JPS58173988A - Differential encoder - Google Patents

Abstract

PURPOSE:To avoid the generation of an excessive error voltage except an error voltage between a predicting value and the sampling value, by sampling and holding an input television signal at a sample hold circuit inserted to the input side of a differential encoding circuit. CONSTITUTION:The television signal from an input terminal 11 is applied to an analog subtraction circuit 13 via a sample hold circuit 12. The signal is subtracted with an analog predicting signal from a D/A conversion circuit 14 at the subtraction circuit 13, the subtraction output is converted into a digital signal at an A/D conversion circuit 15, the digital signal is quantized at a quantization circuit 16, and the prediction signal is formed by using a high-order predicting function at a prediction circuit 17 from the quantized output. This prediction signal is converted into the analog predicting signal at the circuit 14. The encoded output is obtained from the circuit 15.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention particularly relates to a differential encoder among circuits for encoding and decoding frequency multiplexed color television signals for digital transmission.

<Prior art> When differentially encoding a television signal, there is a method in which a prediction signal and a television signal are subjected to differential calculation using an analog calculation circuit, and this differential signal is converted into a digital signal to perform differential encoding. There is.

On the other hand, as a prediction signal generation method, a previous value predictive differential pulse encoding method is known in which a ``sampled'' value at a previous sampling time is used as a predicted signal for a black-and-white television signal. Further, as a predicted signal for an NTSC color television signal signal in which a color signal is frequency-multiplexed using a subcarrier in the high frequency range of a luminance signal, a sample value even earlier than the previous sampling time is used as a predicted signal. There are high-order differential pulse coding systems that provide good prediction especially for relatively low and relatively high portions of the television signal band.

When differentially encoding an NTSC television signal using a high-order differential pulse encoding method, the prediction error is minimized for the sample value in the low frequency band of the NTSC television signal and in the 1fltl carrier band. A predictive signal is generated. For signals in the subcarrier band, since the sampling frequency is several times the frequency of the subcarrier band, the change in the signal within one sampling period is larger than that in the low frequency band. Therefore, the predicted value is output with a small prediction error at the sampling point, but since the predicted value is retained in the -sampling interval, the error voltage between the input signal and the predicted value is small in the interval excluding the sampling point. This value is larger than the error voltage in the low frequency band.

An analog calculation circuit that receives an error voltage such as
A circuit with good linearity is required even for large amplitude inputs. However, an analog arithmetic circuit that satisfies the above-mentioned requirements requires an arithmetic circuit with broadband characteristics and a large dynamic range, making it technically difficult and expensive. In addition, if an arithmetic circuit with small dynamic range characteristics is used, the circuit will become saturated and an accurate error voltage will no longer be output, the signal-to-noise ratio will begin to drop, and color distortion will occur. The disadvantage is that the characteristics deteriorate.

The same problem as the above-mentioned high-order predictive encoder occurs also in a differential encoder (double integral type 6M) having frequency characteristics between the differential circuit and the A/D conversion circuit.

<Summary of the Invention> The purpose of the present invention is to eliminate the above-mentioned conventional drawbacks by inserting a sampling and holding circuit on the input side of the differential encoding circuit to sample and hold the input television signal. , - Keep the voltage value applied to the differential encoding circuit constant during the sampling period, and do not generate any extra error voltage other than the error voltage between the predicted value and the sampled value in the analog calculation circuit of the differential encoding circuit. An object of the present invention is to provide a differential encoder as described above.

According to this invention, a subtraction circuit performs subtraction between an input television signal and a predicted signal, the A/D conversion circuit converts the subtraction output into a digital signal, and the digital signal is converted to a quantization circuit. A prediction circuit generates a prediction signal using a high-order prediction function based on the output of the quantization circuit, and the prediction signal is converted into an analog signal by a D/A conversion circuit and supplied to the subtraction circuit. In a differential encoder for differentially encoding a frequency multiplexed color television signal including a carrier color signal, a sampling and holding circuit is inserted on the input side of the subtraction circuit to sample and hold the input television signal, and the subtraction circuit The deterioration of the encoding characteristics of the encoder due to the saturation of the encoder is improved.

<Example> Next, embodiments of the invention will be described with reference to the drawings.

FIG. 1 shows the configuration of a differential encoder according to the present invention. 1st
In the figure, a television signal from an input terminal 11 is supplied to an analog subtraction circuit 13 via a sampling and holding circuit 12. The subtraction circuit 13 performs subtraction with the analog predicted signal from the D/A conversion circuit 14, the subtraction output is converted into a digital signal by the A/D conversion circuit 15, and the digital signal is quantized by the quantization circuit 16. The prediction circuit 17 uses the membered output to generate a prediction signal using a high-order prediction function. This predicted signal is converted into an analog predicted signal by the D/A conversion circuit 14.

A coded output is obtained from the quantization circuit 15.

In a differential encoder configured as shown in FIG. 2, for example, when a signal indicated by the dotted line 18 in FIG. chain line 1
The sampled hold output shown in 9 is obtained. This sampled holding output and the prediction signal shown by the solid line 21 in FIG. 2A are input to the analog subtraction circuit 13 in FIG. 1, and an error signal shown by the solid line 22 in FIG. 2B is obtained.

The prediction signal indicated by the solid line 21 in FIG. 2A is D/A converted from the prediction circuit 17 in FIG. It is output via the circuit 14.

On the other hand, sample point t1 in FIG. 2A. t*, ts, other than t4e
In the section, there is an error between the predicted signal indicated by the solid line 21 in FIG. 2A and the input signal indicated by the dotted line 18 in FIG. 2A, as shown by the dashed-dotted line 23 in FIG. 2B. In a conventional encoder that does not use the sampling and holding circuit 12, the error signal output from the subtraction circuit 13 is as shown by the dashed-dotted line 23 in FIG. 2B.

However, in this invention, due to the presence of the sampling circuit 12, the signal at point M18 in FIG. 2A is sampled, and the − sampling interval in FIG. ts
.

By holding the sampled value during t4, the error signal becomes the same as the value at the sampling point, as shown by the solid line 22 in FIG. 2B, and a signal with a small error is obtained.

Effects> As explained above, the present invention provides a differential code that performs a differential operation between the prediction (H number and the television signal) in the analog subtraction circuit 13, and converts the differential signal, tb prediction error signal, into a digital signal. By equipping the device with a sampling and holding circuit that samples and holds the input television signal, the error between the input television signal and the predicted signal in the interval other than the sampling point is removed, and the saturation of the analog subtraction circuit is eliminated. Since an accurate error signal can be supplied to the A/D conversion circuit without being affected by this, it is possible to provide a differential encoder with less deterioration in encoding characteristics for color components.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an example of a differential encoder according to the present invention, and FIG. 2 is a voltage characteristic diagram for explaining the operation of the encoder shown in FIG. 1. 11: Input terminal, 12: Sampling holding circuit, 13: Analog subtraction circuit, 14: D/A conversion circuit, 15: A/D
Conversion circuit, 162′j! 17: Prediction circuit. % Pestle applicant Taku Kusano, agent of NEC Corporation

Claims (1)

[Claims] (1) In a differential encoding circuit that differentially encodes a frequency multiplexed color television signal including a carrier color signal using a high-order prediction function, a subtraction circuit that performs subtraction between an input signal and an analog prediction signal; An A/D conversion circuit that converts the output of the subtraction circuit into a digital signal, a quantization circuit that quantizes the A/D converted digital signal according to predetermined rules, and a quantization circuit that converts the output of the quantization circuit into a digital signal. a prediction circuit that generates a digital prediction signal, a D/A conversion circuit that converts the digital prediction signal of the prediction circuit into an analog prediction signal and outputs it to the subtraction circuit, and an input side of the subtraction circuit. and a sampling and holding circuit that is inserted into the input signal and samples and holds the input signal.