JPH0787430B2 - Digital FM modulator - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM modulator for FM broadcasting, and more particularly to a digital FM modulator which directly receives a stereo audio digital signal.

[0002]

2. Description of the Related Art A conventional digital FM modulator, as shown in FIG. 2, stores a stereo audio digital signal (for example, 16 bits, clock frequency 384 kHz) in an accumulator 3.
A direct digital synthesizer 6 having a configuration in which the function converter 4 replaces it with sine wave waveform data and the DA converter 5 converts it into an analog signal after the accumulation processing is used. Since this direct digital synthesizer 6 changes the frequency of the analog signal in proportion to the input data, the output is obtained as an FM modulated wave. For this direct digital synthesizer, SP2 from Pressy
Integrated circuits such as 002 are well known.

[0003]

[Problems to be Solved by the Invention] This conventional digital F
In the M modulator, when the analog stereo original signal is reproduced in combination with the FM demodulator, there is a problem that the separation of the L and R channels of the Stareo signal is poor. this is,
Due to the effect of holding the digital signal by the sampling theorem, the high frequency range of the amplitude frequency characteristic after FM demodulation is attenuated by the holding time of the clock frequency or the output frequency of the direct digital synthesizer, and the L + R signal and L of the current FM broadcast shown in FIG. This is because a level deviation occurs with the −R signal.

An object of the present invention is to provide a digital FM modulator capable of compensating for attenuation at high frequencies and improving channel separation of stereo signals.

[0005]

According to the present invention, a stereo audio digital signal including a pilot signal is received,
In a digital FM modulator for converting an analog FM signal by a direct digital synthesizer, a frequency compensation filter for receiving the digital signal for stereo sound and giving an output to the direct digital synthesizer, and a part of the output of the direct digital synthesizer are received. An FM demodulator, a bandpass filter that extracts only the pilot signal from the output signal of the FM demodulator, a comparator that compares the converted voltage of the pilot signal from this bandpass filter with a reference voltage, and a comparator of this comparator. A digital FM modulator having an AD converter which converts an output voltage into a digital signal and feeds it back as a control signal of the frequency compensating filter is obtained.

[0006]

The present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of the present invention. First, a signal obtained by digitally converting a stereo audio signal including a pilot signal of, for example, 19 kHz passes through an input terminal 1 and a frequency compensation filter, for example, a digital equalizer 2
Entered in. In this digital equalizer 2, the amount of compensation is changed by a control signal. Specifically, it has an operating range for compensating for the high frequency attenuation of the input digital signal. The output signal of the digital equalizer 2 is converted into an analog FM modulated wave by the direct digital synthesizer 6 and output from the output terminal 11.

The direct digital synthesizer 6
Is an accumulator 3 for receiving and accumulating the output signal from the digital equalizer 2, a function converter 4 for converting the output signal of the accumulator 3 into waveform data of a sine wave, and a waveform from the function converter 4. And a DA converter 5 for converting data into an analog signal.

A part of the analog FM modulated wave from the direct digital synthesizer 6 is branched and demodulated by the FM demodulator 7 to output an analog baseband signal. This baseband signal has the spectrum shown in FIG. 3 and is input to the bandpass filter 8 to obtain only the 19 kHz pilot signal. This pilot signal level is converted into a DC voltage by the comparator 9 and then compared with a reference voltage to output an error voltage. This error voltage is AD
It is converted into a digital signal by the converter 10 and becomes a control signal for determining the compensation amount of the digital equalizer 2.

Now, when the clock frequency of the input stereo audio signal is converted from 384 kHz to 256 kHz, for example, due to the holding effect by the sampling theorem, the amplitude frequency near the L-R signal of the baseband signal after FM demodulation. The characteristic is more damped. Also, when it is necessary to change the holding time of the instantaneous frequency of the direct digital synthesizer 6, the similar attenuation characteristic near the high frequency changes. In the present invention, the amount of attenuation near this high frequency is F
After the M demodulation, it is known from the attenuation amount of the pilot signal of 19 kHz, and the compensation amount of the digital equalizer 2 is changed based on this information to flatten the amplitude frequency characteristic of high frequency.

[0011]

As described above, according to the present invention, the level of the pilot signal is detected, the compensation amount of the digital equalizer is changed by the detected voltage, and the amplitude frequency characteristic of the high frequency is corrected. It is not affected by the clock frequency of the signal or the holding time of the instantaneous frequency of the direct digital synthesizer, and as a result, it is possible to compensate for the attenuation in the high frequency range and improve the channel separation of the stereo signal.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a block diagram showing a conventional digital FM modulator.

FIG. 3 is a diagram showing a spectrum of a baseband signal after FM demodulation.

[Explanation of symbols]

 1 Input Terminal 2 Digital Equalizer 3 Accumulator 4 Function Converter 5 DA Converter 6 Direct Digital Synthesizer 7 FM Demodulator 8 Bandpass Filter 9 Comparator 10 AD Converter

Claims (3)

[Claims] 1. A digital FM modulator that receives a stereo audio digital signal including a pilot signal and converts it into an analog FM signal by a direct digital synthesizer, and receives the stereo audio digital signal and outputs the output to the direct digital synthesizer. A frequency compensating filter to be applied, an FM demodulator that receives a part of the output of the direct digital synthesizer, a bandpass filter that extracts only the pilot signal from the output signal of the FM demodulator, and a bandpass filter from this bandpass filter. A digital signal having a comparator for comparing the converted voltage of the pilot signal with the reference voltage, and an AD converter for converting the output voltage of the comparator into a digital signal and feeding it back as a control signal of the frequency compensation filter. FM modulator. 2. The digital FM modulator according to claim 1, wherein the direct digital synthesizer receives an output signal from the frequency compensating filter, performs accumulator processing, and a sine wave output signal of the accumulator. Function converter for converting to the waveform data of D, and D for converting the waveform data from this function converter to an analog signal
A digital FM modulator having an A converter. 3. The digital FM modulator according to claim 1, wherein the frequency compensating filter is a digital equalizer.