JPH07123304B2 - Color signal demodulation circuit for digital television receiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention (Industrial field of application) The present invention relates to a color signal demodulation circuit for a digital television receiver.

(Prior Art) In recent years, with the remarkable progress of semiconductor device and integrated circuit technologies, digital signal processing technology has begun to change from conventional analog signal processing technology. In television receivers, signal processing, which was conventionally difficult, is realized by digitization, and higher quality image reproduction is becoming possible. In such a digital television receiver, luminance / chromaticity (hereinafter referred to as A / C) separation using a digital memory and new issue processing using a digital filter are performed.

FIG. 5 is a structural example of a digital television receiver.

A baseband television signal is input from the terminal 1. The television signal is, for example, an NTSC composite video signal. A signal from the terminal 1 is converted into a digital signal by an analog / digital (hereinafter referred to as A / D) converter 2. The A / D converter 2 operates by the clock from the synchronizing circuit 3. The composite signal output from the A / D converter 2 is a vertical low pass filter (V-LPF) in the vertical direction.
4 separates Y / C in the vertical direction. The luminance signal Y _L output from the vertical low pass filter 4 is added to the adder 5
At, it is subtracted from the original signal. The output of the adder 5 is
It is output as the luminance signal Y _H through the horizontal of the horizontal low-pass filter (H-LPF) 6.

The luminance signals Y _L and Y _H are added by the adder 7, and the adder 7
From the luminance signal Y.

On the other hand, the color signal C is obtained by subtracting the output signal of the horizontal low-pass filter 6 from the input signal of the horizontal low-pass filter 6 in the adder 8. The output of the adder 8 is input to the color signal demodulation circuit 100 surrounded by a broken line.

The operation of the color signal demodulation circuit 100 will be described with reference to FIG. In the color signal C input to the color signal demodulation circuit 100, the i signal and the q signal are quadrature-modulated and expressed by the following equation.

C = i (t) cos (2π _SC t + θ) ・ q (t) sin (2π _SC t + θ)…
(1) However, the frequency of the f _SC color subcarrier and the frequency spectrum of the i (t) and q (t) signals are I
(), Q (). The modulation spectra of I () and Q () are shown in FIG. 6 (a).

As shown in FIG. 6 (a), the i (t) signal is a vestigial sideband signal. Therefore, the color signal C needs to be spectrally shaped by a bandpass filter having a complementary symmetrical characteristic with respect to _SC as shown in FIG. 6B (called Nyquist spectrum shaping). ). This bandpass filter corresponds to the bandpass filter 101 in FIG. This bandpass filter 101
The color signal C input to is output after being spectrum-shaped and input to the synchronous detection circuits 102 and 103.

The synchronous detection circuit 102 uses the carrier 5 having the same phase as the i (t) signal.
1 is input from the synchronizing circuit 3 and demodulates the i (t) signal. Further, the synchronous detection circuit 103 receives the carrier 52, which is shifted by π / 2 phase from the carrier 51, and demodulates the q (t) signal. However, when demodulating the q (t) signal, i
(T) The quadrature component of the signal is mixed in the high frequency band of 0.5 to 1.5 MHz, which is removed by the low pass filter 14 in the subsequent stage.

The frequency spectrum after demodulation is as shown in FIG. 6 (c). If the frequency characteristic of the bandpass filter satisfies the characteristic shown in FIG. 6 (b), i (t), q
The (t) signal can be demodulated without distortion.

Next, the luminance signal and the demodulated I and Q signals are respectively D / A
The signals are input to the converters 9, 11, 13 and converted into analog signals, and then input to the low-pass filters 10, 12, 14. And
The output signals of the low-pass filters 10, 12 and 14 are converted into RGB signals by the matrix circuit 15 and input to the monitor 16 for image reproduction.

(Problems to be Solved by the Invention) As described above, in the color signal demodulation circuit of the conventional digital television receiver, the bandpass filter having the frequency characteristic shown in FIG. 6B is required. . If the characteristics of the bandpass filter are insufficient, waveform distortion will occur when the color signal is synchronously detected and demodulated.

Since this bandpass filter must have a linear phase, it is composed of a transversal digital filter.

Therefore, FIG. 7 shows an example in which the proposer of the present invention designed a linear phase transversal type digital filter.

FIG. 7 shows an example designed as a Nyquist spectrum shaping filter. In other words, the _SC
This is an example designed to pass the conditions of complementary symmetry of the cutoff characteristics through -0.5MHz, _SC + 0.5MHz and minimize the square error with the desired frequency characteristics.

This filter is an example with 11 taps, but the ripple in the pass band is large and sufficient characteristics are not obtained. To limit the ripple to the detection limit (36.3 dB) or less, it is a design method using a window function (Kaiser window)
Requires 65 taps. In order to obtain the characteristic of FIG. 6 (b) by using a filter with a small number of taps, it is necessary to further cascade-connect a high-pass filter to remove unnecessary low-frequency components.

As described above, in order to obtain the bandpass filter having the ideal characteristics shown in FIG. 6B, the hardware is increased. However, in the conventional color signal demodulation circuit, due to price restrictions, the hardware scale is reduced and a bandpass filter with insufficient characteristics is used, and the characteristics of the color signal are deteriorated. This causes distortion of the color signal or impairs demodulation of high frequency components of the i (t) signal.

Therefore, an object of the present invention is to provide a color signal demodulation circuit of a digital television receiver capable of demodulating I signal up to 1.5 MHz and Q signal up to 0.5 MHz without distortion on a practical hardware scale. To do.

[Structure of the Invention] (Means for Solving Problems) The present invention obtains a digital color signal digitally converted by a sampling clock synchronized with the phases of the I axis and the Q axis, and outputs the digital color signal for each predetermined sample. The polarity is inverted to obtain the first signal including the I and Q signals of uniform polarity. Next, the high frequency component of the first signal is removed by a low pass filter to obtain the second signal. Further, the Q signal component is removed from the signal obtained by subtracting the second signal from the first signal, and only the I signal component is doubled in level to obtain the third signal. Then, the signal obtained by adding the third signal and the second signal is sub-sampled at a predetermined sample phase, and I
The signal and the Q signal are separated and demodulated.

(Operation) By the above means, the I and Q signals are processed in the time-division multiplexed signal format, and an independent system of I and Q signal processing is unnecessary. Further, the input / output verification processing of the low pass filter and the extraction of only the I signal component prevent the high frequency region of the Q signal from being disturbed when the I signal high frequency component is added to the output signal of the low pass filter. ing. As a result, a high-quality demodulated chrominance signal can be obtained with a small-scale hardware without using a conventional bandpass filter with a large-scale hardware.

Embodiment An embodiment of the present invention will be described below with reference to the drawings. First
The figure is an embodiment of the present invention. The difference from the conventional television receiver shown in FIG.
The other part is the same circuit. Therefore, the same parts are denoted by the same reference numerals, and the description will focus on the color signal demodulation circuit 200 which is a different part.

The color demodulation circuit 200 is supplied with the color signal from the adder 8 of the luminance / chromaticity (Y / C) separation circuit. Hereinafter, description will be made with reference to the explanatory views of FIGS. 2 and 3.

Now, assuming that the phase of the clock in the analog-digital (A / D) converter 2 matches the phase of the i (t) signal, the color signal C is C = i (t) cos2π _SC t + q (t). sin2π _SC t…
(2) It is expressed by. However, _SC is the frequency of the color subcarrier.

Further, when the sampling frequency is 4 _SC , the color signal C input to the color signal demodulation circuit 200 becomes a data string as shown in FIG. The I and Q signals are alternately located, and two I and Q signals having a sign are inverted to form a pair.

The color signal C input to the color signal demodulation circuit 200 is input to the rectification detection circuit 201. The rectifying and detecting circuit 201 has an input signal shown in FIG. 2 (a) and a frequency shown in FIG. 2 (b).
The product of _{SC and} the square wave signal is calculated, and as shown in FIG. 2 (c), all the positive and negative data strings are converted into positive data strings.

This conversion process can also be realized by detecting the sign bit of digital data and inverting the sign bit for data having a sign bit indicating negative.

The rectified and detected data string shown in FIG. 2C is a data string in which the I signal and the Q signal are time-division multiplexed.
The output signal of the rectification detection circuit 201 is a low-pass filter (IP
F) It is input to 202. Since the input signal of the low-pass filter 202 is a signal as shown in FIG. 2 (d) in which the I signal and the Q signal are time-division multiplexed as described above, the low-pass filter 202 operates as shown in FIG. ) As shown in 4 _SC
D-type flip-flops FF1 and FF that operate with the clock
2, FF3, FF4 can be connected in a cascade connection, and the I signal can be configured by a transversal filter that performs the arithmetic processing of the I signals and the Q signal by the Q signals. The coefficient units K1, K2, K3 are provided for gain adjustment.

As described above, in the low-pass filter 202, by processing the data two by two, it is not necessary to prepare two independent low-pass filters for the I signal and the Q signal, and the hardware can be reduced.

The cutoff frequency of the low-pass filter 202 is 0.5MHz
Then, the output signal of the low-pass filter 202 is as shown in FIG.
As shown in. However, the signal below 0.5MHz of I signal
A signal of L _L , 0.5 MHz or more is represented by I _H , a signal of 0.5 MHz or less of Q signal is represented by Q _L , and a signal of 0.5 MHz or more is represented by Q _H.

Next, the output signal of the low pass filter 202 is subtracted from the input signal of the low pass filter 202 in the adder 204. As a result, the output signal of the adder 204 becomes a data string of I, Q, that is, I _H , Q _H signals of 0.5 MHz or more, as shown in FIG. 2 (g).

The output signal of the adder 204 is doubled by the amplifier 205 (FIG. 2 (h)), and its output is added to the output signal of the previous low-pass filter 202 in the adder 203. Here, the Q signal is a component up to 0.5 MHz, but I _H
Since the quadrature component of the signal (hereinafter referred to as I _H ^* ) is mixed, Q _H
= I _H ^* Therefore, in order to avoid this, in the amplifier 205, gate processing is performed at the timing of the Q _H component, the data is replaced with “0”, and only the I _H component is doubled and output (FIG. 2 (i)). ).

Therefore, the output of the adder 203 becomes a data string such as ..., I _L + 2I _H , Q _L , I _L + 2I _H , Q _L ,.

Here, since I = I _L + 2I _H and Q = Q _L , the output of the adder 203 becomes a data string as shown in FIG. 2 (j).

The output signal of the adder 203 is input to the sub-sampling circuits 206 and 207. In the sub-sampling circuits 206 and 207, as shown in FIG.
Sub-sampling is performed by the synchronization clock CK-Q synchronized with the I and Q signals. As a result, the multiplexed I signal and Q signal are separated, and the demodulation of the color signal is completed (FIG. 2 (l)).

FIG. 3 is a diagram for explaining the above-described signal processing progress using the frequency domain.

The modulated I signal and Q signal each have a region component as shown in FIG. That is, the I signal has a component of _SC- 1.5MHz to _SC + 0.5MHz, and the Q signal has a component of _SC- 0.5MHz to _SC + 0.5MHz.

I signal is a double sideband signal _SC + 0.5 MHz from _SC -0.5MHz, _SC -1.5MHz from _SC -0.5MHz has a single sideband signal. Therefore, when this is detected and converted to a baseband signal, the I _H signal of 0.5 MHz or more has half the amplitude of the I _L signal of 0.5 MHz or less, as shown in FIG. 3 (b). .

Further, when it detects the Q signal into a baseband signal, since the I signal is a single sideband signal from _SC -1.5MHz to _SC -0.5MHz, ^* is the quadrature component I _H of the I _H signal When mixed, the spectrum becomes as shown in FIG. 3 (b).

In the I signal detected as described above, the high frequency component has half the amplitude of the low frequency component, while the high frequency component of the I signal is mixed in the Q signal. Therefore, in this state, that is, if the output signal of the rectifying and detecting circuit 201 is subsampled as it is and separated into the I signal and the Q signal, distortion occurs.

In order to prevent this, the system uses a low-pass filter 202, adders 203 and 204, and the like.

FIG. 3 (c) is the spectrum of the output signal of the low-pass filter 202, and FIG. 3 (d) is the spectrum of the output signal of the adder 204. A low-pass filter with a cut-off frequency of 0.5 MHz for the I signal that is the output signal of the rectifying and detecting circuit 201.
An I _H signal and an I _L signal are separated by 202 and an adder 204. Next, only the I _H signal is doubled by the amplifier 205, and the amplitudes of the I _L signal and the I _H signal are made uniform, as shown in FIG. 3 (e). Then, in the adder 203, the I _H and I _L signals are added, and as shown in FIG.
The signal is being demodulated.

On the other hand, for the Q signal, the low-pass filter 202 removes the quadrature component I _H ^* of the I _H signal, and the Q signal shown in FIG. 6 (d) is demodulated.

4 (a) and 4 (b) show the characteristics of the low-pass filter 202 in terms of frequency vs. level and logarithmic exponent of frequency vs. level.

[Effects of the Invention] As described above, according to the present invention, a color signal is first detected to obtain a time division multiplexed signal of I and Q signals, and then a low pass filter, an adder and a sub-sampling circuit are used. Thus, it is possible to perform color demodulation without using a bandpass filter which is conventionally required. A large-scale hardware band-pass filter is required to demodulate a high-quality color signal with a conventional color signal demodulation circuit, but according to the present invention, a low-pass filter can be constructed with about a dozen or more taps. Therefore, it is possible to significantly reduce the hardware.
Also, since it is processed as a time division multiplexed signal of I and Q signals,
Since it is not necessary to configure separate processing circuits for I and Q signals, it is possible to further reduce the hardware.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is an explanatory diagram shown for explaining the signal processing process of the circuit of FIG. 1, and FIG. 3 is a signal of each part of the circuit of FIG. 4 (a) and 4 (b) are diagrams showing the characteristics of the low-pass filter according to the present invention, FIG. 5 is a block diagram of a conventional digital television receiver, and FIG. Is an explanatory diagram showing the frequency spectrum of each signal of the circuit of FIG. 7, and FIG. 7 is a diagram showing a characteristic example of a digital filter. 2 ... Analog-to-digital converter, 3 ... Synchronous circuit, 4 ...
… Vertical low-pass filter, 5,7,8,203,204 …… Adder,
6 ... Horizontal low-pass filter, 9,11,13 ... Digital-analog converter, 10,12,14 ... Low-pass filter, 15 ...
Matrix circuit, 16 ... Monitor, 200 ... Color signal demodulation circuit, 201 ... Rectification detection circuit, 202 ... Low-pass filter, 205 ... Amplifier, 206, 207 ... Sub-sample circuit.

Claims (1)

[Claims] 1. A means for obtaining a digital color signal converted into a digital signal by a sampling clock synchronized with the phases of the I axis and the Q axis; and an I signal having a uniform polarity by inverting the polarity of the digital color signal for each predetermined sample. And a means for obtaining a first signal including a Q signal, a means for obtaining a second signal to which the first signal is supplied and the frequency band of the Q signal is set and a high frequency component is removed, Means for removing the Q signal component from the signal obtained by subtracting the second signal from the signal and calculating only the I signal component as the third signal by doubling the level, and the third signal and the The signal obtained by adding the second signals is sub-sampled at a predetermined sample phase and I
A color signal demodulation circuit for a digital television receiver, comprising a means for separating and deriving a demodulated output of a signal and a Q signal.