JPH01212126A - Device for removing multi-pass interference - Google Patents

Abstract

PURPOSE:To receive a multiplexing signal without fail by making an interference removing signal corresponding to multi-pass interference from a received sound sub-channel signal and canceling the interference. CONSTITUTION:A band pass filter (BPF2) 30 extracts the sound sub-channel signal from a sound base band signal and a sound sub-channel signal 31 is two-multiplied by a two-multiplying circuit 32. Then, a mu<2>(t) is obtained. Next, the mu<2>(t) is differentiated by a differention circuit 36 and a dmu<2>(t)/dt is obtained and the amplitude of a differentiation signal is corrected by an AGC circuit 38. Then, the delay quantity of a circuit from an input signal 20 is mixed with a signal 24, which is corrected by a delay circuit 22, by a mixer 26 in a reverse phase. A mixed signal 28 obtains a band from 3fH to 4fH and it is obtained as a signal 42 for control by a BPF444 which can remove the part of a sound control signal. Further, the delay circuit 22 is controlled as the delay circuit with tap so that the control output of a detecting circuit 40 to an AGC circuit 39 can be minimum. Thus, a phase change, which is generated by the multi-pass interference is decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a receiving side device that removes multipath interference in multiplex broadcasting.

More specifically, the present invention relates to a device for removing multipath interference that occurs when transmitting predetermined information by multiplexing it with an FM modulated signal. This is to reduce interference caused by harmonics of the audio subchannel signal caused by multipath interference.

[Summary of the Invention] The present invention relates to a receiving side device that removes multipath interference in multiplex broadcasting, and in order to reduce interference due to harmonics of audio subchannels that occur when receiving in a place where there is multipath interference. By creating an interference removal signal corresponding to multipath interference from the received audio subchannel signal and canceling the interference, multiplexed signals can be received without error.

[Prior Art] Research has been carried out to remove interference caused by distortion in the transmission path of FM broadcast waves at the intermediate frequency stage of a receiver (for example, automatic cancellation of multipath distortion in Dategen rFM broadcasts). "Method" (November 30, 1980) Television Society Technical Report TEBS59-1. Takashi Mochizuki "A method for automatically removing FM multipath distortion using an adaptive digital filter"
(1986.3) Television Society Journal Vol. 1.39. No
, 3).

[Problems to be Solved by the Invention] However, as mentioned above, in the conventional technology, research is underway to remove multipath interference in the intermediate frequency stage of a receiver, and this method requires a large amount of calculation for interference removal. In addition, since the signal frequency of the intermediate frequency stage is a high frequency of IMH2 or higher, real-time processing is difficult, and it has not been put to practical use yet.

From this point of view, the present applicant has filed applications dated December 28, 1988 (Japanese Patent Application No. 62-329869) and (Japanese Patent Application No. 62-329870) to obtain low-frequency baseband signals after FM demodulation. The present invention aims to provide an FM multiplex data broadcasting receiver that can relatively easily remove interference waves through real-time processing based on the correlation between interference waves and stereo signals contained in FM multiplex data channels.

Here, in the former application (Japanese Patent Application No. 62-329869), the DC deviation of the data signal due to multipath interference is detected from the demodulated signals of the ■ axis and the Q axis of FM multiplex data broadcasting using an average value calculation circuit. Discloses a configuration in which a signal obtained by demodulating a signal obtained by doubling a stereo signal is used as a fluctuating signal, and interference removal signals for the ■-axis and Q-axis are created by calculation from both signals, and multipath interference components are removed by mixing with the input signal. There is.

In addition, in the latter application (Japanese Patent Application No. 62-329870), the in-phase component of the ■ axis and the orthogonal component of the Q axis are calculated from the signal obtained by doubling the stereo signal, and the signal is used to remove the interference wave signal. The data signals detected by the I-axis and Q-axis carrier waves from the demodulation circuit are averaged by an average value calculation circuit, thereby detecting the amount of multipath interference, and a multipath interference cancellation signal is obtained by calculation from both. This is then mixed with the input signal to remove multipath interference components.

However, the former application (Japanese Patent Application No. 62-329889)
The FM multiplex data broadcast receiver uses the condition that the carrier frequency of the FM multiplex data broadcast is twice the frequency of the stereo signal, so it is difficult to receive terrestrial TV multiplex facsimile broadcasts that differ from this condition. The disadvantage is that it cannot be used on a machine.

Furthermore, the FM multiplex data broadcasting receiver according to the latter application (Japanese Patent Application No. 62-329870) has, in addition to the above-mentioned drawbacks, the drawback that it requires a special circuit, resulting in a complicated circuit configuration.

As described above, a multipath interference removal device for terrestrial television multiplex facsimile broadcasting could not be realized.

Therefore, in view of the above-mentioned points, an object of the present invention is to provide a device that can reduce multipath interference with a simple configuration.

[Means for Solving the Problems] A multipath interference removal device according to the present invention includes a first bandpass means for extracting an audio subchannel signal from an audio baseband signal of an audio detection output, and
a multiplying means for multiplying and generating a second harmonic; a differentiating means for differentiating the second harmonic; a level control means for controlling the signal level of the differentiated signal; and a circuit for delaying the audio baseband signal. a delay means for compensating for the delay amount of the signal; a mixing means for mixing the level-controlled signal and the delayed signal in opposite phases; and a mixing means for extracting a specific frequency region of the mixed output and outputting a control signal for the level control means. and second bandpass means.

[Function] The present invention provides an interference removal signal corresponding to multipath interference from a received audio subchannel signal in order to reduce interference due to harmonics of the audio subchannel that occurs when receiving in a place where there is multipath interference. By creating a signal and canceling the interference, multiplexed signals can be received without error.

[Example] Hereinafter, the present invention will be explained based on Examples.

As a premise for explaining an embodiment of the present invention, first, the influence of terrestrial television audio multiplexed facsimile signals on the transmission path will be described.

FIG. 3 shows the spectrum of a television audio baseband signal. The facsimile signal is multiplexed into a band from 4fH to 5fH (fH: horizontal synchronization frequency).

The facsimile signal is a four-phase DPS signal with a carrier frequency of 70.8 kHz and a transmission speed of 18.0 kb/s. This audio baseband signal is FM modulated and transmitted.

Next, multipath interference of FM waves will be explained.

According to Carlson's theory (FM Radio Engineering 2 Nikkan Kogyosha, edited by Sugawara), -Characteristics U (ΔΩ) of the FM wave transmission path in boat fishing
There is a nonlinear distortion, and U(ΔΩ) becomes U(ΔΩ)−1+α1
(ΔΩ) 10 α2 (ΔΩ) 2 ÷ α, (ΔΩ) 3...+
j(β1(ΔΩ)◆β2(ΔΩ)2◆β3(ΔΩ)3+
), and if the modulation signal of the FM wave is μ(1), then the distortion Ω generated in this transmission path is
d(t) can be calculated from the series α, β and μ(1) as follows.

Now, if the audio subchannel signal is Vcos (Ωst), the modulation signal μ(1) is. (m = modulation index, Ωc: 4i1 transmission frequency) On the other hand, the characteristics of the FM wave transmission path when there is multipath are as follows:
There is only one reflected wave, and the phase difference between the direct wave and the reflected wave is 90°
If so, U(ΔΩ) = 1-r 5in(τΔΩ)-j rc
It can be approximated by os(τΔΩ). Here, r is the level ratio of the reflected wave and the direct wave, and is the delay time of the reflected wave. This U (
ΔΩ) becomes by series expansion. From this μ(1) and U(ΔΩ), the previous Ωd
From the equation (t), the distortion generated by multipath is as follows.

Here, if the term τ3 is omitted as τΩ×1, Ωd
(t) becomes.

That is, a signal obtained by squaring and differentiating the modulation signal μ(1) becomes the main component of multipath interference.

FIG. 4 shows the spread of the doubled signal of the audio subchannel when double harmonic components are generated due to multipath interference. As shown in this figure, it can be seen that the signal spreads over a wide band including the facsimile channel.

FIG. 5 is a system diagram after television voice detection in a conventional facsimile receiver. On the facsimile receiver,
The facsimile channel is extracted from the audio baseband signal (see Figure 3) obtained by FM detection (50) of the TV audio signal, and is subjected to 4-phase PSK detection (54) and differential conversion (56). and received by the facsimile receiver as a facsimile data signal (5
8) Do.

FIG. 1 is a system diagram of a facsimile signal receiving section to which the present invention is applied. In this figure, 2 is an audio detector, 4 is a multipath interference removal circuit (ghost canceller) which will be described in detail below, 6 is a bandpass filter, 8 is a detector for the 4-phase PS, 10 is a differential converter, 12 is a facsimile receiver. In this way, the multipath interference removal circuit 4 is inserted in the audio baseband signal processing section shown in FIG. 5 in this figure.

FIG. 2 shows a detailed configuration of the multipath interference cancellation circuit (ghost canceller) shown in FIG. 1. In this figure, the input signal 20 is the audio baseband signal shown in FIG. BPF 230 extracts the audio subchannel signal from the audio baseband signal. Next, the extracted audio subchannel signal 31 is doubled by a doubling circuit 32 to obtain μ2(t) described above.

After the band of the facsimile channel and the band used for detection are extracted by the PBF 334, they are differentiated by the differentiation circuit 36 to obtain the above dμ2(t)/dt.

Next, the amplitude of the differential signal is corrected by the 366 circuit 38,
A mixer 26 mixes the input signal 20 with a signal 24 whose delay amount in the circuit has been corrected by a delay circuit 22 in reverse phase.

The mixed signal 28 is converted into a control signal 42 by a BPF 444 which has a band of 3 fH to 4 fH and can remove the voice control signal portion.

This control signal 42 is double-wave rectified by the detection circuit 40 and then averaged to become a control signal for the AGC circuit 38, forming a closed loop.

Further, by using the delay circuit 22 as a tapped delay circuit and controlling the control output of the detection circuit 40 to the GG circuit 38 to be minimized, it is possible to reduce the phase change of the interference wave caused by multipath interference.

[Effects of the Invention] When the present invention is applied to terrestrial television audio multiplex facsimile broadcasting, it is possible to reduce the interference waves caused by multipath interference with a simple circuit in order to avoid a large deterioration in transmission characteristics due to multipath interference. This makes it possible to realize an apparatus that is excellent in terms of economy.

The present invention can also be applied to FM multiplex data broadcasting.

[Brief explanation of the drawing]

Fig. 1 is a system diagram of a facsimile signal receiving section to which the present invention is applied. Fig. 2 is a diagram showing a multipath interference removal circuit to which the present invention is applied. Fig. 3 shows an audio baseband spectrum of terrestrial television broadcasting. 4 is a diagram showing the band occupied by the second harmonic of the audio subchannel on the audio baseband spectrum, and FIG. 5 is a system diagram of a conventionally known facsimile signal receiving section. 2...Audio detector, 4...Multipath interference removal circuit, 6...Band pass filter, 8...Detector for 4-phase PS, 1O...Differential converter, 12... Facsimile receiver, 22...Delay circuit, 26...Mixer, 30.36.44...Band pass filter, 32...
- Double multiplier (square) circuit, 36... Differential circuit, 38... AGC circuit, 40... Detection circuit.

Claims (1)

[Scope of Claims] 1) a first bandpass means for extracting an audio sub-channel signal from an audio baseband signal output from audio detection; a multiplication means for doubling the sub-channel signal to generate a second harmonic; differentiating means for differentiating the second harmonic; level control means for controlling the signal level of the differentiated signal; delay means for delaying the audio baseband signal to compensate for the delay amount of the circuit; and a second bandpass means for extracting a specific frequency region of the mixed output and outputting a control signal for the level control means. Multipath interference removal device.