JPH0888578A - Receiver for broadcasting adapted to analog and digital signals - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable high-quality frequency demodulation through one A/D converter by supplying an IF signal to a frequency down converter connecting its output to an A/D converter to operate by subsampling. SOLUTION: An outdoor input IF signal is supplied to a SAW filter 1 and then supplied to a mixer 2 to which a clock is applied by an oscillator LO of 70 MHz. A frequency-down-converted signal is supplied to an input 3 of a circuit 4. The circuit 4 is provided with an amplifier 5 amplitude-controlled by an AGC circuit 6, an A/D converter 7 controlled at a sampling frequency fs of 100 MHz, a BPF 8, a frequency modulator 9 and an LPF 10 for generating a CVBS signal at its output. Further, this circuit is provided with an image carrier reproducer 11 for generating carrier waves with phases at 0 deg. and 90 deg. for QPSK demodulation, synchronous demodulators 12 and 13 for quadrature phase demodulation of QPSK and LPE 14 and 15 for generating channel signals Q and I. Thus, frequency demodulation is enabled through one A/D converter 7.

Description

Detailed Description of the Invention

[0001]

The present invention relates to analog and digital transmission using FM (frequency) modulation and QPSK (quadrature phase shift keying) modulation.
The present invention relates to a broadcasting receiver adapted to a signal converted into an F (intermediate frequency) signal.

[0002]

2. Description of the Related Art A digital TV broadcasting system for transmitting a TV (television) signal in a digital data format via a DBS (direct broadcasting satellite) has been developed in several countries for the purpose of a next-generation TV system. In terms of transmission, the QPSK modulation method is a digital modulation method in which the high frequency spectrum efficiency is most effective. The proposed digital QPSK demodulator still has an analog IF input, which is followed by a quadrature phase modulator with a low pass filter in front of the A / D converter for in-phase and quadrature phase components. Expected to be connected. The above methodology is significant in that it is currently not possible to directly digitize analog IF signals and in any case frequency down conversion is required. However, on the other hand, it is difficult to realize a quadrature demodulator for high IF frequencies. Another significant drawback of the above digital QPSK demodulator is that it requires two A / D converters, each consuming a large amount of power.

Direct IF sampling has made some wireless applications more cost effective than traditional analog signal processing, and is a digital solution to the video IF signal in satellite TV receiver design. It seems that it is no longer necessary to exclude measures. This solution should include conventional analog satellite broadcast reception using FM modulated signals.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to broadcast digital signals of the above type which are suitable for digital and analog input signals and which use only conventional A / D converters for analog and digital transmission over satellites. It is to provide a receiver.

[0005]

According to the present invention, the above I
The F signal is an FM that operates in a digital signal processing system.
The output is connected to the demodulator and the QPSK demodulator, and the output is supplied to the frequency down converter whose output is connected to the A / D converter operating in the sub-sampling. Nowadays,
The multi-standard conforming TV receiver must also function as a satellite broadcast receiver. Thus, the outdoor unit provides satellite channels already downconverted to the frequency band in the range of 950 to 1750 MHz handled by multi-standard tuners. To take advantage of the dual conversion tuner, the first IF frequency is around 2 GHz. For terrestrial and wireline transmission, for analog DBS transmission, the video signal is FM-modulated, so that a very high bandwidth of about 27 MHz is required according to the Carson formula. Therefore, the fixed bandpass filter in the first IF circuit is 2
It needs to be scaled in multiples between 3 and 3. If the expected bandwidth is approximately tripled for ground or wireline reception,
The second IF frequency needs to be tripled to maintain the above relationship. Otherwise, the image frequency components on the first IF band will adversely affect the performance of the system or the required analog filters will be extremely expensive. In consideration of such a situation, the second IF frequency is 130-140.
It must be on the order of MHz. Thus, the envisioned multi-standard tuner provides a second output for the 140 MHz satellite IF signal.

[0006]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described below with reference to the accompanying drawings so that the present invention may be better understood. In FIG. 1, an input IF signal sent from an outdoor station is supplied to a SAW (surface acoustic wave) filter 1 and then 70 MHz.
Is supplied to the mixer 2 clocked by an oscillator LO having a frequency of. The frequency down-converted signal is applied to the input 3 of the circuit 4. The circuit 4 includes an amplifier 5 whose amplitude is controlled by an AGC (automatic gain control) circuit 6, an A / D converter 7 controlled at a sampling frequency fs of 100 MHz, a bandpass filter 8, and an FM modulator 9. And a low-pass filter 10 for generating a CVBS signal at its output. Furthermore,
An image carrier regenerator 11 for generating carrier waves at 0 ° and 90 ° phases for QPSK demodulation, synchronous demodulators 12 and 13 for QPSK quadrature phase demodulation, and signals Q and I at the output of the circuit 4.
And low pass filters 14 and 15 for generating

The operation of the above circuit will be described below with reference to FIG. First, the output signal of the tuner passes through the SAW filter 1 at a center frequency of 140 MHz which suppresses most of the adjacent channel signal components ((a) of FIG. 2,
(See (b) and (c)). The input of the demodulator is generally constituted by the sum of the useful signal and the interfering signals of the common-channel and two adjacent channels.
The next mixer stage 2 operates at a local oscillation frequency between 60 and 70 Hz, which realizes a frequency shift to the IF signal around 75 MHz (see (d) and (e) in FIG. 2). The down-converted IF signal has an internal band pass limit AG before being input to the A / D converter 7.
It is supplied to the C amplifier 5. The AGC amplifier 5 needs to surely increase the entire dynamic range of the A / D converter 7 so as to keep the quantization noise to a minimum. The sampling frequency of the A / D converter 7 is to obtain a sufficiently wide frequency space between the periodic spectra which may be occupied by the residual portion of the adjacent channel signal when the attenuation of the SAW filter 1 is not sufficient. It is necessary to reach up to 100 MHz (see (f) of FIG. 2). A
The / D converter 7 is operated in the sub sampling mode,
Aliasing problems due to band-limited input signals do not occur. Due to the high sampling frequency, A
The efficient dynamic resolution of the D / D converter 7 can be reduced somewhat with respect to the nominal resolution. FM
It is expected that the distortion due to modulation will be negligible. A bandpass filter 8 (see (g) in FIG. 2) having an adaptively controlled bandwidth selects a desired signal in which the residual portion of the signal of the adjacent channel is minimized. To do. The FM demodulator 9 converts the IF signal into a CVBS signal and a subcarrier-modulated acoustic signal, and the CVBS signal and the subcarrier-modulated acoustic signal need to be separated by a low-pass and a band-pass filter. In particular, for the next color decoding unit the sampling rate has to be adapted to its clock frequency. Therefore, the low pass filter is also used as a sampling rate converter.

The DBS-Band plan was defined in the WARC'77 specification. Thereby, the maximum signal bandwidth was fixed at 27 MHz and the DBS satellite transponder bandwidth was designed accordingly. Therefore, 27MHz
The signal bandwidth of the D-DBS system direct TV
Can be considered as a stable specification that allocates a net data rate of about 40 Mbit / sec, which is higher than the data rate described in the proposal. This number is TCM-8 PS
This means that a modulation scheme such as K-modulation is used instead of an amplitude modulation scheme (such as 16QAM) that has good bandwidth efficiency but poor power efficiency. This point is very important when highly nonlinear channels are used, such as satellite transponders.

In order to receive the digitally transmitted video signal, an analog preprocessing block may be left behind. In the digital part, a quadrature mixer and a spatial carrier regenerator are added to demodulate the QPSK modulated signal. The carrier regenerator may be controlled by a digital QPSK demodulator not shown in FIG. A low pass filter is connected after the quadrature demodulator to generate in-phase and quadrature-phase symbol patterns at its output. This has the advantage that digital quadrature demodulation is implemented without any phase and amplitude matching problems between the I and Q channels. Moreover, considering the digitization of analog FM transmission, in the conventional QPSK demodulator, two or, in some cases, three A / D converters are required. In case of 1 A /
Only a D converter is needed.

[0010]

The main advantages obtained by the present invention are as follows. The proposed architecture is based on satellite I
Allows the F signal to be digitized, whether as an analog or digital signal. Due to the robustness of the selected modulation scheme, for normal video applications,
It becomes possible to use the A / D converter with reduced resolution at a higher sampling frequency. Furthermore, the number of A / D converters required for digitization is only one, compared to two or three for conventional solutions that minimize power consumption. On the other hand, by utilizing digital signal processing, FM demodulation and QPSK demodulation are performed with high quality. Nevertheless, the number of analog components required is quite small.

[Brief description of drawings]

1 is a block diagram of a television receiver operating in accordance with the present invention.

FIG. 2 is a diagram showing a frequency spectrum for converting an analog satellite IF signal into a digital IF signal.

[Explanation of symbols]

 1 SAW (surface acoustic wave) filter 2 mixer 3 input 4 circuit 5 amplifier 6 AGC (automatic gain control) circuit 7 A / D converter 8 bandpass filter 9 FM modulator 10, 14, 15 low-pass filter 11 image carrier regenerator 12,13 Synchronous demodulator

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical indication location H04N 7/20

Claims (3)

[Claims] 1. A broadcast receiver adapted for signals which are transmitted analogically and digitally using FM and QPSK modulation and are converted into analog IF signals, said IF signals being both digital signal processed. F operating in the method
A receiver, the output of which is connected to an M demodulator and a QPSK demodulator, and the output of which is supplied to an A / D converter operating in sub-sampling. 2. The A / D converter operates at a sampling frequency of about 100 MHz.
The receiver described. 3. The receiver according to claim 1, wherein the sampling frequency has a value such that two spectra of the signal are below the sampling frequency.