JPH0253305A - Fm demodulator - Google Patents

Abstract

PURPOSE:To attain the control of a feedback gain of an FM detector automatically by setting an FM threshold level in response to a signal corresponding to the error state of a reception data. CONSTITUTION:Correction error bits for (272, 190) or (16, 5) codes sent multiplexedly to a data channel are always counted and control signals 114-116 are outputted in response to the quantity to set a gain alpha of a voltage controlled oscillator (VCO) 115, the band width of a band pass filter 102 and the attenuation of an attenuator 112. Thus, the output signal is amplified by a multiple of (1+alpha). Thus, the threshold level is expanded by nearly 10logalpha and the error due to burst FM noise is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an FM demodulator applicable to satellite broadcast data channel receivers, FM multiplex broadcast receivers, and the like.

[Summary of the Invention] The present invention relates to an FM demodulator that can be applied to, for example, a receiving circuit for a satellite data channel, and is capable of automatically controlling an FM threshold value even when the error rate worsens due to rain or the like. , so that signals are always received under the best reception conditions. In other words, when the reception input terminal becomes low due to rain or the like and bit errors increase, this is detected and this information is fed back to the FM detector (frequency compression feedback type or PLL type, etc.) to lower the FM threshold level. This ensures stable and reliable reception of satellite data channels. A similar concept can also be applied to an FM multiplex broadcast receiver.

[Prior Art] Conventional satellite/data channel receivers have used a method in which the output of FM demodulation is simply demodulated into QPS and decoded. In this method, impulse noise due to the threshold begins to appear when the C/N is about 9 dB, and bit errors begin to occur in the received data. Particularly in data broadcasting that transmits important data such as computer programs, this bit error is fatal. Therefore, in data broadcasting using Japanese satellite data channels, a powerful (272,19
0) C/N=6dB using code error correction method
The system is designed to enable services up to

On the other hand, the (272,190) code has the ability to correct errors of approximately 11 bits out of 272 bits, and for errors larger than this, only about 2% are missed, and all remaining errors can be detected. becomes. Therefore, if you constantly monitor the number of error bits corrected or the number of blocks that cannot be corrected, you can grasp the reception status.
By controlling the feedback gain of the detector using this information, the best received signal can be obtained.

[Problems to be Solved by the Invention] However, although various methods for lowering the FM threshold level have been announced for conventionally known frequency compression feedback detectors or PLL type detectors, this FM A technology that automatically controls the threshold level and always receives signals in an optimal state has not yet been developed.

Therefore, an object of the present invention is to automatically control FM in view of the above points.
An object of the present invention is to provide an FM demodulator configured to control the feedback gain of a detector.

[Means for Solving the Problems] In order to achieve the above object, the present invention includes a monitoring means for outputting a signal corresponding to an error state of received data, and an FM threshold in response to an output from the monitoring means. and control means for setting the level.

[Operations] According to the present invention, the feedback gain of the FM detector is controlled based on information regarding the number of error bits obtained from an error correction circuit or the like, so that the bit error rate of the data channel is optimized.

[Example] Hereinafter, the present invention will be described in detail based on Examples.

FIG. 1 shows an embodiment of a frequency compression feedback type detector to which the present invention is applied. In this figure, 100 is the I of IGHx.
F flaw signal 101 is a multiplication circuit, 102 is a bandpass filter, 1
03 is an amplitude limiter and discriminator, 104 is a baseband filter, 105 is a voltage controlled oscillator (O to V), 1
0B is a data channel decoder, 112 is an attenuator for level adjustment, 107 is a multiplier output signal, 10
8 is a VCO output signal, 109 is a bandpass filter output signal,
110 is a discriminator output signal, 113 is a baseband signal,
114 is a level control signal, 115 is a VCO gain control signal, and 116 is a bandwidth control signal.

FIG. 2 represents a current satellite broadcast baseband signal. The audio signal is 5.73M1 (2 transmitted on z subcarrier,
The remaining time slots are used for data channel transmission. The capacity of the data channel varies depending on the audio transmission mode, but is approximately 200 kbps in the lowest case.

In FIG. 1, the IF signal 100 is, for example, A CO5 (
(+j at+ψ(t)). The center frequency of VCO105 is ω. -ω. shall be. If the output signal frequency ωl of 0105 is set to ω+=(ω0−ω.)+αe (t), the output signal 108 of VC0105 is 60(t)=2
cos((cos((ωc −ωo)t +, 't a
e (t)dt). e (t) is the output signal 110 of the amplitude limiter and discriminator 103, and α is a value corresponding to the gain of the VCO 105.

The output signal 107 of the multiplier 101 is the IF signal 100 and the V
The CO output signal 108 has a difference frequency and a sum frequency component. The sum frequency is 2ω. -ω. Centered on , the difference frequency is ω. Since the signal is centered at , the bandpass filter 102
Well then, ω. The frequency component of the sum is removed around . Therefore, the output of bandpass filter 102 is ep(t
)=A(cos (ω, t+ψ(1)−cho(” e (
L) dt)) becomes.

Since the amplitude limiter and discriminator 103 performs frequency detection on e2, its output signal 110 is as follows. Therefore.

Also, e p(t) = 8(cos(ω, t+□ψ(1)))
1÷α. This indicates that the angular deviation ψ(1) of the human input signal is reduced to □1+α. Therefore, the bandwidth of the bandpass filter 1θ2 may be 2 (ΔF+28)X−1+α. Here, ΔF represents the maximum frequency deviation, and B represents the baseband bandwidth. Also, the output e (
Since t) becomes ψ/(1+α), the level simply becomes □.

1+α Next, let's talk about noise.

Since the amplitude change is removed by the amplitude limiter, it becomes 1″m e (t) = −[ψ(1) +Δψ(t)]l+α. However, Δψ is the frequency change due to noise. Therefore, The S/N at the output is the same as that of a normal demodulator.However, since the passband width of the bandpass filter 02 is □, the C/N when the discriminator power is increased by 1+α becomes larger by (l+α). Therefore, the threshold level will be expanded by approximately 10 log α, reducing errors due to bursty FM noise.

In one embodiment of the present invention, the correction error bits of the (272.1!IQ) code or (16,5) code multiplexed on the data channel are constantly counted, and the control signals 114, 115, 1lli By outputting , the gain α of VCOIQ5, the bandwidth of 1F band filter 102, and the attenuation rate of attenuator 112 are set. This increases the output signal by (1◆α).

In addition, in the configuration shown in FIG. 1, if α cannot be adjusted in multiple stages, a sufficient improvement effect can be obtained by simply making it possible to digitally switch α between 1 and 5, for example. .

Although the above description is an example in which the present invention is applied to a satellite data channel receiver, application to other systems is also possible. For example, the Japanese system for FM multiplex broadcasting is the same as satellite data channels (272
, 190) code as the error correction code, it is possible to configure the system with the same circuit configuration as that shown in FIG.

Figure 3 shows the baseband signal of Japanese FM multiplex broadcasting. It is possible to transmit data at 4Bk bps using differential QI'SK using subcarriers 76k)lz. FIG. 4 shows a circuit configuration when the present invention is applied to this FM multiplex broadcast. Although the circuit configuration is the same as in FIG. 1, the respective parameters are naturally different.

Also, multiplexed signal decoder 400 is quite different from data channel decoder 106 of FIG.

The European FM multiplex broadcast RDS differs from the system shown in Figure 3 in that it uses a subcarrier at 57kl (x).

The error correction code uses a (26,16) shorted cyclic code. In this case as well, the gain of the vCO, the constant of the bandpass filter, and the output gain can be controlled from the decoding information of the (26,16) code to optimize reception.

Furthermore, if the multiplexed signal is a frequency modulation signal such as MS, a sufficient limiter n is applied for decoding the multiplexed signal, so the attenuator 112 in FIG. 1 changes only the video signal. The configuration may be such that the multiplexed signal is not changed.

[Effect of the invention] As described above, according to the present invention, the optimal FM
Since a threshold value can be set, the best reception condition can be maintained. More specifically, the following effects can be obtained.

■ In satellite data channels or FM multiplex broadcasting, the number of error bits or packets obtained when decoding an error correction code is counted, and the vCO gain of the frequency compression feedback type FM detector is controlled based on the counted number. If there are many FM threshold levels, the FM threshold level can be controlled to be lower.

(2) At the same time, the output gain also decreases, so bit errors can be suppressed as much as possible by increasing the gain by a corresponding amount and controlling the bandpass filter.

■ The circuit can be easily configured and integrated into an IC.

■ Naturally, the range of services will expand, and TV, which is a popular program, will
The quality of the screen or FM stereo broadcast will also improve.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an embodiment of the present invention; Fig. 2 is a spectrum diagram showing a satellite broadcasting baseband signal; Fig. 3 is a spectrum diagram showing a baseband signal of FM multiplex broadcasting; FIG. 3 is a diagram showing an example of application of the invention to an FM multiplex receiver. ...IF signal, ...multiplier circuit, ...bandpass filter, ...amplitude limiter and discriminator, ...baseband filter, ...Vfl:O (voltage controlled oscillator), 106... - Data channel decoder, 112...attenuator.

Claims (1)

[Claims] 1) A monitoring device for outputting a signal corresponding to an error state of received data; and a control device for setting an FM threshold level in response to an output from the monitoring device. Features of FM demodulator.