JPH0720276B2 - Analog / digital converter differential phase characteristic evaluation method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method of evaluating a differential phase (differential phase) characteristic of an analog / digital converter.

[Prior Art] In an analog / digital converter for video, it is necessary to guarantee a differential phase as well as a differential gain. If the differential phase of the analog / digital converter is large, the color tone of the converted video signal will change. Therefore, the analog / digital converter for video needs to evaluate the differential phase characteristic.

In the conventional differential phase characteristic evaluation method, the measurement video signal in which the color subcarrier is superimposed on each steer of the stepwise luminance signal is input to the analog / digital converter to be evaluated,
The converted output data is temporarily stored in the memory. And
By software processing, the phase value of the color subcarrier at the same position of each steer of the luminance signal is calculated from the converted output data, and the difference between the maximum value and the minimum value is divided by 2 to differentiate the analog / digital converter. Find the phase. The differential phase characteristic of the analog / digital converter is evaluated by comparing and judging this differential phase with a certain threshold value.

[Problems to be Solved] In the case of such an evaluation, the analog / digital converter has an integral multiple (for example, 4 times the period of the fundamental wave of the color subcarrier).
A sampling clock having a period of 2 times).

If the frequency of this sampling clock is deviated, its effect appears in the phase of the color subcarrier at each steer of the digitized video signal, and naturally, the value of the differential phase is also affected. If this influence is large, it becomes difficult to accurately evaluate the differential phase characteristic of the analog / digital converter. Therefore, an oscillator having good frequency stability is conventionally used for generating a sampling clock.

However, there is a limit to the improvement of the frequency stability, and it is extremely difficult to completely suppress the frequency deviation of the sampling clock for a long period of time. Further, it is difficult to completely eliminate the frequency shift due to the adjustment error.

Therefore, conventionally, there is a problem that the evaluation accuracy of the differential phase characteristic of the analog / digital converter is relatively poor.

[Object of the Invention] Accordingly, an object of the present invention is to provide a differential phase characteristic evaluation method of an analog / digital converter capable of achieving high evaluation accuracy by compensating for the influence of the frequency shift of the sampling clock. is there.

[Means for Solving the Problems] In order to achieve this object, according to the present invention, an analog / digital converter for measuring video signals in which a color subcarrier is superimposed on each steer of a stepwise luminance signal is provided. To calculate the phase value of the color subcarrier at the specific position of each steer from the converted output data, and to calculate the frequency deviation of the sampling clock of the analog / digital converter from the difference between the phase values in at least two steers. The phase compensation amount for each steer that is dependent (proportional to the phase variation amount due to the frequency shift) is calculated, the phase value of each steer is corrected by this phase compensation amount, and the analog / analog based on the corrected phase value is calculated. Evaluate the differential phase characteristics of the digital converter.

[Operation] The position value of the color subcarrier at the specific position of each steer calculated from the converted output data of the measurement video signal is affected by the frequency deviation of the sampling clock, and the influence amount is accumulated in later steers. And grow bigger.

Here, in general, the frequency fluctuation of the sampling clock in a short period is sufficiently small, and the frequency of the sampling clock can be regarded as constant within one cycle of the measurement video signal.
Therefore, it can be considered that the influence of the frequency shift of the sampling clock is sequentially accumulated from the first steer to the last steer by an approximately constant amount with respect to the phase value of each steer.

Therefore, as described above, the phase compensation amount for each steer proportional to the amount of phase fluctuation depending on the frequency shift of the sampling clock can be calculated from the difference between the calculated phase values in at least two steers. Therefore, if the phase value of each steer is corrected by this phase compensation amount, the influence of the frequency shift of the sampling clock can be almost completely compensated, and the differential of the analog / digital converter is calculated based on the corrected phase value. The phase characteristics can be evaluated with high accuracy.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic block diagram showing a simplified functional configuration of an embodiment of the present invention. In this figure, 10 is an analog / digital converter to be evaluated. A video generator 12 generates a measurement video signal to be input to the analog / digital converter 10.

FIG. 3 shows a simplified video signal for measurement. As shown in this figure, the measurement video signal has a constant number of cycles of the color subcarrier 15 for each steer of the 8-step staircase luminance signal 14.
Is a superimposed signal. The phase of the color subcarrier 14 superimposed on each steer of the luminance signal 14 is adjusted so as to start from the beginning of the steer (so that the phase becomes zero there).

Referring again to FIG. 1, 16 is an analog / digital converter 10
It is a clock generator that generates a sampling clock for input to. This sampling clock is a pulse having a period four times (generally an integral multiple) the period of the fundamental wave of the color subcarrier 14 of the measurement video signal. Therefore, the analog / digital converter 10 samples the video signal for measurement at a cycle four times the fundamental wave cycle of the color subcarrier, converts it into a digital signal, and outputs it.

A storage unit 18 includes a memory circuit and its control circuit. The conversion output data of the analog / digital converter 10 and the sampling clock are input to the storage unit 18, and the conversion output data of the analog / digital converter 10 are sequentially stored.

20 is a processing unit. It generally comprises a microprocessor 22, a memory 24, an interface circuit 26, and a bus 28. A program 24a for differential phase characteristic evaluation processing is stored in advance in the memory 24, and storage areas 24b to 24h used for the processing are prepared, which will be described later.

FIG. 2 is a schematic flowchart showing the outline of the processing by the program.

Referring again to FIG. 1, reference numeral 30 denotes a display keyboard, which can exchange information with the microprocessor 22 via the interface circuit 26. Microprocessor
22 can access the storage contents of the storage unit 18 via the interface circuit 26, and can also control the start / stop of writing of the storage unit 18.

Next, the differential phase characteristic evaluation processing will be described with reference to the drawings.

The analog / digital converter 10 to be evaluated is set,
When the display / keyboard 30 is instructed to start the processing in the connected state as shown in FIG. 10, the microprocessor 22 starts executing the program 24A.

First, in step 100, the microprocessor 22 gives a write permission to the storage unit 18 through the interface 26 for a certain period of time. The converted output data of the analog / digital converter 10 is sequentially written and stored in the storage unit 18 only during the time when the write permission is given.

When the writing of the converted output data is completed after a certain time has passed, the storage data of the storage unit 18 is read into the input buffer storage area 24b of the memory 24 (step 102).

Next, in step 104, the input buffer storage area 24b
By referring to a series of converted output data of the video signal in the above, the sine component (sinA, sinB) of the fundamental wave of the color subcarrier at the same position of each steer of the video signal is obtained by the well-known Fast Fourier Transform (FET) method. , sinC, sinD, sinE, sin
F, sinG, sinH) and cosine component (cosA, cosB, cosC, cos
D, cosE, cosF, cosG, cosH) are calculated respectively, and the storage area 2
Written to 4c.

In the next step 106, using the sine and cosine components, P1, P2, ...
.., P8 is calculated.

P1 ＝ tan ^-1 (sinA / cosA) P2 ＝ tan ^-1 (sinB / cosB) P3 ＝ tan ^-1 (sinC / cosC) P4 ＝ tan ^-1 (sinD / cosD) P5 ＝ tan ^-1 (sinE / cosE) P6 ＝ tan ^-1 (sinF / cosF) P7 ＝ tan ^-1 (sinG / cosG) P8 ＝ tan ^-1 (sinH / cosH) These P1 to P8 are the phase of the color subcarrier at the specific position of each steer from the beginning to the end. Although it is a value (calculated value), P1 is set to zero and P2 to P8 are corrected to values based on P1. That is, P2 ~
The value obtained by subtracting P1 from P8 is set again as P2 to P8 and is stored in the storage area 24d together with P1 = 0.

In the next step 108, the value of Δ is calculated by the equation Δ = (P8−P1) / 7, using the phase values P1, P8 calculated for the first and last steer. This Δ is a phase compensation value for the second steer and is stored in the storage area 24e.

The amount of phase compensation for the third steer to the seventh steer is obtained by multiplying this Δ by an integer. This is because the frequency of the sampling clock can be regarded as constant within one cycle of the video signal, and the influence of the frequency shift can be considered to increase by a substantially constant amount from the previous steer.

In the next step 110, the correction value of the phase value of each steer is obtained according to the following equations.

CP1 = 0 CP2 = P2-Δ CP3 = P3-2Δ CP4 = P4-3Δ CP5 = P5-4Δ CP6 = P6-5Δ CP7 = P7-6Δ CP8 = P8 These corrected phase values CP1 to CP8 of each steer are Storage area
It is stored in 24f.

In the next step 112, the differential phase DP of the analog / digital converter 10 is calculated by dividing the difference between the maximum value and the minimum value of CP1 to CP8 by 2 and stored in the memory area 24g.

In the next step 114, the calculated differential phase DP
And the determination threshold value TH pre-stored in the storage area 24h are compared.

If DP ≦ TH, the analog / digital converter 10 determines that the differential phase characteristic is good, a pass message is displayed on the keyboard display 30 together with the differential phase DP (step 116), and the evaluation process ends. On the other hand, if DP> TH, it is determined that the differential phase characteristic is defective, and a reject message is displayed on the keyboard display 30 together with the DP (step 118), and the processing ends.

In this embodiment, the evaluation is performed for each analog / digital converter and the evaluation result is displayed on the keyboard display 30.
The identification information of the digital converter may be stored together with the external storage device or may be printed out.

Further, the conversion output data of the plurality of analog / digital converters are temporarily stored in the external storage device, and the necessary conversion output data of the analog / digital converters are sequentially read into the memory 24 to perform the evaluation as described above. You may

In addition, the phase compensation amount for each steer is calculated when evaluating one or more analog / digital converters, and then the phase compensation amount is re-calculated when evaluating other analog / digital converters within a fixed time. Don't calculate
It is also possible to use the previously obtained amount of phase compensation. This is applied when the frequency of the sampling clock can be regarded as almost constant within a certain time, and the evaluation processing time can be shortened.

The present invention is not limited to the above-described embodiment even with respect to the configuration other than the above, and various modifications can be made without departing from the scope of the invention.

As described above in detail, according to the present invention, the measurement video signal in which the color subcarrier is superimposed on each steer of the stepwise luminance signal is input to the analog / digital converter, and The phase value of the color subcarrier at a specific position of each steer is calculated from the converted output data, and for each steer that depends on the frequency shift of the sampling clock of the analog / digital converter from the difference between the phase values of at least two steers. The phase compensation amount is calculated, the phase value of each steer is corrected by this phase compensation amount, and the differential phase characteristic of the analog / digital converter is evaluated based on the corrected phase value. The influence of the shift can be almost completely eliminated, and the evaluation accuracy of the differential phase characteristics of the analog / digital converter can be greatly improved. .

Further, since the influence of the sampling clock frequency deviation is compensated, it becomes possible to use a relatively inexpensive oscillator for generating the sampling clock.

[Brief description of drawings]

FIG. 1 is a schematic block diagram showing a simplified functional configuration of an embodiment of the present invention, FIG. 2 is a schematic flowchart of differential phase characteristic evaluation processing, and FIG. 3 is a simplified waveform of a measurement video signal. It is a figure. 10 …… analog / digital converter, 12 …… video generator, 14 …… luminance signal, 15 …… color subcarrier, 16 …… clock generator, 18 …… storage section, 20 …… processing section, 22
… Microprocessor, 24… memory.

Claims (1)

[Claims] 1. A video signal for measurement in which a color subcarrier is superimposed on each steer of a stepwise luminance signal is input to an analog / digital converter, and the phase of the color subcarrier at a specific position of each steer is output from the converted output data. The value is calculated, and the phase compensation amount for each steer depending on the frequency deviation of the sampling clock of the analog / digital converter is calculated from the difference between the phase values in at least two steers. A differential phase characteristic evaluation method for an analog / digital converter, wherein the phase value is corrected and the differential phase characteristic of the analog / digital converter is evaluated based on the corrected phase value.