JP2976435B2 - Cable compensation circuit for digital video signal - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to cable compensation for compensating for deterioration of characteristics of digital video signals serially transmitted via a coaxial cable or the like due to cable attenuation. The present invention relates to a digital video signal cable compensating circuit for performing the following.

[Prior Art] Conventionally, when transmitting a video signal in a television station, a coaxial cable is used and transmitted as an analog signal.
Analog signal transmission is accompanied by deterioration of frequency amplitude characteristics due to attenuation by a cable. Therefore, such a deterioration is compensated to flatten the frequency amplitude characteristics. So-called cable compensation is performed.

On the other hand, recently, devices for digitally processing a video signal in a television station are increasing, and accordingly, a video signal is transmitted as a digital serial signal.

[Problem to be Solved by the Invention] Since the transmission rate when serially transmitting a digital video signal is, for example, 143 Mbit / s, the so-called eye pattern closes due to attenuation in the coaxial cable, and a data error occurs on the receiving side. Occur. To solve this problem,
Again, cable compensation is needed.

The present invention has been made in view of such a problem, and an object of the present invention is to provide a cable compensation circuit capable of performing cable compensation for improving an eye pattern with respect to a serially input digital video signal. Aim.

[Means for Solving the Problems] A cable compensation circuit for a digital video signal according to the present invention provides a gain between a digital video signal input serially and a compensation signal obtained by performing a predetermined cable compensation on the digital video signal. A gain control circuit for mixing and outputting at a mixing ratio according to a control signal; a level detection circuit for detecting an amplitude level of an output signal of the gain control device; and a gain control signal having an output signal of the level detection circuit as an input. A control signal amplification circuit including a loop filter that outputs
The control signal amplifier circuit increases the compensation signal component in the output signal of the gain control circuit as the output signal level of the level detection circuit decreases.

[Operation] The frequency spectrum of the digital video signal is dispersed on average over a wide band, and the frequency spectrum becomes flat by defining the rise time of the digital video signal by the signal source. Also, the digital video signal has data “1” or “0” during serial transmission.
Are scrambled so as not to be continuous. Furthermore, since the attenuation of high-frequency components increases in a coaxial cable, when the amplitude of a digital video signal on the receiving side is considered on average, as the cable length increases, data “1”,
The level is remarkably reduced in a portion where the change of “0” is sharp.

As in the present invention, the amplitude level of the digital video signal output through the gain control circuit is detected, and based on this, the mixing ratio of the digital video signal and the compensation signal in the gain control circuit is changed, and the cable compensation amount is changed. , The automatic cable compensation is performed so that the eye pattern of the digital video signal at the output is always the best.

Example Next, an example of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a block diagram of an automatic cable compensation circuit according to an embodiment of the present invention. In the figure, a digital video signal transmitted serially via a coaxial cable (not shown) is input to an input terminal 1. The digital video signal supplied to the input terminal 1 and the compensation signal via the cable compensation element 3 are supplied to the gain control circuit 4. The gain control circuit 4 mixes the two input signals at a mixing ratio controlled by a gain control signal from a control signal amplification circuit 6 including a loop filter and sends the mixed signal to the output terminal 2. Level detection circuit 5 receives the output signal of gain control circuit 4, performs amplitude detection, and sends the output signal to control signal amplification circuit 6.

Here, the circuit of FIG. 1 is configured such that when the output signal level of the level detection circuit 5 decreases, the compensation signal component from the cable compensation element 3 in the output signal of the gain control circuit 4 increases. As described above, the level detection circuit 5 detects the amplitude level of the output signal of the gain control circuit 4 and automatically performs cable compensation based on the detected level. Therefore, the cable compensation amount (amplitude of the compensation signal) in the cable compensation element 3 becomes the maximum compensation amount of the cable compensation circuit.

FIG. 2 is a diagram showing a schematic configuration of the gain control circuit 4 in FIG. The digital video signal and the compensation signal supplied to the input terminals 41 and 42, respectively, are multiplied by the multiplication circuits 43 and 44.
The output signal of the adding circuit 45 is sent to the output terminal 46. The multiplication circuits 43 and 44 receive the control signal supplied to the control signal input terminal 48 and the control signal output from the inverting amplification circuit 47, and the gain is controlled complementarily.

FIG. 3 more specifically shows the configurations of the gain control circuit 4, the inverting amplifier circuit 47, and the cable compensation element 3 in FIG. 1 shown in FIG. The terminal 51 corresponds to the control signal input signal 48 in FIG. 2, and the terminal 52 corresponds to the digital video signal input terminal in FIG.
Resistors R2 and R3 and capacitors C1 and C2 are cable compensating element 3.
And the transistors TR1 to TR6 and the resistors R1, R11 to R1
Reference numeral 5 denotes a gain control circuit 4. It is apparent that the function of the inverting amplifier circuit 47 shown in FIG. 2 is substantially included in this. Further, 53 is an output terminal, 54 is a positive power input terminal, and 55 is a negative power input terminal.

FIG. 4 shows an example of a digital video signal attenuated by serial transmission through a long coaxial cable. As shown in this figure, the amplitude of the portion where "1" or "0" is continuous is large, and the amplitude decreases where "1" and "0" are repeated quickly. By passing this signal through a level detection circuit to obtain the average value of the hatched portion in the figure, it is possible to easily detect the amplitude level of the digital video signal.

It should be noted that the present invention is not limited to the above-described embodiment. For example, it is apparent that the circuits shown in FIGS. 2 and 3 can be used even when performing continuous variable cable compensation by remote control without performing automatic cable compensation. It is.

[Effect of the Invention] As described above, the present invention detects the amplitude level of a digital video signal output via a gain control circuit,
Based on this, automatic cable compensation is performed on the digital video signal serially transmitted by controlling the mixing ratio of the digital video signal in the gain control circuit and the compensation signal through the cable compensation element by the negative feedback loop. It is possible.

In serial transmission of a digital video signal, an eye pattern is emphasized rather than a frequency amplitude characteristic. However, according to the present invention, there is an advantage that cable compensation that optimizes an eye pattern can be performed.

When the gain of the digital video signal is slightly reduced, the output signal tends to be overcompensated as cable compensation. However, the eye pattern is not largely disturbed, and there is no practical problem.

[Brief description of the drawings]

FIG. 1 is a block diagram of a digital video signal cable compensation circuit according to an embodiment of the present invention, FIG. 2 is a block diagram showing a schematic configuration of a gain control circuit in FIG. 1, and FIG.
FIG. 2 is a circuit diagram more specifically showing the configuration of the main part of FIG. 2 and FIG.
FIG. 4 is a diagram showing an example of a digital video signal in which high frequency components have been attenuated by a coaxial cable. 1; input terminal, 2; output terminal, 3; cable compensation element, 4; gain control circuit, 5; level detection circuit, 6: control signal amplifier circuit, 4
1, 42; input terminal, 43, 44; multiplication circuit, 45; addition circuit, 46; output terminal, 47; inverting amplifier, 48, 51; control signal input terminal, 52;
Digital video signal input terminal, 55; negative power supply input terminal, R1
~ R3, R11 ~ R15; Resistor, C1, C2; Capacitor, TR1 ~ TR6;
Transistor

Claims (1)

(57) [Claims] A gain control circuit for mixing and outputting a digital video signal input serially and a compensation signal obtained by subjecting the digital video signal to predetermined cable compensation at a mixing ratio according to a gain control signal; A level detection circuit that detects an amplitude level of an output signal of the gain control circuit, and a control signal amplification circuit including a loop filter that receives the output signal of the level detection circuit as input and outputs the gain control signal, A digital video signal cable compensation circuit, wherein the control signal amplification circuit increases the compensation signal component in the output signal of the gain control circuit as the output signal level of the level detection circuit decreases.