Television receiver with circuits for recovering two audio signals

Abstract: A television receiver for receiving a first audio signal which is frequency modulated (FM) and which has a carrier with a first predetermined frequency difference from a carrier of a video signal. A second audio signal is pulse code modulated (PCM). The carrier has a second predetermined frequency difference from the carrier of the video signal. A tuner (2) is provided for receiving an RF signal and for generating an IF signal. A video detecting means (6) detects a video IF signal. An FM audio detecting means (13) detects the first audio signal. A PCM audio detecting means (16) detects the second audio signal. An audio IF detecting means (10) is connected between the tuner (21) and the FM audio detecting means (13) or PCM audio detecting means (16).

Description: This invention relates to television receivers. In Europe, there is known a method of television broadcasting called NICAM (Near Instantaneous Compound Audio Multiplex). With the NICAM system, as shown in Figure 1, a carrier CV of the video signal, a first audio signal carrier C1 and a second audio signal carrier C2 which have different frequencies compared with the frequency f CV of the carrier CV of the video signal are provided. The above first audio signal carrier C1 is a carrier of a frequency modulated (FM) analogue audio signal. The second audio signal carrier C2 is a carrier of a digitally modulated DQPSK - differential 4-phase PSK PCM audio signal. The above analogue audio signal is a first audio signal SV1 and the signal obtained by modulating the first audio signal carrier C1 by the first audio signal SV1 is a first modulated audio signal SV100. The PCM audio signal is a second audio signal SV2. The signal obtained by modulating the second audio signal carrier C2 by the second audio signal SV2 is a second modulated audio signal SV200. An intermediate frequency (IF) is set to 38.9 MHz. Frequencies f C1 and f C2 of the first and second audio signal carriers C1 and C2 are specified as follows. That is, the IF of 38.9 MHz is set as a reference, and in a direction such that the frequency becomes low. Accordingly to the B system of the VHF band and the G system of the UHF band the frequency f C1 of the first audio signal carrier C1 is set to a position of 5.5 MHz and the frequency f C2 of the second audio signal carrier C2 is set to a position of 5.85 MHz, respectively. According to the I system, the frequency f C1 of the first audio signal carrier C1 is set to a position of 6.0 MHz and the frequency f C2 of the second audio signal carrier C2 is set to a position of 6.552 MHz. In the television receiver of the NICAM system, a SAWF (Surface Acoustic Wave Filter: hereinafter referred to as a SAW filter) is used. The usual frequency characteristics of a SAW filter are, for instance, as shown by a solid line L2 in Figure 1. According to the characteristics shown by the solid line L2, the level of the frequency f C1 of the first audio signal carrier C1 and the level of the frequency f C2 of the second audio signal carrier C2 are set to be almost equal. Level ratios of the first and second audio signal carriers C1 and C2 to the carrier CV of the video signal as mentioned above are decided as follows by a standard. That is, assuming that the levels of the carriers CV, C1 and C2 are set to LCV, LC1 and LC2, respectively, it is determined that LCV/LC1 = 13 dB and LCV/LC2 = 20 dB. Therefore, the above level ratios are used as input conditions of the SAW filter to detect them. However, under such input conditions, in the case where the frequency characteristics of the RF signal change by a standing wave ratio (SWR) of the transmission system, and upon demodulation of the second modulated audio signal SV200, the level of the second audio signal carrier C2 decreases, the level ratio (LC1/LC2) increases, and the level of the first audio signal carrier C1 relatively increases. Therefore, interference of the first audio signal carrier C1 with the second audio signal carrier C1 relatively increases, so that an error rate of the second audio signal SV2 increases. On the other hand, to avoid the interference of the first audio signal carrier C1 with the second audio signal carrier C2, the sensitivity of a detecting circuit to detect the second audio signal SV2 cannot help being lowered. Radio Receivers, 1986, pages 389-430, Peter Peregrinus Ltd, London, GB; S M Edwardson : "The broadcast receiver (2) : vision" discloses a NICAM television received of the above described general type. According to the present invention there is provided a television receiver as claimed in claim 1. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a graph of frequency characteristics of carriers in a SAW filter in the NICAM system; Figure 2 is a block diagram of an embodiment of television receiver according to the present invention; Figure 3 is a graph of frequency characteristics of a trap circuit which is used in an embodiment of television receiver according to the present invention; and Figure 4 is a graph of frequency characteristics of a SAW filter which is used in another embodiment of television receiver according to the present invention. The embodiment of Figures 2 and 3 is applied to a television receiver of the NICAM system. In the drawings, symbols which are common to those in the known apparatus are the same, and overlapping descriptions are omitted. In Figure 2, a television signal from an antenna 1 is supplied to a tuner circuit 2. The television signal of a desired channel is selected and amplified by the tuner circuit 2. This television signal is converted into an IF signal by a mixing circuit of the tuner circuit 2. The IF signal from the tuner circuit 2 is supplied to a SAW filter 3 and a trap circuit 4, respectively. In the SAW filter 3, a video IF signal is separated from the IF signal supplied from the tuner circuit 2. After the video IF signal has been amplified by a video IF amplifying circuit 5, it is supplied to a video detecting circuit 6. The video detection is executed so that a composite colour video signal SYC is extracted and taken out from a terminal 7. On the other hand, in the trap circuit 4, the level of the frequency of the first audio signal carrier C1 of the IF signal which is supplied from the tuner circuit 2 is attenuated in a range such that sensitivity to a monaural audio signal does not deteriorate. That is, as shown in Figure 2, the trap circuit 4 functions to reduce the level of the first audio signal carrier C1 by about 10 dB. After the IF signal transmitted through the trap circuit 4 has been amplified by an audio IF amplifying circuit 8, it is supplied to a SAW filter 9. The characteristics of the SAW filter 9 are set to be similar to those in the known one. The above IF signal is supplied to an audio IF detecting circuit 10. A signal in which the first modulated audio signal SV100 and the second modulated audio signal SV200 are multiplexed to a luminance signal in a base band is obtained from the audio IF detecting circuit 10. This signal is supplied to a filter 11 and to a differential 4-phase PSK filter 12. The filter 11 is constructed so that a signal having a frequency of about, for example 5.5 MHz, can pass. Therefore, the first modulated audio signal SV100 which has a frequency of 5.5 MHz and was frequency modulated is separated and supplied from the filter 11 to a frequency demodulating circuit 13. In the frequency demodulating circuit 13, frequency demodulation is executed for the first modulated audio signal SV100, and the frequency demodulated first audio signal SV1 is obtained at a terminal 14. On the other hand, in the differential 4-phase PSK filter 12, the second audio signal SV200 is separated and supplied to a demodulating circuit 15. In the demodulating circuit 15, the second audio signal SV2 is demodulated, and the demodulated output is supplied to a PCM decoder 16. In the PCM decoder 16, the supplied second audio signal SV2 is supplied to an D/A converter (not shown) provided in the PCM decoder 16, so that the second audio signal SV2 is converted into the analogue audio signal SAV which is extracted from a terminal 17. With the embodiment, since the trap circuit 4 decreases the level of the first audio signal carrier C1 by about 10 dB relative to the level of the second audio signal carrier C2, interference of the first audio signal carrier C1 with the second audio signal carrier C2 is eliminated. Therefore, even if the level of the second audio signal carrier C2 is decreased, the interference of the first audio signal carrier C1 can be eliminated, so that the error rate of the second audio signal SV2 does not deteriorate and there is no need to lower the sensitivity of the circuit to detect the second audio signal SV2. Another embodiment of the invention will now be described. it differs from the above embodiment in that the trap circuit 4 is eliminated and a SAW filter having frequency characteristics as shown in Figure 4 is used in place of the SAW filter 9. As shown in Figure 4, the SAW filter is set so that the first audio signal carrier C1 is attenuated by about 10 dB relative to the second audio signal carrier C2. Therefore, even in this embodiment, in a manner similar to the above embodiment, the level of the first audio signal carrier C1 can be reduced by about 10 dB relative to a level of the second audio signal carrier C2, so that interference of the first audio signal carrier C1 with the second audio signal carrier C2 is eliminated. In other respects the second embodiment is similar to the first embodiment. As shown in the first and second embodiments, a trap circuit 4 having the frequency characteristics shown in Figure 3, or the SAW filter having the frequency characteristics shown in Figure 4 is provided. Assuming that the levels of the carriers CV, C1 and C2 are respectively set to LCV, LC1 and LC2, when a television signal of LCV/LC1 = 13 dB, LC1/LC2 = 7 dB according to the standard of the NICAM system is received, LVC/LC1 = 20 dB and LC1/LC2 = 0 dB can be set on the input side of the audio IF detecting circuit 10. Thus, the effect as mentioned above can be obtained. Since the means for attenuating the first audio signal carrier is provided at the front stage of the audio IF detecting circuit provided in the television receiver, there is the effect such that the interference of the first audio signal carrier relative to the second audio signal carrier can be eliminated. Due to this, even in the case where the level of the second audio signal carrier is decreased, the interference of the first audio signal carrier is eliminated. Thus, there are effects such that the error rate of the second audio signal does not deteriorate and there is no need to lower the sensitivity of the detecting circuit to obtain the second audio signal.

Claims: A television receiver for receiving a first audio signal which is frequency modulated (FM) and which has a first carrier of a first predetermined frequency difference relative to a carrier of a video signal, and a second audio signal which is pulse code modulated (PCM) and which has a second carrier of a second predetermined frequency difference relative to said carrier of said video signal, the receiver comprising: tuner means (2) for receiving an RF signal and for generating IF signals; video detecting means (6) connected to receive said IF signals and for detecting a video IF signal; FM audio detecting means (13) for detecting said first audio signal; PCM audio detecting means (16) for detecting said second audio signal; and audio IF detecting means (10) for receiving said IF signals connected between said tuner means (2) and said FM audio detecting means (13) and PCM audio detecting means (16); characterized by means (4,9) connected upstream of said audio IF detecting means (10) for providing a lower frequency response at a frequency corresponding to said first audio signal compared to a frequency response at a frequency corresponding to said second audio signal such that sensitivity to said first audio signal is substantially undeteriorated whilst interference of said first audio signal with said second audio signal is substantially eliminated. A television receiver according to claim 1, wherein said means (4, 9) connected upstream of said audio IF detecting means comprises: a SAW filter (9) connected upstream of said audio IF detecting means (10); and a trap circuit (4) connected upstream of said SAW filter (9) and which has a lower frequency response at a frequency corresponding to said first audio signal compared to a frequency response at a frequency corresponding to said second audio signal. A television receiver according to claim 2 wherein said SAW filter (9) has frequency responses for said first and second audio signals such that a gain at a frequency corresponding to said first audio signal is approximately the same as a gain at a frequency corresponding to said second audio signal. A television receiver according to claim 2 wherein said trap circuit (4) has a frequency response for said first audio signal which is 10 dB lower than a frequency response for said second audio signal. A television receiver according to claim 1, wherein said means connected upstream of said audio IF detecting means comprises: a SAW filter (9) connected upstream of said audio IF detecting means (10) and having a frequency response such that a gain at a frequency corresponding to said second audio signal is greater than a gain at a frequency corresponding to said first audio signal. A television receiver according to daim 5 wherein said SAW filter (9) has a 10 dB lower frequency response for said first audio signal than a frequency response for said second audio signal. A television receiver according to daim 1 wherein said RF signals received by said tuner means (2) of the television receiver comprise NICAM signals broadcast in Europe.

Inventors: OHTA MASASHI, SAKAI YUJI, OHTA, MASASHI, SAKAI, YUJI, OHTA, MASASHI, C/O PATENTS DIVISION, SAKAI, YUJI, C/O PATENTS DIVISION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1990
Main classification: H04N5
IPC codes: H04B1, H04N5