAL and NTSC color burst signals. The NTSC color burst signal of Figure 3b has a fixed phase shift of 180 ° with respect to the U chrominance signal. Figure 3a
PAL color burst signal has a phase shift of 135 ° with respect to the U chrominance signal during the first line I and a phase shift of 225 ° (ie -135 °) with respect to the U chrominance signal during the next line II. .

FIG. 4 is a diagram for explaining the detection of the PAL burst phase according to the present invention. During both the first line I (shown in FIG. 4a) and the second line II (shown in FIG. 4b), the reference signal REF is 90 ° with respect to the phase of the PAL burst phase signal in these lines. Has a phase shift of.

FIG. 5 shows a first embodiment of the chrominance demodulation circuit according to the present invention, in which the burst phase detection function and the chrominance demodulation function are realized using common components. The input signal is applied to the first branch with the multiplier 3 for multiplication (demodulation) by the sine-phase reference signal -SIN and the low-pass filter 501 for eliminating all 2ω components. The input signal is also fed to a second branch having a multiplier 5 for multiplication (demodulation) by the cosine phase reference signal COS and a low-pass filter 503 for eliminating all 2ω components. Supplied by low pass filter 503
Since the PAL (Phase Alternating Line) ± V chrominance signal has an alternating phase, a multiplier 505 for multiplying the line alternating ± 1 signal Sw shown in FIG. 9a is provided after the low pass filter 503.

According to this aspect of the invention, the output of the U-branch low-pass filter 501 is multiplied by the signal Sw in the multiplier 507, the output signal of which is the adder 7 in the V-branch low-pass filter. It is added to the output signal of the pass filter 503. Adder 7
Is applied to low pass filter 9 through burst gate 509 which conducts only during the burst period.
Burst gate 509 is controlled by burst gate pulse BG. The output of the low pass filter 9 is given to the voltage controlled oscillator 511, and the sine phase reference signal -SIN and the cosine phase reference signal COS are given from the oscillator 511 to the multiplier 3 and the multiplier 5, respectively.

The embodiment of FIG. 5 differs from the embodiments of FIGS. 1 and 2 in that the output signal of the U-branch is multiplied by the line replacement ± 1 signal Sw to achieve the line replacement. The operation of the embodiment of FIG. 5 will be further described later with reference to FIG.

FIG. 6 shows a second embodiment of the chrominance demodulation circuit according to the present invention. In FIG. 6, the output of the low pass filter 501 and the output of the low pass filter 503 are directly connected to the input of the adder 7. There is no multiplier 505. The voltage controlled oscillator 611 supplies a sinusoidal phase reference signal -SIN ', which is line alternating during the burst period instructed by the burst gate signal BG and controlled by ± 1 signal Sw. Is ± sin (ωt), and is simply sin (ωt) outside the burst period. Cosine phase reference signal COS '
Is ± cos (ωt) line-alternately outside the burst period when the burst gate signal BG controls the ± 1 signal Sw.
Which is simply cos (ωt) during the burst period. Since the multiplier 5 is controlled by the line alternation reference signal COS ', the multiplier 505 of FIG. 5 has become redundant. The multiplier 507 of FIG. 5 has become redundant because the reference signal -SIN 'is line alternating during the burst period.

FIG. 7 is a phase diagram of a color burst signal having no phase error, which corresponds to the U signal and the ± V signal demodulated by the demodulation circuit of FIG. 5 during the reception of the color burst signal. The PAL burst phase diagram in Figure 7a is identical to that shown in Figure 3a. Figure 7b shows that the U output of the circuit always has the same sign, but the sign of the demodulated ± V signal before the PAL switch 505 alternates.

FIG. 8 is a phase diagram of a color burst signal with a phase error θ, which corresponds to the U signal and the ± V signal demodulated by the demodulation circuit of FIG. 5 during the reception of the color burst signal. In this case, the U pulse and ± V pulse no longer have a fixed amplitude, but instead the "starting amplitude ±
Error component ”.

FIG. 9 shows some of the signals that occur in the chrominance demodulation circuit of FIG. FIG. 9a shows the line changeover switching signal Sw. FIG. 9b is the distorted U pulse of FIG. 8b (output of low pass filter 501) multiplied by the switching signal Sw as provided by multiplier 507. Distorted ± shown in Figure 9c
The V-pulses are the same as shown in FIG. 8b, the pulses being provided by the low pass filter 503. Figure 9d
Is the sum of the pulse of FIG. 9b and the pulse of FIG. 9c, which is obtained by the adder 7. The amplitude of the error pulse shown in FIG. 9d contains information about the magnitude of the phase error θ, which is used in low pass filter 9 to adjust VCO 511 after low pass filtering. Figure 9e illustrates what would happen if the distorted U pulse was multiplied by a switching signal Sw with the wrong phase due to the wrong line number being determined: ie alternating line by line, phase error A fixed amplitude pulse is obtained that has no information about θ.

Those skilled in the art will be able to design numerous alternatives to the embodiments described above without departing from the scope of the claims.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of a color subcarrier frequency reproduction circuit according to the present invention.

2 is a diagram illustrating an alternative embodiment of a burst phase detection circuit for use in the color subcarrier frequency recovery circuit according to the present invention of FIG.

FIG. 3 is a diagram showing a phase diagram of a PAL burst and an NTSC burst.

FIG. 4 is a diagram illustrating detection of a PAL burst phase according to the present invention.

FIG. 5 is a diagram showing a first embodiment of a chrominance demodulation circuit according to the present invention.

FIG. 6 is a diagram showing a second embodiment of the chrominance demodulation circuit according to the present invention.

7 is a diagram showing a phase diagram of a color burst signal having no phase error, which corresponds to the U signal and the ± V signal demodulated by the demodulation circuit of FIG. 5 during the reception of the color burst signal. Is.

8 is a diagram showing a phase diagram of a color burst signal with a phase error θ, which corresponds to the U signal and ± V signal demodulated by the demodulation circuit of FIG. 5 during the reception of the color burst signal. It is a thing.

9 is a diagram showing some of the signals that occur in the chrominance demodulation circuit of FIG.

[Explanation of symbols]

1,1 'Burst phase detection circuit 3, 5, 15, 505, 507 Multiplier 7,13 adder 9 Low pass loop filter 11,511,611 Voltage controlled oscillator 501, 503 Low pass filter 509 Burst Gate

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Henrikus Johannes Heerkens Netherlands 5621 Beer Eindowen Frühne Wautzwach 1 (56) References JP-A-60-150394 (JP, A) JP-A-63-117587 (JP) , A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H04N 9/45

Claims (4)

(57) [Claims] 1. A chrominance demodulation circuit for a PAL signal having a color burst period, a color burst frequency, and a line frequency, wherein an input signal is multiplied by a sine wave phase reference signal of the color burst frequency to produce a first output. Multiplying means for supplying a signal; multiplying means for multiplying the input signal by a cosine wave phase reference signal of the color burst frequency to supply a second output signal; the first output signal and the second output signal A chrominance demodulator circuit comprising: summing means for adding the output signal; and means coupled to the summing means for generating the sine wave phase reference signal and the cosine wave phase reference signal. A chrominance demodulation circuit, wherein the positive and negative signs of the sine wave phase reference signal alternate at half of the line frequency during a cycle. 2. A chrominance demodulation circuit for a PAL signal having a color burst frequency and a line frequency, wherein an input signal is multiplied by a sine wave phase reference signal of the color burst frequency to supply a first output signal. Multiplier means for multiplying the input signal by the cosine wave phase reference signal of the color burst frequency to provide a second output signal; the first output signal and the second output signal A summing means for adding; and means for generating the sine wave phase reference signal and the cosine wave phase reference signal coupled to the addition means; A chrominance demodulation circuit characterized by multiplying a switching signal of half of the line frequency before adding to the other output signal. 3. A receiver for a PAL video signal having a line frequency, a color burst period, a color burst frequency, and a chrominance signal modulated on a subcarrier at the color burst frequency, the receiver comprising: A chrominance demodulation circuit for demodulating the chrominance signal, and multiplication means for multiplying the input signal by the sinusoidal phase reference signal of the color burst frequency to provide a first output signal; Multiplier means for multiplying the input signal by a cosine wave phase reference signal of the color burst frequency to supply a second output signal; an adding means for adding the first output signal and the second output signal; And a means for generating the sine wave phase reference signal and the cosine wave reference signal in combination with the summing means; In the color burst period, the receiver for PAL video signal, wherein a positive or negative sign of the sinusoidal phase reference signal alternates at half the line frequency. 4. A line frequency, a color burst frequency,
A receiver for a PAL video signal having a chrominance signal modulated on a subcarrier at the color burst frequency, the receiver having a chrominance demodulation circuit for demodulating the chrominance signal, and A multiplying means for multiplying the input signal by the sine wave phase reference signal of the color burst frequency to supply the first output signal; and 2 by multiplying the input signal by the cosine wave phase reference signal of the color burst frequency. A multiplying means for supplying a second output signal; an adding means for adding the first output signal and the second output signal; and a sine wave phase reference signal and a cosine wave phase coupled to the adding means A receiver for a PAL video signal, including means for generating a reference signal, wherein one of the output signals is added to the output signal before the other is added. Receiver for PAL video signal, characterized by multiplying half the switching signal emission frequency.