GB2237710A - CATV terminal extracts digital stereo audio signal - Google Patents

Abstract

A second audio IF signal obtained from an audio IF signal is detected to obtain an analog baseband audio signal, which is then subjected to sound volume control and muting control. A digital stereo audio signal is extracted from the second audio IF signal by a band pass filter 30, and then combined with a baseband video signal and a sound-controlled analog baseband audio signal by circuitry 21, 31. <IMAGE>

Description

CATV TERMINAL APPARATUS The present invention generally relates to a CATV terminal apparatus, and particularly relates to a CATV terminal apparatus having functions of, for example, sound volume control and muting control on an audio signal.

In the CATV terminal apparatus of the above kind, in order to perform sound volume control and muting control on an audio signal, it is necessary to convert the audio signal into a baseband signal once. To this end, a conventional CATV terminal apparatus having such a configuration as shown in Fig.

5 has been generally used. In this figure, reference numeral 11 designates an input terminal of the CATV terminal apparatus to be connected to a center (head end) through a transmission line (not shown) such as a coaxial cable. Reference numeral 12 designates a converter for selecting a desired one from television signals transmitted through a number of channels in a frequency band of from 50 MHz to 500 MHz, and for converting the frequency of the television signal of the selected channel so as to obtain an intermediate-frequency (IF) signal. The IF signal converted by the converter 12 is branched by a branching device 13, and the resultant output signals are supplied to a variable attenuator 14 and a band pass filter (BPF) 15, respectively.

The BPF 15 has a pass band for passing an audio IF signal component in the IF signal. The audio IF signal thus passed through the BPF 15 is supplied to a demodulator 16 and an AM detector 17. From the audio IF signal, the AM detector 17 extracts, through AM-detection, scrambling-concerned data which has been superimposed by AM modulation onto the audio IF signal as in-band data. The data extracted by the AM detector 17 is supplied to a descramble controller 18. The descramble controller 18 generates an expansion signal at predetermined timing to thereby control the attenuation (actually amplification) factor of the variable attenuator 14 on the basis of the scrambling-concerned data supplied thereto so that the variable attenuator 14 expands the video IF signal which has been compressed by scrambling. Thus, the video IF signal is descrambled.The video IF signal descrambled by the variable attenuator 14 is amplified by an amplifier 19, and the amplified video IF signal is supplied to the demodulator 16 through a BPF 20 which passes a video carrier.

In the demodulator 16, a synchronous detection circuit 16a performs synchronous detection on the video IF signal supplied thereto through the BPF 20 so as to output a baseband video signal which is in turn supplied to an RF modulator 21, as shown in Fig. 6. Further, an inter-carrier detection circuit 16b detects the audio IF signal supplied thereto through the BPF 15 so as to output a second intermediate-frequency (SIF) signal obtained by the detection.

The SIF signal is supplied to a BPF 22, so that a specified frequency component of the SIF signal is passed through the BPF 22 and supplied to the demodulator 16 again. An FM detection circuit 16c FM-detects the specified frequency component of the SIF signal to thereby obtain a baseband audio signal. Then, after being subjected to volume control and muting control by means of a sound control circuit 16d, the baseband audio signal is supplied to the RF modulator 21.

The RF modulator 21 up-converts the baseband video signal and the baseband audio signal supplied from the demodulator 16 into an output signal having a frequency of a specified channel of a television receiver and outputs the output signal through an output terminal 23.

Recently, in Europe, the NICAM system has been employed as a sound-multiplex television broadcasting system utilizing an artificial satellite. According to this system, a digital-modulated stereo audio signal having a carrier frequency at a position 6.552 MHz upper side from a video signal carrier frequency is transmitted, as shown in Fig. 7 for the PAL-I system. In the PAL-B, G system, on the other hand, the carrier frequency is selected to be 5.85 MHz.

In the foregoing CATV terminal apparatus configured so as to perform volume control and muting control on an audio signal, however, there has been a problem that a digital stereo audio signal cannot be produced at all even if a television signal of the NICAM system is received.

According to the present invnetion, a CATV terminal apparatus comprises: converter means for converting a received CATV signal into an intermediate-frequency signal; video detector means for detecting a video intermediate-frequency signal in the intermediate-frequency signal from the converter means to produce a baseband video signal; second intermediate-frequency circuit means for converting an audio intermediate-frequency signal in the intermediate-frequency signal into a second audio intermediatefrequency signal; sound detector means for detecting the second audio intermediate-frequency signal from the second intermediatefrequency circuit means to produce an -analog baseband audio signal; sound control means for performing a sound control operation on the analog baseband audio signal from the sound detector means;; filter means for extracting a digital stereo audio signal from the second audio intermediate-frequency signal; and combining means for combining the baseband audio signal from the video detector means, a sound-controlled analog baseband audio signal from the sound control means, and the digital stereo audio signal from the filter means.

A CATV terminal apparatus in accordance with the invention is capable of performing sound volume control, muting control, and the like on the analog audio signal, and further capable of producing the digital stereo audio signal.

A particular embodiment of a CATV terminal apparatus according to the present invention will now be described with reference to the accompanying drawings; in which: Fig. 1 is a block diagram showing a CATV terminal apparatus according to an embodiment of the present invention; Fig. 2 is a circuit block diagram showing in detail a part of Fig. 1; Fig. 3 is a block diagram showing in detail another part of Fig. 1; Fig. 4 is a block diagram showing a modification of a part of Fig. 1; Fig. 5 is a block diagram showing an example of a conventional CATV terminal apparatus; Fig. 6 is a block diagram showing in detail a part of Fig. 5; and Fig. 7 is a graph showing the frequency composition of a channel of the NICAM system.

Fig. 1 is a block diagram showing a CATV terminal apparatus according to an embodiment of the present invention.

In this figure, portions equivalent to those described with respect to Fig. 5 are correspondingly referenced.

In Fig. 1, television signals transmitted through a number of channels in a frequency band of from 50 MHz to 500 MHz are supplied from a center, which is a head end, to an input terminal 11 of the CATV terminal apparatus. A converter 12 selects a desired one of the television signal channels and frequency-converts the television signal of the selected channel so as to output an intermediate-frequency signal (IF signal). The IF signal converted by the converter 12 is branched by a branching device 13, and the resultant output signals are supplied to a variable attenuator 14 and a band pass filter (BPF) 15, respectively.

The BPF 15, which is supplied with the IF signal from the branching device 13, has such a pass band as passes an analog FM audio signal carrier fAl and a digital audio signal carrier fu as shown in the drawing. After passed through the BPF 15, the audio signal carriers fXl and fK are supplied to a demodulator 16 and an AM detector 17. From the audio IF signal, the AM detector 17 extracts, through AM-detection, scrambling-concerned data which has been superimposed by AM modulation onto the IF audio signal as in-band data, and supplies the extracted data to a descramble controller 18. The descramble controller 18 generates an expanding signal at predetermined timing to thereby control the attenuation (actually amplification) factor of the variable attenuator 14 on the basis of the scrambling-concerned data supplied thereto so that the variable attenuator 14 expands the video IF signal which has been compressed by scrambling. Thus, the video IF signal is descrambled.

The video IF signal descrambled by the variable attenuator 14 as described above is amplified by an amplifier 19, and then supplied to the demodulator 16 through a BPF 20 which passes a video carrier. The video carrier to be passed through the BPF 20 includes a video signal carrier fv and a chrominance signal subcarrier fc as shown in the drawing.

As shown in Fig. 2, the demodulator 16 has a synchronous detection circuit 16a which performs synchronous detection on the video IF signal supplied thereto through the BPF 20 to thereby output a baseband video signal, and an intercarrier detection circuit 16b which performs inter-carrier detection on the audio IF signal supplied thereto through the BPF 15 to thereby output a second intermediate-frequency (SIF) signal. The SIF signal detected by the inter-carrier detection circuit 16b is output from the demodulator 16.

The SIF signal from the demodulator 16 is supplied to BPFs 22 and 30. The BPF 22 has a pass band the center frequency of which is a difference frequency (fv - fAl) between the video signal carrier frequency fv and the analog FM audio signal carrier frequency fAl. The BPF 30, on the other hand, has a pass band the center frequency of which is a difference frequency (fv - fA2) between the video signal carrier frequency fv and the digital audio signal carrier frequency fA2.

The frequency component (fv - fAl) passed through the BPF 22 is supplied to the demodulator 16 again so as to be subjected to FM detection by means of an FM detection circuit 16c so that a baseband analog audio signal is obtained.

Thereafter, after being subjected to volume control and mutingcontrol by means of a sound control circuit 16d, the baseband analog audio signal is supplied to an RF modulator 21. The frequency component (fv - fA2) which has been passed through the BPF 30, on the other hand, is supplied to one input terminal of a combining device 31 so as to be combined with the video signal supplied thereto through a sound trap circuit 32 as shown in the drawing, and the output composite signal of the combining device 31 is supplied to the RF modulator 21.

The RF modulator 21 has a video/audio signal processing circuit 21a supplied with the analog audio signal from the demodulator 16 and the composite signal (video + fv fA2) from the combining device 31, a combining device 21b for combining the signals processed in the video/audio signal processing circuit 21a with each other, a modulator 21c for modulating the output composite signal of the combining device 21b, and an oscillator (OSC) 21d, as shown in Fig. 3.In the video/audio signal processing circuit 21a, the audio signal is subjected to AM modulation by means of a sound VCO 21at, and the output composite signal (video + fv - fA2) from the combining device 31 is clamped by means of a video clamp circuit 21a2 and processed with respect to the video signal level by means of a white clip circuit 21a3. The signals processed by the video/audio signal processing circuit 21a are combined with each other by the combining device 21b, and then up-converted by the modulator 21c so as to have a frequency of a specified channel of a television receiver. The thus obtained signal is output from an output terminal 23.

Although, in the above embodiment, the frequency component (fv - fA2) passed through the BPF 30 is combined with a video signal by means of the combining 31 and supplied to the RF modulator 21, the configuration may be changed such that, as shown in Fig. 4, the frequency component (fy - fA2) passed through the BPF 30 is directly supplied to the RF modulator 21 so that the frequency component (fv - f) passed through the BPF 30 is combined, by means of the combining device 21b in the RF modulator 21, with video and audio signals which have been subjected to signal processing in the video/audio signal processing circuit 21a.Such a configuration has an advantage that interferences can be prevented from being brought into the video signal because the composite signal (video + fv - f) is no longer subjected to the signal processing in the video/audio signal processing circuit 21a.

Further, although the above description has been made to the PAL-I system and the PAL-B, G system, the present invention can be applied to any system in which a digital sound is multiplexed in addition to an analog sound.

As described above, according to the present invention, in a CATV terminal apparatus in which a second audio IF signal obtained from an audio IF signal is detected to obtain an analog baseband audio signal, which is then subjected to sound volume control, muting control, and the like, a digital audio signal is extracted from the second audio IF signal and then is combined with a baseband video signal. It is therefore possible to provide a CATV terminal apparatus in which not only a video signal and an analog audio signal but a digital stereo audio signal can be obtained.

Claims (5)

1. A CATV terminal apparatus comprising: converter means for converting a received CATV signal into an intermediate-frequency signal; video detector means for detecting a video intermediate-frequency signal in the intermediate-frequency signal from the converter means to produce a baseband video signal; second intermediate-frequency circuit means for converting an audio intermediate-frequency signal in the intermediate-frequency signal into a second audio intermediatefrequency signal; sound detector means for detecting the second audio intermediate-frequency signal from the second intermediatefrequency circuit means to produce an analog baseband audio signal; sound control means for performing a sound control operation on the analog baseband audio signal from the sound detector means;; filter means for extracting a digital stereo audio signal from the second audio intermediate-frequency signal; and combining means for combining the baseband audio signal from the video detector means, a sound-controlled analog baseband audio signal from the sound control means, and the digital stereo audio signal from the filter means.

2. A CATV terminal apparatus according to claim 1, wherein the combining means comprises sub-combining means for combining in advance the baseband video signal with the digital stereo audio signal.

3. A CATV terminal apparatus according to claim 1 or 2 further comprising modulator means for up-converting an output signal of the combining means to produce a television signal having a frequency of a specified channel.

4. A CATV terminal apparatus according to any one of claims 1, 2 and 3, wherein the sound control operation performed by the sound control means includes sound volume control and muting control.

5. A CATV terminal apparatus substantially as described with reference to Figures 1 to 4 of the accompanying drawings.