JPS59218051A - Automatic waveform equalizer - Google Patents

Abstract

PURPOSE:To obtain a reference waveform having a spectrum band similar to that of an input digital signal by supplying an output signal of a transversal filter to a level discriminating circuit to generate a rectangular simulated reference waveform thereby waveform-shaping by means of a band stop filter. CONSTITUTION:The input digital signal 2 is led to the transversal filter 3. The filter 3 consists of an analog delay element 4 having a tap at each 1/2 of the sample interval of the signal 2, a multiplier 5 and an adder 6. The signal 2 is subjected to convolution arithmetric with a gain value at a tap gain memory 7 via the filter 3, becomes an output signal 8 reduced with waveform distortion and is outputted. The output signal 8 is inputted to a reference waveform generating circuit 10 comprising a level discriminating circuit 11 and a band stop filter 13. The band stop filter 13 forms the simulated reference waveform 12 in a waveform 14. The waveform 14 is subtracted with the output signal 8 by a subtractor 15 to form an error signal 16. The interrelation between the signals 16 and 2 is calculated by a correlation operating circuit 17 so as to correct the tap gain of a memory 7.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an automatic waveform equalizer for equalizing digital No. 48 signals.

[Technical background of the invention and its problems]

In digital signal transmission, an automatic waveform equalizer is generally used on the receiving side to remove transmission path distortion. There are various types of automatic waveform equalizers, but a typical example is the one described in the document "Lucky" Automatic Equa.
llzation for Digital Commu
nication' Be1l System Te
ehnlcalJournal p 547-p 58
8 April, 1695 J.
Some use a transversal filter with variable tag gain. This is a transversal filter that introduces an input digital signal into a transversal filter and corrects its tag gain based on the cross-correlation calculation result between the error signal, which is the difference between the output signal and the reference waveform, and the input signal. This is to obtain a digital signal whose waveform has been equalized, that is, from which transmission line distortion has been removed.

Here, the reference waveform corresponds to the output waveform when waveform equalization is performed correctly, and according to the above-mentioned literature, this reference waveform is determined by level judgment of the output signal using a level judgment circuit consisting of a comparator. generated by. The reference waveform thus obtained is a binary rectangular wave.

However, since such a reference waveform usually has a spectrum that is wider than the spectral band of the input digital signal, the equalizer is forced to generate spectral components that are not included in the input signal at the output of the transversal filter. It works like that. As a result, when a transversal filter having a tap interval shorter than the sampling interval of the input digital signal is used, there is a risk that its tap gain will not stably converge and may diverge.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide an automatic waveform equalizer that does not cause the tag gain of a transversal filter to diverge and has stable operation.

[Summary of the Invention] According to the present invention, a reference waveform used for tag gain correction of a transversal filter has a Nyquist characteristic similar to that of an input digital signal.

That is, the present invention first generates a rectangular pseudo reference waveform using a level judgment circuit that judges the level of the output signal of a transpersal filter in the reference waveform generation circuit, and then shapes this pseudo reference waveform using a band limiter and a filter. It is characterized by obtaining a reference waveform having a spectrum band similar to that of the input cadinographic signal.

〔Effect of the invention〕

According to the present invention, the rectangular output waveform of the output signal level determination circuit is not used as the reference waveform as it is, but a waveform that approximates the transmitting side digital signal spectrum via a band-limiting filter is used as the reference waveform. When correcting the tap gain based on the error waveform, which is the difference between this reference waveform and the output signal of the transversal filter, it is said that the 4taf0 gain diverges in the case of a transversal filter with a tag interval shorter than the sample interval. [Implementation of the Invention] FIG. 2 is a block diagram of an automatic waveform equalizer according to an embodiment of the present invention. A binary digital signal 2 is input to the input terminal. However, the waveform of this digital signal 2 is
As shown in the figure (,), it is assumed that the signal is distorted due to the band limit. An example of such a digital signal is a television multiplexed text signal. This input r digital signal 2 is introduced into a transversal filter 3. The transpersal filter 3 includes an analog delay element 4 (DI*D2, p...Dn) having a tag at every 1/2 sample interval of the input digital signal 2.
), a multiplier 5 connected to each tag output of the delay element 4, and an adder 6 for adding the outputs of the multiplier 5.

The input digital signal 2 is passed through the transversal filter 3 to the tap gain memory 7 (Ml).

M2. 2 (b
) is sent to the output terminal 9 as an output signal 8 as shown in FIG.

The output signal 8 of the transversal filter 3 is input to a reference waveform generation circuit 10 consisting of a level determination circuit 11 and a band limiting filter 13. In the reference waveform generation circuit 10, first, a level determination circuit 11 consisting of a comparator to which a predetermined reference level vref is applied is applied to the transversal filter 3.
The output signal 8 of the second
A rectangular pseudo reference waveform 12 as shown in Figure (c) is generated. Since this pseudo reference waveform 12 is a binary digital signal, its spectrum Sω extends over a wide band up to infinity in principle. On the other hand, the digital signal spectrum Rω set on the transmitting side eliminates code amount interference at the sample point. It is standard. That is,
The reference waveform assumed on the transmitting side is, for example, as shown in Figure 2 (d).
As shown in the figure, this has a spectrum of a cosine roll-off filter that attenuates by 6 dB at a frequency that is 1/2 the reciprocal of the sandal spacing of the digital signal. The band-limiting filter 13 is a shaping filter that shapes the rectangular pseudo reference waveform 12 into a waveform 14 as shown in FIG. 2(d) that is received when there is no waveform distortion. Here, when the spectrum of the pseudo reference waveform 12 is 8(1), and the frequency characteristic of the cosine roll-off filter that attenuates by 6 dB at a frequency of 1/2 of the reciprocal of the sample interval of the digital signal is Cω, then the near-limit filter 130 frequency characteristic L(1) is L
Any characteristic of a low-pass filter given by (f)=CWS(1') may be used.

The reference waveform 14 generated in this way is subtracted by number 8 from the output of the transparsal filter 3 by a subtracter 15.
An error signal 16 is created.

Then, the cross-correlation between this error signal 16 and the input digital signal 2 is calculated by the correlation calculation circuit 17, and the tag gain held in the tap gain memory 7 (M1 to Mn) is successively corrected based on the calculation result. .

In this way, by simply adding a new band-limiting filter 13 to the hardware, a stable automatic waveform equalizer t'I? with no malfunction can be created. ? to be accomplished.

be able to.

It should be noted that the present invention is not limited to the above-described embodiment; for example, the reference waveform generation circuit may be one in which the level determination circuit 11 and band-limiting filter 13 shown in FIG. 1 are integrated. An example of this is shown in FIG. That is, amplifier 3
An element 33 that provides filter characteristics is added to 0, so that level determination and band-limiting filter characteristics can be performed at the same time. A terminal 41 is provided with the output signal 8 of the transpersal filter 3, and a terminal 42 is provided with a reference level Vyef for level determination. Resistor 3 (R1) and resistance, ? 2 (R2) determines the gain of the amplifier 31, and the larger R2βl is, the larger the gain becomes. The element 33 that provides band-limiting filter characteristics provides the amplifier 31 with filter characteristics. The level-determined and band-limited reference waveform is sent to the terminal 43. In this embodiment, since the level determination circuit (comparator) and the band-limiting filter are integrated, the configuration can be simplified. If strict characteristics are not required for the band-limiting filter, a simple low-pass filter can be used instead. In that case, a capacitor may be used as the filter element 33 that imparts the characteristics.

FIG. 4 shows another embodiment of the present invention, in which a delay circuit 1 corrects a signal delay occurring in the reference waveform generation circuit 10.
8 is inserted into the other input side of the subtractor 15.

When the signal delay caused by the level judgment circuit and band-limiting filter in the reference waveform generation circuit 10 is large enough to be ignored, the delay 1 circuit 18 having a delay characteristic equal to the amount of delay produces a correct error signal. This makes it possible to obtain a more stable equalization operation.

Further, in the above embodiments, the case where the input digital signal is a binary signal has been described, but the present invention can also be applied to a case where the input digital signal is a ternary signal, and more generally, an n-value digital signal. The level determination circuit may be of any type as long as it determines the level of the output signal of the transversal filter using n-1 reference levels and generates a pseudo reference waveform of n values. The band-limiting filter may be configured in the same way as in the binary case.

Furthermore, in the embodiment, the tag gain is modified sequentially by calculating the cross-correlation between the input signal and the error signal, but the tag gain is modified by an algorithm called a zero forcing method that uses only the r difference signal. You may. Further, in a feedback type (cyclic type) configuration, a cross-correlation between an output signal and an error signal may be performed, and the present invention is also effective when such an algorithm is used.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of an automatic waveform equalization amount according to an embodiment of the present invention, FIG. 2 is a waveform diagram for explaining its operation, and FIG. 3 is another example of the configuration of the reference waveform generation circuit. 4 are configuration diagrams of other embodiments of the present invention. 3... Transversal filter, lθ... Reference waveform generation circuit, 14... Reference waveform, 16... Error signal.

Claims (3)

[Claims] (1) Introduce the input digital signal to a transpersal filter with variable tap gain that has a tag interval shorter than its sample interval, and calculate the tag gain of this transparsal filter from the output signal of the 1-transversal filter and this output signal. In an automatic waveform equalizer that equalizes the waveform of the input digital signal by correcting it based on an error signal that is the difference from the generated reference waveform, the level of the output signal of the transversal filter is determined. An automatic waveform equalizer comprising a reference waveform generation circuit including a level determination circuit and a filter for band-limiting the output signal of the level determination circuit. (2) The band-limiting filter is characterized in that it has a cosine roll-off characteristic that attenuates the frequency spectrum of the output signal of the level judgment circuit by a frequency of 1/2 of the reciprocal of the fundamental period of the input digital signal - t"6ag. An automatic waveform equalizer according to claim 1. (3) The automatic waveform equalizer according to claim 1, wherein the level determination circuit and the band-limiting filter are integrally formed by sharing one operational amplifier.