JPH0473838B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present system relates to an automatic gain control circuit provided near the first stage of a MUSE system decoder.

[Conventional technology]

Research and development of high-definition television is nearing the stage of practical use. MUSE (Multiple Sub-Nyquist
The Sampling Encoding method is a method developed by the Japan Broadcasting Corporation, which encodes high-definition television signals with a baseband bandwidth of approximately 30 MHz to approximately 8M
Bandwidth compression to Hz bandwidth. Such band compression not only makes it possible to broadcast on one channel of satellite broadcasting, but is also expected to be applied to so-called package media such as video disks and home cameras.

In this way, from an application standpoint, it has a variety of supply sources.
The level of the MUSE signal varies considerably.
Therefore, it is necessary to make the signal level constant before signal processing by the decoder.

[Problem that the invention seeks to solve]

When decoding the MUSE signal, first synchronous separation is performed, but this synchronous separation is performed using the HD
(Horizontal drive) Performs waveform pattern recognition.
The HD waveform is sent from the 1st sample to the 12th sample of the transmission sample number for each line, and the part where the waveform changes suddenly is used as a synchronization point. In the case of pattern recognition, detection errors are likely to occur if the signal level deviates from a specified level.

In view of the above circumstances, an object of the present invention is to provide a circuit that automatically controls a signal level using an HD waveform as a reference.

[Means for solving problems]

The MUSE signal input to the MUSE decoder undergoes dispersal processing and nonlinear de-emphasis processing, is A/D converted, and is transmitted to the video signal processing system and control system. In the present invention, a variable gain amplifier is provided before the MUSE baseband signal subjected to the above processing is input to the A/D converter, and an automatic gain control signal is generated by referring to a reference level determined for the HD waveform. create,
controlling the variable gain amplifier;

The device of the present invention has an HD
An absolute value processing circuit that provides the absolute value of the amplitude with respect to the midpoint of the waveform, a time window average value circuit that averages the HD period for the absolute value, and a difference circuit that extracts the difference signal between the average value and the amplitude reference value. and an integrating circuit for the differential signal, and a D/A converter for D/A converting the output of the integrating circuit, and the output of the D/A converter is used as an automatic gain control signal for the variable gain amplifier. .

[Effect]

An automatic control loop is formed to keep the amplitude of the MUSE baseband signal constant using the midpoint of the HD waveform as a reference. An A/D converter is placed after the variable gain amplifier, and a control amount is obtained through digital processing, and then a control signal is given to the variable gain amplifier as an analog amount by a D/A converter. Therefore, the A/D conversion diagram input is held at a constant value and is effectively A/D converted.

〔Example〕

Hereinafter, one embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a circuit block diagram, and FIG. 2 shows waveforms at various parts. However, although the waveform is a digital signal after A/D conversion, it is shown in analog form for easy understanding. From input terminal 10
A MUSE baseband signal is input, passes through a variable gain amplifier 11, and is converted into a digital signal by an A/D converter 12. The output signal 12a is transmitted from a terminal 20 to a MUSE type signal processing system/control system. Output signal 12b, on the other hand, enters the feedback path of the automatic control loop of the present invention. This feedback path includes an absolute value processing circuit 13 and a time window average value circuit 1 as shown in the figure.
4. It is composed of a differential circuit 15, an integrating circuit 16, and a D/A converter 17, and the signal 17a controls the gain of the variable gain amplifier 11 as an automatic gain control signal.

Next, the circuit operation of this embodiment will be explained.
MUSE baseband signal is transmitted through variable gain amplifier 1
1, the signal is amplified and input to the A/D converter 12 to become an n-bit digital signal. Here n=8. The A/D converter 12 includes a pre-filter, .
Includes D converter. The output of the A/D converter 12 is the second
Consider the case where the input signal shown in the solid line is smaller than the input signal shown in the dotted line as shown in FIG. Note that the dotted line in Figure 2
The solid line is for the regular input signal/actual input signal. This conversion output is given as a binary code, so if the absolute value processing circuit 13 takes the two's complement and converts it to an absolute value, the output value will be the midpoint of the HD waveform.
2n ^-1 is assumed to be zero amplitude, as shown in Figure b. Next, in the time window average value circuit 14, the HD point (waveform change point)
The average value for the period is determined through the time window 141 with the center at the center (c, d in the same figure). This time window looks at both ends of the HD waveform and averages over 8 samples. Next, set the above average value to the amplitude reference value 2 ^n-2 × 7/
Take the difference from 8. Here, 2 ^{n -2} × 7/8 is a value calculated from the normal waveform, and since the amplitude is zero at the central reference value 2 ^{n -1} , the number of samples to be added for averaging is seven. The differential signal shown in e of the figure is integrated by an integrating circuit 16. Here, 161 is an adder circuit, and 162 is a latch circuit, which latches the signal for one line period. The latch circuit 162 resets its output by the signal 1 when the power is turned on, and returns the gain of the variable gain amplifier 11 to the initial value. Note that the input of the difference signal to the adder circuit 161 is set to be negative, thereby making this feedback loop a negative feedback. Therefore, the output of the integrating circuit 16 is as shown in FIG. This output is converted into an analog signal by a D/A converter 172 in the D/A converter 17, and a low-pass filter 171 removes noise components and prevents sudden changes in the D/A output, making it variable as an automatic gain control signal 17a. It is supplied to a gain amplifier 11.

The variable gain amplifier 11 operates to increase the gain when the input signal is smaller than the amplitude value of the reference waveform, and decreases the gain when the input signal is large so that the output 12a of the A/D converter 12 is It is always transmitted to the video signal processing system and control system as an HD waveform with a specified amplitude.

〔Effect of the invention〕

As explained above in detail, the MUSE signal decoder detects the level of the HD waveform pattern, and controls the gain of the variable gain amplifier installed before the A/D converter based on the difference signal from the amplitude reference value. A signal always at a constant level is input to the A/D converter and sent as a digital signal to the digital processing system. Therefore, when detecting synchronization from an HD waveform, this circuit automatically maintains the HD waveform at an optimal level, so that synchronization can always be detected correctly.

Furthermore, in the MUSE method, since the blanking level is given as a digital value, there is an effect that the MUSE signal of the digital processing system is always kept in a normal state with respect to the blanking level.

[Brief explanation of the drawing]

FIG. 1 is a circuit block diagram of an embodiment of the present invention.
FIG. 2 is an analog diagram of the waveforms of each part of the circuit of FIG. 1, and FIG. 3 is an HD waveform diagram of the MUSE system. 11... Variable gain amplifier, 12... A/D conversion section,
13... Absolute value processing circuit, 14... Time window average value circuit, 15... Difference circuit, 16... Integrating circuit, 17...
D/A conversion section, 10...MUSE baseband signal input terminal, 20...video signal system/control system input terminal.

Claims (1)

[Claims] 1 A/D converting the MUSE baseband signal,
A device that automatically controls the gain of the variable gain amplifier by providing a variable gain amplifier in front of the A/D converter in a path for transmitting the video signal to a video signal processing system/control system, wherein the output of the A/D converter is an absolute value processing circuit that provides the absolute value of the amplitude with respect to the midpoint of the HD waveform;
a time window average value circuit that averages the HD period for the absolute value; a difference circuit that extracts a difference signal between the average value and an amplitude reference value; and an integration circuit for the difference signal;
A D/A converter for D/A converting the output of the integrating circuit, and the output of the D/A converter is used as an automatic gain control signal for the variable gain amplifier. Control device.