JPS6369353A - Data signal demodulation system - Google Patents

Abstract

PURPOSE:To prevent a crosstalk with an adjacent channel even if a frequency deviation by an outdoor down-converter takes place by providing an AFC loop adjusting the oscillated frequency of a local oscillator to obtain an intermediate frequency in a way that the intermediate frequency is an object frequency in an FSK demodulator. CONSTITUTION:An FSK data sent in a carrier frequency 109.025MHz is amplified by a high frequency amplifier 301 via an input terminal 300. The output of a high frequency amplifier 301 is mixed with a local signal from the local oscillator 303 in terms of frequency at a mixer 302 and converted into an intermediate frequency signal (10.7MHz). The oscillation frequency of the local oscillator 302 is adjusted automatically by the control of a microcomputer 311 and the oscillator 302 acts like decreasing the frequency deviation (nearly + or -150KHz) of the outdoor down-converter at the pre-stage of the FSK demodulator.

Description

DETAILED DESCRIPTION OF THE INVENTION [Objective of the Invention (Field of Industrial Application)] The present invention relates to a data signal demodulation method for improving the signal demodulation function of a terminal in, for example, a satellite broadcasting system.

(Prior Art) In a satellite broadcasting system, a method is adopted in which a signal transmitted from a broadcasting satellite is received and processed at a terrestrial retransmission station, and then transmitted again to the receiver side.

FIG. 8 shows the structure of the transmitting flII (terrestrial retransmission station) and the receiving side (outdoor down converter and subscriber's home terminal). The explanation will start from the terrestrial retransmission station, which is the transmitting side. Television broadcasts transmitted from broadcasting satellites.

The signal is received by a terrestrial retransmission station and sent to each subscriber terminal.
It is retransmitted at a frequency of 2500 MHz to 2644 MHz in the HF band, but when retransmitted, N
If it is retransmitted as a TSC signal, it will be retransmitted as a TSC signal.

A special telepic that compresses and inserts two channels of store movie signals and FM audio signals into a 6 MHz band corresponding to one channel of an FM signal.

may be retransmitted as a link signal. The former is used, for example, to retransmit free programs, and the latter is used to retransmit paid programs. To view special television signals, an external decoder is required in addition to the normal receiving terminal.

As shown in FIG. 8, as television signal sources on the transmission side, a first television signal source 101 for band compression, a second television signal source 102 for band compression, and an NTSC signal source 103 are installed. .

First and second television signal sources 101 for band compression.
Video baseband signal V of each NTSC system from 102
l and V2 are input to the video processing circuit 104, where they are band-compressed. Band-compressed video signals VJa, Vj
m is input to the intermediate frequency conversion circuit 105, where the video signal Vja is AM-modulated with carrier waves of 41.30 MHz, and the video signal VJm is AM-modulated with carrier waves of 45 and 75 MHz, respectively.

On the other hand, first and second television signal sources 1 for band compression
Each audio signal AJ from 01,102.

A2 is 46 in the intermediate frequency conversion circuit 105, respectively.
．． 75 Mllz, FM modulated with a carrier wave of 46.45 MHz. Furthermore, the transmission data from the data generator 100 is FM modulated using a 46.75 MHz carrier wave. In other words, when transmitting data to subscriber terminals,
A method is adopted in which data is superimposed on the carrier wave of the audio signal A1 and transmitted along with the video signals VJa, V2m and the audio signal A2 through a specific allocated channel. A computer is used as the data generator 100, and the transmitted data includes a channel map for each subscriber.

As a result of the above, the frequency stiffness of the band-compressed television signal output from the intermediate frequency conversion circuit 105 is as follows:
The result is as shown in FIG. Further, an intermediate frequency conversion circuit 105
The intermediate frequency signals VIP and AIF outputted from the
Converted from 0 MHz to 450 MHz. The bandwidth of one channel of television signal is 6 Ml (z,
In this case, two channels are compressed in this band as shown in FIG. This high frequency signal VRF,
The ARP is further supplied to the ap converter 10B via the mixer 107, and is converted to a frequency of 2500 MH in the SHF band.
z ~ 2644 MHz. The signals converted in this way are transmitted as air waves from the transmitting antenna 109 to each subscriber side.

On the other hand, the normal NTS transmitted from the NTSC signal source 103
In the case of a C signal, the video signal is AM modulated using a 45.7 MHz carrier wave in the intermediate frequency conversion circuit 110, and the audio signal is AM modulated using a 41.25 MHz carrier wave.
M modulated. The intermediate frequency signals VIP and AIF output from the intermediate frequency conversion circuit 110 are transmitted to the high frequency conversion circuit 11.
1 and is converted into high frequency signals VRF and ARP. After this, the process up to the transmitting antenna 109 is the same as in the case of a band-compressed television signal.

As mentioned above, the television signal and data transmitted from the terrestrial retransmission station are sent to the receiving antenna 112 on the receiving side.
Received at.

The received signal is transmitted by an outdoor down converter 113.
The frequency is converted from the SHF band to a VHF or UHF band signal. Then, it is introduced to the subscriber's satellite broadcasting terminal 114 via a cable.

The distributor 115 of the terminal device 114 is connected to the tuner 116 and the F
The signal is guided to the SK demodulator 117. The channels 116 can select the television signal of each channel based on the channel selection data from the microcomputer 118.

When the selected television signal is a normal NTSC signal, the output signal from the tuner 116 is supplied to a video intermediate frequency circuit 120 and an audio intermediate frequency circuit 121, and is converted into an intermediate frequency signal of a predetermined frequency. Ru. In this case, the switches 122 and 123 select the outputs of the video intermediate frequency circuit 120 and the audio intermediate frequency circuit 121 under the control of the microcombi 118. The audio intermediate frequency signal is subjected to FM detection by the FM detector 124 and reproduced as an FM signal included in the original telepino signal. This audio FM signal is supplied to a high frequency remodulation circuit 126 via a volume adjustment circuit 125.

This high frequency remodulation circuit 126 is connected to the video intermediate frequency circuit 12.
The output terminal 1 converts the video intermediate frequency signal from 0 and the audio FM signal into a 3-channel or 5-channel high frequency signal that can be received by a normal television receiver.
27. Therefore, this output terminal 12
7 is connected to the antenna terminal of the subscriber's indoor television receiver.

On the other hand, when a band-compressed television signal is selected, the microcombi 118 switches to the switch 122.
．． 123 to the external decoder 128 side. The external decoder 128 receives the output of the tuner 116 and outputs a band-compressed television as shown in FIG. 9 and FIG.
decodes the signal into an intermediate frequency signal. The video signal is input to the high frequency remodulation circuit 126 via the switch 122, and the audio intermediate frequency signal is input to the high frequency remodulation circuit 126 via the switch 123, the FM detector 124, and the volume adjustment circuit 25.

Next, in the above broadcasting system, for example, one of the audio signal channels shown in FIG. 10 may be used for FSK data transmission as shown in FIG. 12. In this case, the switch 207 in the FSK demodulator 117 is manually switched to the FM detector 124 side, for example.

Therefore, the data decoded by the decoder 128 and subjected to FM detection is introduced into the microcomputer 118 via the data identification circuit 208. Note that the microcomputer 11B determines which of the two band-compressed television signals to select.
It is determined by transmitting predetermined data to B. Data communication between the external decoder 128 and the microcomputer 11B is performed via the dedicated connector 210.

Next, if the carrier frequency of data transmitted to the terminal 114 is 109.025 W (l), the FSK demodulator 11
7 is used. At this time, switch 207 is set to FM
It is switched to the detector 206 side.

FSK transmitted on a carrier frequency of 109.025MHz
The data is transmitted to the high frequency amplifier 201 in the FSX demodulator 111.
The local oscillator 203 is amplified by the frequency mixer 202.
is mixed with the output from and frequency converted to an intermediate frequency of 10.7 MHz. This intermediate frequency signal is amplified by an intermediate frequency amplification circuit 204, subjected to band limitation by a bandpass filter 205, and then supplied to an FSK detector 206. The bandpass filter 205 is set to have characteristics as shown in FIG. 11, as will be explained later.

The demodulated output of FSK detector 206 is input to data identification circuit 208 via switch 207. The data identified here is read by the microcomputer 118.

Next, the characteristics of the FSK demodulator IJ7 will be explained. The high frequency amplifier 201 has automatic gain control (AGC) fed back from the F'SK detector 206! The gain is controlled by the pressure.

This is because the frequency characteristics of the bandpass filter 2050 are set to those shown in FIG. That is, the characteristic of the bandpass filter 205 is that the center frequency is 10.7 MH
At frequencies higher than z, the signal level is attenuated as the frequency increases. Therefore, when a strong level signal passes through this filter, signal distortion occurs, so it is necessary to suppress the gain to prevent distortion from occurring.

The reason why the frequency characteristics of the bandpass filter 2050 are set as shown in FIG. 11 is as follows.

The frequency shift in outdoor down compaction 113Vc is
About ±150 KEz, and F on the transmitting side
Since the frequency shift of SK modulation is ±75 KEz, the frequency band as a filter is required to be 450 KHK0.
Next, the FSK data is transmitted on the carrier wave of the first audio signal ^1 of the band-compressed television signal, for example, and a second audio signal A1 is placed on the carrier wave of the first audio signal There is a carrier wave of the audio signal A2 (see Figs. 1θ and 12), and according to [, in the outdoor down converter 113, ±
When a frequency shift of 150 KHz occurs, the second audio signal also causes interference with 4501Q (which should not be mixed into the filter band of s). Therefore, at frequencies higher than the center frequency of 10.7 MHz, the bandpass filter 205 is designed so that the signal level is attenuated as the frequency increases.
At 10.7 MHz + 150 KHz, the attenuation is 8
It is expressed as dB.

Note that the accuracy of the outdoor down converter 113 deviates by as much as ±150 kHz as described above because the frequency of the transmitted signal is in the SHF band from 2500 MHz to 2644 MHz.
When converting a signal with such a frequency as MHz, even if you use something with high frequency accuracy such as a crystal oscillator, the worst-case accuracy is about 50 ppm, so at most it is ±125 KHz. This is because accuracy can only be maintained up to Therefore, outdoor down converter 1
It is difficult to significantly improve the 130 frequency accuracy from the current situation.

(Problems to be Solved by the Invention) As described above, in the conventional system, the bandpass filter 205 in the FSK demodulator 111 is set to have the characteristics as shown in FIG. Temporarily, this bandpass filter 205
If the frequency characteristic of K is flat, then the carrier frequency is 109
．． 025 MHS, carrier frequency 109.325 MHS (frequency difference + 300
kHz) will be mixed in with the audio signal. As a countermeasure for this, conventionally, the center frequency is 10.7 M.
) At frequencies higher than Is, a 450 KHz band filter is used that attenuates the signal level as the frequency increases. However, there is a problem that this bandpass filter 2051fl causes signal distortion due to group delay.

Therefore, an object of the present invention is to provide a data signal demodulation method that can prevent interference with adjacent channels (FSK data and audio signal channels) without causing group delay even if a frequency shift occurs due to an outdoor down converter. shall be.

[Structure of the Invention] (Means for Solving the Problems) In the present invention, in an FSX demodulator, the oscillation frequency of a local oscillator for obtaining an intermediate frequency is adjusted so that the intermediate frequency becomes a target frequency. An AFC route is provided.

(Function) With the above means, even if there is a down converter frequency shift, the shift in the intermediate frequency can be corrected, and the bandwidth of the intermediate frequency band filter is narrowed to improve the selectivity.
Moreover, the factor of group delay distortion can be eliminated as a flat frequency characteristic.

(Example) Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows one embodiment of the present invention, which is an FSK demodulator on the receiving side of a satellite broadcasting system.

F transmitted at carrier frequency 109.025 MHz
The SX data is sent to the high frequency amplifier 3 via the input terminal SOO.
It is amplified by 0. The output of the high frequency amplifier 301 is frequency mixed with a local signal from a local oscillator 303 in a mixer 302 and converted into an intermediate frequency signal (10.7 MHz).

In ζζ, the oscillation frequency of the local oscillator 302 is not fixedly adjusted as in the past, but is automatically adjusted by the control of the microcombi controller, which will be described later. ±150KHz)
functions to reduce

The 10.7 MHz intermediate frequency signal from mixer 302 is
After being amplified by an intermediate frequency amplifier 304, the signal is supplied to an FSX detector 306 via a bandpass filter 305. The detected output of FSX detection here is Sui.

The data is supplied to a data identification circuit 308 via a channel 302.

The data '01 or l'' identified by this circuit is input to the microcomputer 311.

As shown in FIG. 7, the band width of the bandpass filter 305 is 280 KHz, which is much narrower than the conventional 450 KHx, improving selection characteristics. The reason why the performance is improved in such a narrow band will be explained later.

Furthermore, in the FSK detector 306, a shift in the intermediate frequency 10.7M) Lx is detected due to the circuit configuration shown in FIG. That is, the frequency of the intermediate frequency signal is compared with the oscillation signal (10.7 MHz) of the intermediate frequency oscillator 402 in the frequency comparison circuit 401.

Therefore, from this frequency comparison circuit 401, a DC voltage (hereinafter referred to as AFC voltage) corresponding to the frequency difference between the actual input intermediate frequency signal and the target intermediate frequency signal is generated.
is obtained.

The relationship between the AFC voltage and the intermediate frequency described above has a so-called figure-eight curve characteristic as shown in FIG.

The origin of this curve (VAFC=Vr-Δf=o CJ point) is called the zero-crossing point. This point is
This is the point at which the target intermediate frequency and the actual input intermediate frequency match.

The AFCl[pressure having such a relationship is input to a comparator 309 as shown in FIG. 1, and is compared with the voltage from a constant voltage source 310 set to the target AFC voltage. As shown in FIG. 4, this comparator 309 outputs a level (+5V) if the AFC voltage is higher than the target value, and outputs a low level (Ov) if it is lower. This AFC
The detected voltage is the port P1 of the microcomputer 311.
is input.

As a result, the microcomputer 311 can detect a shift in the intermediate frequency, a shift in the frequency of the outdoor down converter.

The microcomputer 311 controls the oscillation frequency of the local oscillator 303 based on AFC detection based on pressure.

Now, the carrier frequency of the FSK data input to the mixer 302 is ftpt, the intermediate frequency output from this mixer 302 is fty, and the oscillation frequency of the local oscillator 303 is fL.
ocal, the following relationship holds true.

fXp = fly-fLocal ・
...(1) What can be learned from this equation is that the local frequency fL
If ocal is increased, the intermediate frequency fty will be decreased,
Conversely, if fLocal is decreased, the intermediate frequency fly will increase. Therefore, adjusting the value of the local oscillation frequency fLocal by the microcomputer 311 means that FM signals other than FSX data can also be received.

Now, the microcomputer 311 is the local oscillator 303
In controlling the local oscillation frequency, the phase synchronization processing circuit 3
12 to maintain the intermediate frequency output from the mixer 302 at a constant frequency. That is, port P2. P3
．． The i microcontrollers 311 from P4 provide frequency data and adjustment data to the phase synchronization processing circuit 312.

In this case, the microcomputer 311
The AFC detection signal of is sampled at regular intervals. 1st
If the AFC detection signal is at a high level in the second sampling, it means that the intermediate frequency has shifted higher than the target value of 10.7 MHz. Therefore, the microcomputer 311 sets the local frequency to a certain value Δf from the relationship of equation (1).
The frequency data is varied in the direction of increasing the height. If the AFC detection signal is at a high level in the second sampling, the frequency data is similarly varied. In this way, the local frequency is varied step by step, and sampling is not performed until the AFC detection signal changes from high level to low level, that is, until the zero cross point is found, and then the local frequency is varied by Δf. Figure 5 shows this situation.

Conversely, if the AFC detection signal is at low level during the first sampling, sampling and frequency variation are repeated until this signal changes from low level to high level.

Note that the initial value (corresponding to the oscillation frequency f0) given to the phase synchronization processing circuit 312 at the time of the first sampling is:
In equation (1), f 1 y = 10.7 MHz
, the value when fra=109.025MHz, that is, the value to set fLocal=98.325 kmlz.

In addition, the variable range of the local frequency is the outdoor down converter frequency shift of ±150 KHz and the FS on the transmitting side.
X modulation degree ±7510 (considering z, it is assumed to be ±225 KHz. If the zero cross point is not found within this frequency range, it is programmed to end the sanding operation with the frequency shifted by +225 KHS. .

Further, the microcomputer 311 C. While sampling the AFC detection signal, the FSX signal is demodulated by the FSX detector 306 and waveform-shaped by the data identification circuit 308.
It must be determined whether the data has the correct format to be processed by the microcomputer 311. In other words, the microcomputer 311 finds the zero crossing point from the AFC detection signal and determines whether the data is correct. It is programmed to continue AFC adjustment until it receives formatted FSX data.

However, settings are made to ensure that FSX data in the correct format can be received even if a zero-crossing point is not found. If the format of the FSX data is incorrect, AFCill adjustment is performed by changing the initial value or initial value of the same frequency data. FIG. 6 is a flow chart thereof.

Finally, when AFC adjustment is started, the initial value of frequency data is given to the phase synchronization processing circuit 312 (steps sz, s2). Next, a delay time is given until the oscillation of the local oscillator 303 stabilizes (Step 7'83), and the FSK
Determine whether the data format and format are correct (
Step 4).

If the FSK data format is incorrect, change the initial value (Stella fs5) and return to step S2. If the format is correct, determine whether the AFC detection signal is high level or low level. (Stella f
s6).

If the AFC detection signal is at a high level, the frequency data is changed by 41 minutes (steer, fs7.s8.89), and the FSK
It is determined whether the data format is correct (step 510). If the format is incorrect, Stella f
Return to s5, and if it is correct, determine the level of the AFC detection signal (Stella f S J J) 6 If the AFC detection signal is high level, the frequency variable range (+225 K
HS) is determined (step, fS12),
If it has not exceeded Stella 7'', the process returns to S7; if it has exceeded it, it ends once and prepares for the next AFC operation after a certain period of time (Step, fs13>).

- If the AFC detection signal is low level in Stellar fs6, change the frequency data by -31 minutes (steps sx4.szs, s1g) and check whether the FSX data format is correct. A determination is made (step 517). If the format is incorrect, step S5
If it is correct, the level of the AFC detection signal is determined (step 518).

If the AFC detection signal is low level, it is determined whether the frequency variable range (-225KHz) has been exceeded (
Ste, gu 519), if it has not been exceeded, Ste.

Return to step 814. Moreover, if it exceeds, once the intermediate frequency of the translator is finished, it is corrected to exactly the predetermined frequency* (10.7 MHz). Therefore, the band of the bandpass filter 305 does not need to take into account the frequency shift of the outdoor down converter as in the conventional case, and is set to 28010 (bandwidth of z) as shown in FIG. It has a flat frequency characteristic and can improve the FSK data detection sensitivity of the FSK detector 306. Furthermore, the bandwidth of this bandpass filter is 280.
By setting the frequency to 1 GIz, it is possible to effectively prevent interference with the audio signal carrier wave existing at +3001 GIz position of the FSK 7'-night carrier frequency. Furthermore, since the frequency characteristics are flat, no group delay distortion occurs.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is a circuit diagram showing an embodiment of the present invention.
Figure 3 is a diagram showing an example of the FC voltage detection circuit, Figure 3 is a diagram showing the output characteristics of the circuit in Figure 2, Figure 4 is an output characteristic diagram of the comparator in Figure 1, and Figure 5 is the diagram shown in Figure 1. 6 is a flowchart shown to explain an example of the operation of the circuit in FIG. 1, and FIG. 7 is a frequency characteristic diagram of the bandpass filter in FIG. 1. , Fig. 8 is an explanatory diagram of the configuration of a satellite broadcasting system, Figs. 9 and 10 are frequency spectrum diagrams shown to explain the transmission method of band compression television signal, and Fig. 11 is a diagram showing the band of Fig. 8. FIG. 12, which is a frequency characteristic diagram of the filter, is a frequency spectrum diagram showing an example of transmission of FSX data. 301... High frequency amplifier, 302... Mixer, 30
3... Local oscillator, 304... Intermediate frequency amplifier, 3
05...Band filter, 306...FSX detector,
308...Data identification circuit, 309...Comparator, 3
11... Microcomputer, 312... Phase synchronization processing circuit. Applicant's representative Patent attorney Takeshi Suzue National policy Figure 2 Figure 3 Figure 4 Urinary loss (ds) 1 loss (dB)! Figure 11

Claims (1)

[Claims] By converting the high frequency signal into a second high frequency signal in a down converter and further mixing this second high frequency signal with the local oscillation output of a local oscillator in a mixer, the FSK data carrier signal is intermediated. means for converting into a frequency signal; a bandpass filter whose band frequency characteristic is set to be flat, the filter taking the intermediate frequency signal as an input and supplying an output to an FSK detector; and an output intermediate frequency of the bandpass filter. means for obtaining a detection signal by comparing the frequencies of the signal and a target intermediate frequency from an oscillator and identifying whether the frequency deviation of the output intermediate frequency signal is increasing or decreasing compared to the target frequency value; phase synchronization processing means forming a phase lock loop with a local oscillator and setting the oscillation frequency of the local oscillator according to given frequency data; and sampling the detection signal, when the detection signal is at one level. , comprising means for resampling the detection signal by varying the frequency data, and repeating the sampling and variation of the frequency data until the detection signal changes from one level to the other level. data signal demodulation method.