JPH0793697B2 - Level adjuster - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for removing sag and automatically adjusting a DC level in analog-to-digital conversion of analog signals, particularly television signals.

[Conventional technology]

Conventionally, when an analog signal such as a television signal is sampled and converted into a digital signal, the DC level setting is mainly performed by clamping the signal before A / D conversion (analog-digital conversion) to remove sag or the like. .

Further, in order to avoid the influence of DC drift caused by power supply fluctuations, etc., the digital conversion output is compared with a set value and feedback-controlled so as to be in agreement (for example, Japanese Patent Publication No. 61-4234). Gazette).

[Problems to be solved by the invention]

In the conventional level adjustment described above, an analog clamp circuit for removing sag and two circuits for adjusting the level by performing feedback based on digital judgment are required.
The hardware was getting bigger. On the other hand, the sag cannot be removed by using only the conventional feedback level adjusting circuit to simplify the hardware.

[Means for solving problems]

The level adjusting device of the present invention compares the coded output D _X with respect to the level of interest in the blanking period of the analog signal A with a set value by sampling and coding the analog signal A having a blanking period. Means for reducing the gain when the difference value of the comparison result is small, and increasing the gain when the difference value is large, for converting and outputting the comparison result, and the analog signal based on the converted comparison result. Means for changing the DC level of A and feeding back so that the encoded output D _X matches the set value.

〔Example〕

 Next, the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention. A level adjusting device when paying attention to a blanking level of a television signal having a sag will be described. In the figure, an analog signal A, which is a television signal, enters an A / D converter 3 through a differential amplifier 2, is sampled, further encoded, and converted into a digital signal D. In the digital signal D, color subcarriers are trapped by the digital low-pass filter 4, high-frequency components are removed, and only low-frequency luminance signal components are output. Therefore, in the back porch, the color subcarrier is blocked and the signal waveform has a flat blanking level.

The low-passed digital signal D is taken out by the gate 5 only during the back porch period, that is, the period when the analog signal A is at the blanking level. In the figure, 10 is a gate pulse generator for this purpose, which separates the sync signal from the analog signal A and generates a gate pulse corresponding to the back porch period with the sync signal as a reference. The PCM coded output D _X for the back porch extracted by the gate 5 is compared with the set value D ₀ by the comparator 11. The result of this comparison is fed back to the other input of the differential amplifier 2, and the DC level is controlled so that the encoded output D _X for the back porch matches the set value D ₀ . The gain of the DC level is small when the comparison result is small, and is large when the comparison result is large.

A specific configuration example of the comparator 11 will be described. The coded output D _X for the back porch is subtracted from the set value D ₀ by the subtracter 6, and the error Δ from the set value D ₀ is calculated. When the analog signal A is not transformer-coupled or capacitor-coupled so that the frequency in the low frequency band is not sufficiently extended, a sag is generated in the vertical synchronizing portion, and in this case, the error Δ shows a large value. On the other hand, in the place where the correlation between lines is strong, the value of the blanking level of the back porch does not change greatly, and the error Δ becomes a small value or 0. In order to sufficiently suppress sag, the amount of feedback to the DC level is increased and the encoded output is promptly output.
Control the DC level so that D _X matches the set value.

If the total gain of the loop is 1 or more, oscillation is likely to occur and the loop becomes unstable. No sag, error △
If is small, unnecessary fluctuations due to noise, etc. are not generated, and the gain of the feedback amount is small so that the feedback loop is stable. To do. Thus, the nonlinear amplifier 7
Then, when the error Δ is small, the gain is reduced, and when the error Δ is large, the gain is increased and the output is performed.

The output of the nonlinear amplifier 7 is integrated by the integrator 8, and the integrated value is
It is converted into an analog signal by the D / A converter 9 and fed back to the other input of the differential amplifier 2 so that the error Δ approaches 0. Prompt feedback is required to sufficiently suppress sag, and the time constant of the loop is set to be equal to or less than the period of one horizontal scanning line. If the processing of the comparator 11 is performed at the cycle of the sampling clock or N times the cycle of the sampling clock, the period of the back porch in which the feedback control can be performed can be performed at tens of sampling cycles. Since it is sufficient that the integrated result in the integrator 8 has a total gain of 1, the amount of one-time feedback can be reduced, and the feedback loop can be more stably configured. In this case, the integrator 8 and the D / A converter 9 are also the sampling clock cycle or its N
The time constant of the feedback loop becomes equal to or less than the period of one horizontal scanning line by operating about twice. Also, if the time constant of the loop is reduced, if the error Δ is small, it is equivalent to feeding back an averaged value in line units at the end of the back porch, and the gain does not have to be changed depending on the size of the error Δ. Will be equivalently done.

In the above explanation, we focused on the back porch of the color TV signal and described the method of adjusting this level to the set value. However, the focus level is not limited to the back porch.
It can also be applied to the tip of the sync pulse.

〔The invention's effect〕

As described above, according to the present invention, the comparison result of the encoded output and the set value is non-linearly amplified and fed back, so that a feedback type level adjustment configuration is applied to a sag-like TV signal. Has the effect of being able to sufficiently suppress.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention. 1 ... Input terminal, 2 ... Differential amplifier, 3 ... A / D converter, 4 ... Low-pass filter, 5 ... Gate, 6 ... Subtractor, 7 ... Non-linear amplifier, 8 ... Integrator, 9 ... D / A converter, 10 ... Gate pulse generator, 11 ... Comparator,
A ... Analog signal, D ... Digital signal, D _X ... Coding output, △ ... Error.

Claims (1)

[Claims] 1. A means for sampling and encoding an analog signal having a blanking period, a low-pass filter for band limiting the encoded signal, and a predetermined range of the blanking period in advance. Means for obtaining a difference value by comparing a set value of a predetermined level with the output of the low-pass filter, and reducing the gain when the difference value is small, and increasing the gain when the difference value is large. Means for non-linearly amplifying the difference value using a non-linear gain characteristic, and D / A conversion after integrating the output of the non-linear amplification means,
Means for varying the level of the analog signal according to the D / A converted signal and performing feedback control so that the encoded signal matches the set value of the predetermined level. Level adjuster.