GB2238213A

GB2238213A - Multiplex stereophonic demodulator for A2 and NICAM

Info

Publication number: GB2238213A
Application number: GB9024855A
Authority: GB
Inventors: Il-Gu Na
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 1989-11-15
Filing date: 1990-11-15
Publication date: 1991-05-22
Anticipated expiration: 2010-11-15
Also published as: DE4010590C2; KR910010309U; GB2238213B; GB9024855D0; KR920004944Y1; DE4010590A1

Abstract

The demodulator is incorporated in a television or a video cassette recorder for receiving both an A2 broadcast and a NICAM broadcast available in PAL B and G broadcast regions in Europe. The circuit comprises a video signal processor 40; a main channel audio signal processor 50; a sub-channel audio signal extractor 60; an A2 broadcast processor 70; a NICAM broadcast processor 80; a matrix circuit 23; a selector means 31; a first cut-off switch SW1 coupled to output terminals of said matrix circuit; a second cut-off switch SW2 coupled to output terminals of said selector means; and a display 32. The extractor 60 shifts the frequency of one of the audio sub-channels of A2 or NICAM broadcasts so that they are both output at the same frequency. <IMAGE>

Description

MULTIPLEX STEREOPHONIC DEMODULATING CIRCUIT The present invention

generally relates to a multiplex stereophonic demodulating circuit incorporated in a television or a video cassette recorder, and more particularly to a demodulating circuit for receiving both an A2 broadcast and a NICAM broadcast available in the PAL B and G broadcast regions in Europe.

The european broadcast system of PAL B and G type is generally divided into two multiplex stereophonic broadcasts: the A2 broadcast system of two carriers, and the NICAM (Near Instantaneously Companded Audio Multiplex) broadcast system.

The NICAM broadcast system recently introduced in Europe is undergoing a test broadcasting stage in some countries, and is likely to be adopted in Sweden, Norway, Finland, Denmark, Spain, etc., and to spread to more countries.

Therefore, in Europe, each broadcast system, A2 and NICAM, requires its own separate receiving apparatus to listen to and view that particular broadcast.

It is an object of the present invention to provide a multiplex stereophonic demodulating circuit compatible with the A2 and NICAM broadcast systems which can selectively receive the A2 broadcast and the NICAM broadcast in a PAL B/G region.

According to the invention, there is provided a multiplex stereophonic demodulating circuit for receiving both an A2 broadcast and a NICAM broadcast of PAL B and G regions, comprising:

a video signal processing unit for proces'sing intermediate frequency signals from a tuner, and generating a common video signal of the A2 broadcast and the NICAM broadcast; a main channel audio signal processing unit for processing said intermediate frequency signal from the tuner, and generating a common main channel audio signal of the A2 broadcast and the NICAM broadcast through a 1 0 first output terminal and a sound intermediate frequency signal through a second output terminal; a sub-channel audio signal extracting unit for shifting a sub-carrier frequency of a sub-channel for, either A2 or NICAM broadcast present in said sound intermediate frequency generated from said main channel audio signal processing unit, and extracting a subchannel audio signal of a single sub-carrier frequency component for both A2 and NICAM broadcast; an A2 broadcast processing unit for generating an A2 broadcast audio signal in response to said sub-channel signal extracted from said sub-channel signal extracting unit, if as for A2 broadcast; a NICAM broadcast processing unit for generating respective left and right audio signals of a NICAM broadcast in response to said sub-channel signal extracted from said sub-channel signal extracting unit, if as for NICAM broadcast; a matrix circuit for generating left and right audio signals in response to said audio signal outputted from said A2 broadcast processing unit and said audio signal outputted from said main channel audio signal processing unit; a selector means receiving said left and right audio signals outputted from said NICAM broadcast processing unit and said audio signal outputted from said main channel audio signal processing unit through input terminals thereof; a first cut-off switch coupled to output terminals of said matrix circuit, said first switch being controlled by a broadcast state discriminating'.9ignal from said NICAM broadcast processing unit on receiving a NICAM broadcast, and muting noise outputs of said A2 broadcast processing unit appearing at the output terminals of said matrix circuit; and a second cut-off switch coupled to output terminals of said selector means, said second switch being controlled by a broadcast state discriminating signal from said A2 broadcast processing unit on receiving an A2 broadcast, and muting noise outputs of said NICAM broadcast processing unit appearing at the output terminals of said selector means. 5 The circuit preferably further comprises a display unit for indicating a mode and a kind of the received broadcast in response to the respective broadcast state discriminating signals generated from said A2 and NICAM broadcast processing unit. Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram of one embodiment of multiplex stereophonic demodulating circuit according to the present invention; Fig. 2 shows RF signal spectra for A2/NICAM broadcasts; and Fig. 3 shows graphical illustrations of the frequency-shifted spectra of A2/NICAM broadcasts owing to 0.11MHz oscillator signals.

Referring to Fig. 1, there is shown a preferred embodiment of the present invention for compatibly receiving both A2 and NICAM broadcasts. As the A2 broadcast system and the NICAM broadcast system have common video carrier bands and common main channel carrier bands of the audio signals, but different subchannel bands of the audio signals, the circuit of the present invention comprises a common video processing unit 40, a common main channel signal processing unit 50, and two different sub-channel signal processing units 70 and 80. % -% In more detail, the video signals of the selected channel are processed in a video processing means 40. The respective main channel audio carrier signals of both A2 and NICAM broadcasts are detected by and outputted from a main channel signal processing means 50, while the sub-carrier signals of the audio signals are shifted in phase by a sub-channel audio signal extracting means 60 where a predetermined sub-channel frequency will be extracted for either A2 or NICAM broadcast. Further, the extracted sub-channel signal, if for an A2 broadcast, is used to generate the audio signal for an A2 broadcast'by means of A2 broadcast processing means 70, and also enables a second cut- off switch SW2 to be operative in order to mute the noise outputted from NICAM broadcast processing means 80. The NICAM broadcast audio signal output from the NICAM broadcast processing means 80 comprises left and right audio signals, which are applied together with an output signal of the main channel signal processing means 50 into a selector 31. The selector 31 selects and outputs two kinds of signals in response to an external switching control signal.

Receiving of both A2 and NICAM broadcast information can be performed as follows. First, the main channel audio signal is processed as in the ordinary audio signal processing method. The sub- channel audio signal is mixed with a predetermined frequency and is shifted in phase, so that the common frequencies, which is extracted from the sub-channel signal extraction means 60, is applied to both A2 broadcast processing means 70 and NICAM broadcast processing means 80. Therefore, A2 broadcast processing means 70 or NICAM broadcast processing means 80 produces certain audio signals in response to the inputted signals of the common frequencies. When A2 broadcast processing means 70 is operative, the signals outputted from NICAM broadcast processing means 80 should be arranged to be cut off, on the other hand, when NICAM broadcast processing means 80 is operative, the signals outputted from A2 broadcast processing means 70 are cut kf.

Referring to Table 1, the respective baseband characteristics of the A2 and NICAM broadcast systems can be seen. In Fig. 2 the RF (Radio Frequency) signal spectra of the respective A2 and NICAM broadcasts are represented, and further, in Fig. 3 it will be seen that the A2 broadcast sub-channel carrier of 5.74MHz and the NICAM broadcast sub-channel carrier of 5. 85OMHz have been - 5 shifted in phase through the mixing with a separate oscillating frequency of 0.11MHz.

Table 1

Baseband characteristic of NICAM system Modulation FM Analog Sound Sub-carrier freq. 5.5 MHz Signal level (Video/Audio) 20:1 Modulation QPSK bit rate 728Kbit/sec Digital Sound Sub-carrier freq. 5.85 MHz Signal level (Video:QPSK) 100:1 Transmission filter roll- 40% off ratio Baseband characteristic of A2 system CH 1 CH 2 Sub-carrier freq. 5.5 MHz 5.742185 MHz Video/FM sound power ratio 13 dB 20 dB Pre-emphasis 50 ps 50 gs Pilot carrier freq. 54.6875 KHz Pilot carrier mod. AM Pilot carrier mod. factor 50% -Pilot freq. Mono 0 Hz Stereo 117.5 Hz Bi-L 274.1 Hz Audio signal Mono Mono (Ml) Mono(Ml) Stereo -l,(L+R)=M R Bi-L Mono 1 Mono 2 Now, the overall operations of the apparatus according to the present invention will be described in detail.

Referring to Fig. 1, when the user of broadcast - receiving apparatus, for example a TV or VCR, pushes keys for selecting a desired TV channel, microcomputer 1 installed in the receiving apparatus detects such a key selection and supplies a tuner 2 with a predetermined phase-locked loop (PLL) data. The tuner 2, in turn, converts an RF signal into an IF (Intermediate Frequency) signal, and the IF signal is commonly applied to VIF SAW (Video IF Surface Acoustic Wave) filter 3 and SIF (Sound IF) SAW filter 7.

A signal from VIF SAW filter 3 is applied to VIF amplifier and video detector 4, which produces a composite video signal. The composite video signal is further delivered to SIF trap 5 for trapping audio intermediate frequencies of 5.5MHz, 5.74MHz and 5.85MHz, and therefore, only the video intermediate frequency is supplied to video processor 6 for outputting resultant video signals.

Furthermore, IF signals from the above-mentioned tuner 2 are delivered through SIF SAW filter 7 to SIF detector 8 so that SIF (Sound Intermediate Frequency) could be detected.

Because the main channel audio carrier of both A2 and NICAM broadcasts is at the frequency of 5.5MHz, as appreciated from the baseband characteristics of Table when the main audio carrier from SIF detector 8 is transferred through a first 5.5MHz filter 9 to an FM detector 10, the FM detector 10 produces the mLlfn channel audio signal and delivers it to a matrix circuit 23.

Moreover, the SIF signal-from SIF detector 8 passes through a band pass filter 11 and is supplied to a second filter 12 of 5.74MHz and a third filter 13 of 5.85OMHz.

The band pass filter 11 should have such features to pass both the subchannel carrier of an A2 broadcast of 5.74MHz, and the sub-channel carrier of a NICAM broadcast - 7 of 5.85MHz. And thus, the sub-channel audio carriers of A2 and NICAM broadcasts pass through the band pass filter 11, and are applied to the second filter 12 of 5.74MHz and the third filter 13 of 5.85OMHz connected in parallel. A mixer 14 is adapted to selectively receive the sub-channel audio carrier of the A2 or NICAM broadcast of 5.74MHz or 5.85OMHz through one input terminal, and receive a O.11MHz oscillating frequency from an oscillator 15 through the other input terminal, the two input signals of the mixer 14 being mixed to produce a frequency shift as shown in Fig. 3.

Therefore, whether the received signal of the tuner 2 is A2 or NICAM broadcast, the mixer 14 should be selectively supplied with the corresponding sub-channel audio carrier of 5.74MHz or 5.85OMHz, which is in turn mixed with 0.11MHz oscillating frequency from the oscillator 15. The output frequency from the mixer 14 is applied to 5.85OMHz peaking unit 16, and thus the peaking unit 16 enables the gain of the audio carrier of 5.85OMHz to peak. Successively, the output of the peaking unit 16 is transferred through a fourth filter 17 of 5.85OMHz to produce the sub-channel audio carrier of 5.85OMHz. Here, the 5.85OMHz filter 17 can produce an inherent subchannel audio carrier of 5.85OMHz for the NICAM broadcast signal, while an inherent sub-channel audio carrier of 5.74MHz, for the A2 broadcast signal can be transduced into a 5.85OMHz signal at the output of the fourth filter 17. In other words, the sub-channel audio carrier of 5.85OMHz should be produced at the output of the filter 17 in respect to both A2 and NICAM broadcast signals.

In case of the A2 broadcast, the sub-chariiiii-1 audio carrier from the fourth filter 17 is delivered through SIF/FM detector 18 to matrixcircuit 23 and a 54.7KHz band pass filter 19. At this time, because the audio sub-channel carrier from the fourth filter 17 of 5.85OMHz even for the A2 broadcast is applied to a phase demodulator 24 within NICAM broadcast processing -means 80 and also the characteristics of the A2 broadcast are different from those of the NICAM broadcast processed in the NICAM broadcast processing means 80, noise signals appear in the left and right audio output lines through the respective low pass filters 29 and 30 within the 5 NICAM broadcast processing means 80.

Further, the audio output signal from FM detector 18 is applied to the matrix circuit 23 and, at the same time, is passed through a 54.7KHz band pass filter 19 so as to produce a pilot carrier for discriminating the broadcast mode. The passed pilot signal, which has already been modulated in Amplitude Modulation method, is demodulated by means of AM detector 20, and then applied to a broadcast state discriminator 21 in which the present broadcast state can be figured out to be a bilingual broadcast or a stereo broadcast. The broadcast discriminating signal from the discriminator 21 is supplied to the matrix circuit 23 and causes the left and right audio signals to be produced. The detector 21 also provides a signal to display 32 to inform the user of the present broadcast state.

Furthermore, the broadcast state discriminating signal from the discriminator 21 is delivered into A2 detector 22 so that A2 detector 22 can generate a predetermined signal, i.e. high levels even in the two modes of bi-lingual broadcast and stereo broadcast. The signal generated from the A2 detector 22 is adapted not only to drive a display unit 32 for notifying the users that this apparatus is being tuned to the A2 broadcast system, but also turn off second cut-off switch SW2 for muting the noise signals from the NICAM broadcast processing system 70.

However, in case of the multiplex stereophonic broadcast for the NICAM broadcast system rather than the A2 broadcast system, the sub-channel audio carrier passed through the fourth filter 17 of 5.85OMHz is impressed on the phase demodulator 24, and the phase demodulator 24 produces digital sound data and clock signals to be forwarded to a NICAM decoder 25.

By determining the present broadcast state, the NICAM decoder 25 controls a mode discriminating unit 27 with mode information signals and also supplies the digital sound data and clock signals to a digital-to-' analog (D/A) converter 26. Thus the digital-to-analog converter 26 produces analog stereophonic signals corresponding to the input digital signals, the left and the right sound signals of the stereophonic signals being formed into final sound signals via the respective low pass filters 29 and 30.

And the mode of the received NICAM broadcast is discriminated to be any of a first monophonic broadcast, a second monophonic broadcast and a stereophonic broadcast by the mode discriminating unit 27 so as to generate a display driving signal for the broadcast mode and supply it to a display unit 32.

Whenever such a NICAM broadcast being inputted, the NICAM detector 28 generates a predetermined signal i.e. high level for notifying the user that this apparatus is being tuned to the NICAM broadcast, irrespective of the discriminated mode of the NICAM broadcast. Also, the high level output of the NICAM detector 28 enables a first cut-off switch SW1 to be turned off so that the noise output from the matrix circuit 23, which may be generated at the A2 broadcast processing means 70 in case of NICAM broadcast tuning, can be muted or disconnected.

Left and right audio output signals passed through low pass filters 29 and 30, together with FM sound output of FM detector 10, are applied to the input terminals of selector means 31, and only two of them selectively appear at two output terminals of selector meaks' 31'based on the external switching controls. Thus, in case of NICAM broadcast tunings, audio signals applied to the input terminals of selector means 31 comprise two left and right analog sound signals and one FM sound signal, and the selector means 31 is adapted to select and produce only two of three inputs in accordance with the contents of the external switching control signals. At the same time, A2 detector 22 generates a predetermined signal (for example, a low level signal) for turning on the second cut-off switch SW2 and enables the audio signals selected by the selector means 31 to be transferred through the second cut-off switch SW2.

The 5.850MHz peaking unit 16 and the fourth filter 17 of 5.850MHz shown in Fig. 1 may be respectively replaced with a 5.74MHz peaking unit and a 5.74MHz filter without departing the scope of the present invention; such a modified embodiment according to the present invention could be operated normally in the same manner as the embodiment of Fig. 1 to receive selectively both A2 broadcast and NICAM broadcast.

As described hereinbefore, multiplex stereophonic demodulating apparatus according to the present invention can receive the video and audio signals derived from both A2 broadcast system and NICAM broadcast system available in the PAL B and G region, in spite of the difference in the broadcast mode thereof so that the apparatus of the present invention might be widely utilized in Europe.

l.' 11 -

Claims

CLAIMS:

1. A multiplex stereophonic demodulating circuit for receiving both an A2 broadcast and a NICAM broadcast bf PAL B and G regions, comprising: a video signal processing unit for processing intermediate frequency signals from a tuner, and generating a common video signal of the A2 broadcast and the NICAM broadcast; a main channel audio signal processing unit for processing said intermediate frequency signal from the tuner, and generating a common main channel-audio signal of the A2 broadcast and the NICAM broadcast through a first output terminal and a sound intermediate frequency signal through a second output terminal; a sub-channel audio signal extracting unit for shifting a sub-carrier frequency of a subchannel for either A2 or NICAM broadcast present in said sound intermediate frequency generated from said main channel audio signal processing unit, and extracting a sub channel audio signal of a single sub-carrier frequency component for both A2 and NICAM broadcast; an A2 broadcast processing unit for generating an A2 broadcast audio signal in response to said sub-channel signal extracted from said sub-channel signal extracting unit, if as for A2 broadcast; a NICAM broadcast processing unit for generating respective left and right audio signals of a NICAM broadcast in response to said sub-channel signal extracted from said sub-channel signal extracting unit, if as for NICAM broadcast; a matrix circuit for generating left and right audio signals in response to said audio signal outputted from said A2 broadcast processing unit and said audio signal outputted from said main channel audio signal processing unit; a selector means receiving said left and right audio signals outputted from said NICAM broadcast processing unit and said audio signal outputted from said main channel audio signal processing unit through input terminals thereof; a first cut-off switch coupled to output terminals of said matrix circuit, said first switch being controlled by a broadcast state discriminating signal from said NICAM broadcast processing unit on receiving a NICAM broadcast, and muting noise outputs of said A2 broadcast processing unit appearing at the output terminals of said matrix circuit; and a second cut-off switch coupled to output terminals of said selector means, said second switch being controlled by a broadcast state discriminating signal from said A2 broadcast processing unit on receiving an A2 broadcast, and muting noise outputs of said NICAM broadcast processing unit appearing at the output terminals of said selector means.

2. A circuit as claimed in claim 1 further comprising: a display unit for indicating a mode and a kind of the received broadcast in response to the respective broadcast state discriminating signals generated from said A2 and NICAM broadcast processing units.

3. A circuit as claimed in claim 1 or 2, wherein said main channel audio signal processing unit comprises: 25 a sound intermediate frequency surface acoustic wave (SIF SAW) filter for receiving said intermediate frequency signal generated from said tuner to produce SIF signals; a SIF detector for detecting SIF signals from said SIF SAW filter and forwarding a detected signal to input terminals of said sub-channel audio extracting-ilnit-and a first filter, the latter having characteristics for passing a FM main channel sub-carrier audio signal; and a FM detector for detecting the output signal from said first filter and forwarding a detected signal to the respective input terminals of said matrix circuit and said selector means.

c

4. A circuit as claimed in claim 3 wherein said subchannel audio signal extracting unit comprises: a band pass filter for receiving a detected SIF signal from said main channel signal processing unit to pass a predetermined frequency band including sub-carrier frequencies of A2 and NICAM sub-channel audio signals; a second filter and a third filter coupled in parallel to an output terminal of said band pass filter, and for filtering sub-carriers of the A2 and NICAM sub- channel audio signals respectively; an oscillator for generating a predetermined oscillating frequency corresponding to the difference between the two sub-carrier frequencies of A2 and NICAM sub-channel audio signals; a mixer having a first input terminal connected to a common output terminal of said second and third filters and a second input terminal connected to an output terminal of said oscillator, said mixer being operative to mix output signals from said second or third filter with oscillating signals from said oscillator and make the output frequency of said second or third filter be shifted; a peaking unit receiving an output signal from said mixer and peaking the gain of a predetermined sub-carrier frequency component of sub-channel audio signals from said mixer; and a fourth filter for filtering said predetermined sub-carrier frequency component outputted from said peaking unit in order to produce a single sub-carrier frequency component for A2 and NICAM sub-channel audio signals. % A.. -

5. A circuit as claimed in any preceding claim, wherein said A2 broadcast processing unit comprises:

a SIF/FM detector for receiving said sub-carrier frequency component from said sub-channel and audio signal extracting unit, and producing a sub-channel audio signal for the A2 broadcast; a band pass filter for passing only an AM pilot carrier signal among the A2 sub-channel audio signals produced from said SIF/FX detector; an AM detector for detecting said AM pilot carrier signal from said band pass filter; and a broadcast state discriminating unit for identifying the state or mode of the A2 broadcast based on the pilot carrier signal from said AM detector.

6. A circuit as claimed in any preceding claim, wherein said NICAM broadcast processing unit comprises: a phase demodulator for demodulating sub-carrier frequency components from said sub-channel audio signal extracting unit to produce digital sound data and clock signals; a NICAM decoder for generating a mode information signal applied to a mode discriminating unit and supplying said digital sound data and clock signals of said phase demodulator to output terminals thereof; a digital-to-analog converter for producing analog stereophonic signals corresponding to input digital signals thereof; left and right low pass filters for filtering analog outputs from said digital-to-analog converter to produce left and right audio signals for the NICAM broadcast; 25 a mode discriminating unit for identifying a mode of the NICAM broadcast based on said mode information signal from said NICAM decoder so as to generate a display driving signal for the NICAM broadcast mode; and a NICAM detector for generating a predetermined level signal in accordance with operations of said mode discriminating unit, said level signal enablir4'said first cut-off switch to be operative so as to mute the outputs from said matrix circuit, on receiving of a NICAM broadcast.

7. A circuit as claimed in claims 2 and 6 wherein said level signal is also operative to activate said display.

8. A multiplex stereophonic demodulating circuit, substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

Published 199 1 atThe Patent Office. State House. 66/71 High Holborn. London WCIR47P. Further copies may be obtained from Sales Branch. Unit 6. Nine Mile Point, Cwmichniach. Cross Keys. Newport. NPI 7HZ- Printed by Nlult'Plcx techniques Rd. St Mary Cray. Kent f