JP2794570B2 - Television audio multiplex signal demodulator - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a television audio multiplex signal demodulator, and more particularly, to a second audio signal and a third audio signal on a higher frequency side than a main television audio signal band. The present invention relates to a demodulation device for demodulating a television audio multiplex signal in which a signal is multiplexed.

[Summary of the Invention]

The present invention relates to a demodulation device for demodulating a television audio multiplex signal obtained by frequency-multiplexing second and third audio signals to a higher frequency side than a first television audio signal band. As a filter for extracting the audio signal, a trap including a part of the band-pass filter for extracting the third audio signal and a subtractor is used, and the output signal from the trap and the harmonics are suppressed. The circuit configuration is simplified without deteriorating the detection characteristics of the second audio signal by multiplying the signal of the second carrier frequency by synchronous detection of the second audio signal.

[Conventional technology]

For example, in the television audio multiplex broadcasting in the United States, the frequency spectrum of the audio signal after FM demodulation is as shown in FIG. 4, and as apparent from FIG. 4, the frequency spectrum corresponds to the main (main) audio. to a higher frequency side than the first frequency band of the audio signal S _1, and second multiple audio signals S ₂ corresponding to the stereo difference signal or the like, the third multi-audio signal S ₃ corresponding to the foreign language audio signal, etc. And are arranged.
The first audio signal S ₁ is disposed on the lower frequency side than the horizontal scanning frequency f _H of the television signal,
the position of f _H is the pilot signal S _PL for identifying the presence or absence of the second audio signal are arranged. It said second multiple audio signal S ₂ is a carrier signal of a second frequency 2f _H second
The signal is AM-modulated with a stereo difference signal (LR), which is an audio signal, and has a frequency band of, for example, 21 kHz to 41 kHz. The third multiplexed audio signal S ₃ of the third frequency 5f _H
Is FM-modulated with a third audio signal, for example, a foreign language (so-called bilingual) audio signal. Furthermore the high frequency side, sometimes telemetry signal S _TL center frequency 6.5F _H is disposed. An example of such a television audio transmission system for multiplexing three or more audio signals is disclosed in U.S. Pat. No. 4,405,944.

As a demodulator for demodulating such a television audio multiplex signal, for example, a demodulator as shown in FIG. 5 is conventionally known. 5, an input terminal 1 is supplied with a television audio multiplex signal having a frequency spectrum as shown in FIG. 4, which is separated from a television video signal and subjected to FM demodulation. This input signal is the frequency band of the _second multiplexed audio signal S2.
The first audio signal S ₁ and the second multiplexed audio signal S ₂ are sent to an LPF (low-pass filter) 2 having a cutoff frequency sufficiently higher than k to 41 kHz, for example, 50 kHz, and are taken out. The signal is sent to the circuit 4 and the second audio signal detection circuit 5, respectively. The output from the first audio signal processing circuit 4 is only the first audio signals S ₁ is sent to LPF6 is taken out, the first audio signals S ₁ is sent to a matrix circuit 7. In the above-described second audio signal detection circuit 5 is arranged on the lower frequency side than the second multiplexed audio signal S ₂ is low frequency converted to for example f _H by, for example, synchronous detection or the like, a is the down-conversion by LPF 8 2 Only the audio signal component is extracted and sent to one selected terminal a of the changeover switch 9. The signal from the input terminal 1 is sent to pass center frequency of 5f _H BPF (band pass filter) 10 is the third multiplex audio signal S ₃ is taken out, FM third speech signal detection circuit 11 After the detection, the LPF 12 extracts only the third audio signal component. The output from the LPF 12 is sent to the other selected terminal b of the changeover switch 9. The output from the changeover switch 58 is sent to a noise low-frequency circuit 13 such as a so-called dbx (registered trademark) decoder, and is subjected to noise low-frequency processing. For example, stereo left and right channel audio signals are extracted from the matrix circuit 7.

Here, noise low-pass processing such as dbx (registered trademark) is required for each of the second and third audio signals. However, providing a noise low-pass circuit for each audio signal system requires a circuit. Since the scale becomes large and the cost increases, the noise reduction circuit 13 is shared by the second and third audio signal systems as shown in FIG. , The third audio signal can be selected and received.

[Problems to be solved by the invention]

By the way, in the second audio signal detection circuit 5, the frequency
A rectangular wave signal of 2f _H as a carrier signal, the second
By multiplying this carrier signal to multiplex the audio signal S _2, but not being performed synchronous detection as to convert the following lower frequency f _H, sufficiently the third multiplexed audio signal S ₃ by LPF2 If this signal is not attenuated, this signal S ₃ (center frequency 5f _H )
There third harmonic of the 2f _H square-wave carrier signal (frequency 6
f _H ) is converted to the lower frequency side by multiplication with the component,
It adversely affects the demodulated signal of the second audio signal. Also, LPF2
Of the phase characteristic is not sufficiently flat to near 3f _H, there is a possibility that the degree of stereo separation after demodulation is deteriorated.

Therefore, specifically, for example, the configuration shown in FIG. 6 is required as the LPF2. That is, in this Figure 6, LPF2 includes a secondary phase equalizer 31, the secondary trap filters or traps of the trap frequencies 6f _H 3
2, has a total of 6-order filter configuration and the secondary trap 33 are connected in series in the trap frequencies 6f _H. This LP
The output from F2 is sent to the second audio signal system via the first audio signal system and the second audio detection circuit 5, respectively. The carrier input terminal 34 of the second voice detection circuit 5, a rectangular wave signal of the frequency 2f _H is supplied as a carrier signal. Here, BP for retrieving third multiplexed audio signal S ₃ of
F10 includes a BPF36 second order example pass center frequency 5f _H,
A secondary trap 37 traps frequency 6f _H, and the secondary trap 38 traps frequency 4f _H has a series-connected six-order filter configuration.

FIG. 7 shows the frequency spectrum of the signals of the respective parts A to C in FIG. 6, where A is the frequency spectrum of the output signal from the LPF 2 and B is the frequency spectrum supplied to the terminal 34.
Frequency spectrum of 2f _H square wave carrier signal, C is B
7 is a frequency spectrum of an output signal from PF10.

In such a specific configuration shown in FIG. 6, the order of the required filter is increased, and the filter (LPF2 or BPF10) is used as an IC (integrated circuit) for the demodulation circuit system.
In the case where it is built in, the chip area increases due to an increase in the number of elements, and when an external filter configuration is adopted, the number of components increases, which leads to an increase in cost.
In addition, when trying to simplify the configuration by reducing the filter order, the S / N deteriorates in performance, especially when the IC is used,
Deterioration of stereo separation occurs.

The present invention has been made in view of such circumstances, and can simplify a filter configuration without deteriorating performance, and can prevent an increase in a chip area and an increase in the number of external components when an IC is formed. It is an object of the present invention to provide a television television multiplexed signal demodulation device.

[Means for solving the problem]

In order to solve the above-described problem, the television audio multiplexed signal demodulation apparatus according to the present invention includes a second television audio multiplexed signal demodulator set to a higher frequency side than the first television audio signal band.
A demodulation device for demodulating a television audio multiplex signal obtained by modulating a carrier signal of a third frequency with second and third audio signals, respectively, and demodulating the frequency multiplexed signal, receives the audio multiplex signal, A band-pass filter having a carrier frequency as a center pass frequency, a subtractor for subtracting an output signal of the band-pass filter from the input audio multiplex signal, and a second signal in which an output signal from the subtractor and a harmonic are suppressed. And a third audio signal circuit to which an output signal from the band-pass filter is supplied.

[Action]

An output signal from a trap including a part of a band-pass filter for extracting the third audio signal and a subtractor is multiplied by a signal of the second carrier frequency in which the harmonics are suppressed to perform synchronous detection. By doing so, the circuit configuration can be simplified without deteriorating the detection characteristics of the second audio signal.

〔Example〕

FIG. 1 is a block circuit diagram showing a main configuration of a television audio multiplex signal demodulation apparatus according to an embodiment of the present invention.

In FIG. 1, an input terminal 1 is supplied with a television sound multiplex signal having a frequency spectrum as shown in FIG. That is, the second, third frequency (2f _H, 5 which are set to the side higher than the first main (primary) frequency band of the television audio signals S ₁
Each second carrier signal f _H), the third second obtained by modulating the audio signal, the third multi-audio signals S _2, S ₃ television is superimposed John sound multiplex signal input terminal 1 Is supplied to This audio multiplexed input signal has the third carrier frequency (5f _H ) as a center pass frequency, for example, 2
The following BPF are sent to (bandpass filter) 41, an output signal from the BPF41 includes a subtractor 42 and, second trap filters or traps of the trap frequencies 6f _H 4
And sent to 3 respectively. Subtractor 42, which subtracts the output signal of the BPF41 from the input signal of the input terminal 1, these BPF41 and secondary trap filters or traps of the trap frequency 5f _H by the subtracter 42 is constituted You. Therefore, the output signal from the subtractor 42 is
It will have a frequency spectrum near the frequency 5f _H is greatly attenuated as Figure 2 A. This subtractor 42
Is sent to the second audio signal detection circuit 5, and is multiplied by the carrier signal of the second carrier frequency (2f _H ) from which the harmonics from the carrier input terminal 45 are suppressed. . As this carrier signal, a so-called pseudo sine wave signal is used, and its frequency spectrum is, as shown in FIG. 2B, for example, where the third harmonic (6f _H ) component is particularly greatly suppressed. For example, the level is lower than the main frequency 2f _H component by 40 dB or more. This pseudo sine wave carrier signal can be obtained by suppressing, for example, the third harmonic of a triangular wave, but may also be obtained by using a staircase wave or the like.

The output signal from the BPF 41 is, for example, sent to a series connection circuit of the secondary trap 44 of the secondary trap 43 and trap frequency 4f _H trap frequency 6f _H, these BPF 41, a trap 43 by and trap 44, a total of 6-order BP for retrieving third multiplexed audio signal S ₃ of the aforementioned
F10 is configured. That is, the BPF 41 and the traps 43 and 44 correspond to the BPF 36 and the traps 37 and 38 constituting the BPF 10 in FIG. 4, respectively. Therefore, the output from the trap 44 has a frequency spectrum as shown in FIG. 2C, similar to the above-mentioned FIG. 7C.

The components other than the configuration shown in FIG. 1 are not the main components of the present invention, and may be configured, for example, in the same manner as the respective components of the circuit in FIG. 5 described above.

According to the configuration shown in FIG. 1, the level of the third harmonic (6f _H ) component of the carrier signal for synchronous detection of the second audio signal is greatly suppressed (for example, 40 dB or more). the third conversion amount is suppressed small to the low frequency side of the multiple audio signals S _3, as the filter BPF41 secondary subtractor 42
Only a trap consisting of Moreover, if the trap 1 stage only, it is easy to sufficiently flat phase characteristics to near 3f _H, it is eliminated conventional phase equalizer (31 of FIG. 4), the demodulated The degree of stereo separation can be secured to the same degree as in the past.

Next, FIG. 3 is a block circuit diagram showing another embodiment of the television audio multiplex signal demodulator according to the present invention.

In FIG. 3, a television audio multiplex signal having a frequency spectrum as shown in FIG. 4 is input to an input terminal 1, and the input audio multiplex signal is
A center pass frequency of the carrier frequency 5f _H of the third speech signal S ₃ for example quadratic BPF (bandpass filter) 41
Has been sent to The output signal from the BPF 41 is supplied to a subtractor (adder) 42 and a trap filter or trap.
The output signal from the trap 43 is sent to the trap 44. Subtractor 42, which subtracts the output signal of BPF41 from the input signal of the terminal 1, trap filter or trap to remove the carrier frequency 5f _H component is formed by these BPF41 and a subtracter 42. The output signal from the subtractor 42 is sent to a pilot signal removing (so-called pilot canceling) circuit 3.

The pilot signal removal circuit 3 is for removing the pilot signal (frequency f _H ) from the input signal. The adder 51 performs a pilot operation on the signal input to the pilot signal removal circuit 3. The pilot signal is removed or attenuated by adding a signal having a phase opposite to the signal (a pilot cancel signal). That is, first, the frequency f _H in the input signal is
Removed signal (pilot signal) with phase-synchronized form by using a PLL circuit or the like, the amplitude in the opposite phase to the input pilot signal by sending a pilot signal of frequency f _H to the amplitude adjustment circuit 55 of the multiplier or the like Is proportional to the signal (pilot
A cancel signal) is sent to the adder 51. The pilot signal detecting circuit 53 detects the level of the pilot signal in the input signal by synchronous detection with a signal of a frequency f _H obtained a signal input to the circuit 3 from the carrier reproduction circuit 52 (pilot signal) The obtained detection DC output signal is sent to a coefficient multiplier 62 and a level discriminator (comparator) 63 via a level adjustment circuit 61. The output from the coefficient multiplier 62 is sent to the amplitude adjustment circuit 55 to adjust the amplitude of the pilot cancel signal. The level discriminator 63 discriminates a pilot detection DC output signal obtained via the level adjustment circuit 61 based on, for example, a reference level from a reference voltage source 64, and determines the presence or absence of a pilot signal.
The output of the presence / absence determination of the pilot signal is sent to, for example, the matrix circuit 7 and the like, and switching control between stereo reproduction and monaural reproduction is performed.

An output signal from the pilot signal removal circuit 3 is sent to a first audio signal processing circuit 4 and a second audio signal detection circuit 5, and an output signal from the first audio signal processing circuit 4 is supplied to a matrix circuit 7 via an LPF 6. Sent to The second audio signal detection circuit 5, are used so-called synchronous detection circuit, as the carrier signal to be multiplied to the signal input, the pseudo sine wave signal having a frequency 2f _H from the carrier reproduction circuit 52 Used. And the pseudo-sine wave signal, for example, from the triangular wave and stepped wave is the third harmonic frequency 6f _H
A component removed or suppressed signals to the level of the 6f _H component is, eg, from the level of the fundamental wave (frequency 2f _H) and that decreased by more than 40 dB. Input signal, i.e. 5f _H carrier component from the television sound multiplex signals in a trap consisting of a BPF41 subtracter 42. The third multiplexed audio signal S ₃ of such a pseudo-sine wave signal to the second audio signal detection circuit 5 Is suppressed and the pilot signal removing circuit 3
By By f _H component is multiplied with a signal which is removed, while suppressing the amount of the third multiplexed audio signal S ₃ is converted to the low frequency side, the second multiplexed audio signal S ₂ and the low-band converting It becomes possible to detect. This second audio signal detection circuit 5
Is sent to one selected terminal a of the changeover switch 16. The changeover switch 16 is controlled to be switched in response to a reception changeover signal (a changeover selection signal indicating whether to receive the second sound signal or the third sound signal) supplied from the terminal 15, and receives the second sound signal. In some cases, the terminal is switched to the selected terminal a side and to the selected terminal b side in receiving the third audio signal.

Next, the traps 43 and 44 of the third sound signal system are
F41 constitutes a BPF10 of the FIG. 5 with, for example, secondary trap circuit of the trap frequency, respectively 6f _H and 4f _H is used. The output signal from the trap 44 is detected by the third audio signal detection circuit 11, and the changeover switch
It is sent to 16 selected terminals b.

The output signal from the changeover switch 16 is sent to an LPF (low-pass filter) 17 having a variable frequency characteristic.
The switch 17 is controlled to switch to a filter characteristic suitable for each sound reception according to the reception switch signal from the terminal 15. That is, at the time of receiving the third audio signal, taking into account that the frequency characteristic of the output signal from the detection circuit 11 tends to decrease in the high frequency range,
The filter characteristics are switched so as to have a peak near the cutoff frequency. The output signal from the LPF 17 is sent to the matrix circuit 7 via the noise low frequency circuit 13.

Also, at the time of receiving the third audio signal, noise generated when the RF electric field is a weak electric field or the like becomes hysteric noise having many high-frequency components due to the characteristics of the LPF 17, but the noise is input by the noise detection circuit 25 in order to prevent this. The noise level of the signal is detected and sent to the LPF 17 so that the peak near the filter characteristic cutoff is reduced.

According to the television audio multiplex signal demodulation apparatus having the configuration as shown in FIG. 3, the LPF 17 switches the frequency characteristics in accordance with each of the reception modes of the second audio signal and the third audio signal, whereby Can be shared by the respective audio signal systems, the circuit configuration can be simplified, and the cost can be reduced. Also, when the RF electric field becomes a weak electric field and the noise increases when the third audio signal is received, the noise level is detected by the noise detection circuit 25 and the high frequency component of the noise is reduced by controlling the frequency characteristic of the LPF 17,
I try to eliminate the discomfort of hearing. Further, the LPF2 of the first and second audio signal systems and the BPF10 of the third audio signal system in the related art share a part of the internal circuit configuration and can be configured with three secondary filter circuits. Not only can the circuit configuration be simplified and the cost can be reduced, but also the S / N and separation characteristics can be improved. Further, the output from the pilot signal detection circuit 53 is adjusted in level by a level adjustment circuit 61, and then sent to a coefficient multiplier 62 for setting the level of the pilot cancel signal and a level discriminator 63 for determining the presence or absence of the pilot signal. Because
The number of adjustment points is small, which contributes to simplification of adjustment work and cost reduction.

Note that the carrier frequencies and modulation forms of the second and third audio signals in the present invention are not limited to those of the embodiment. In addition, it goes without saying that various changes can be made without departing from the spirit of the present invention.

〔The invention's effect〕

According to the television audio multiplexed signal demodulation apparatus of the present invention, the second multiplexed audio signal is synchronously detected by the second multiplexed audio signal.
Since a so-called pseudo sine wave signal in which harmonic (particularly third harmonic) components are suppressed is used as a signal (carrier signal) having a carrier frequency of Even if a relatively low-order (for example, second-order) trap for trapping the third carrier frequency is used, the amount of conversion of the third multiplexed audio signal to the low-frequency side in the second audio signal detection circuit can be reduced. The filter configuration can be simplified without deteriorating the performance, that is, without deteriorating the S / N and the degree of stereo separation, and the chip area for IC integration or the number of external components can be reduced, resulting in cost reduction. Down can be planned.

[Brief description of the drawings]

FIG. 1 is a block circuit diagram schematically showing a main part of an embodiment of a television audio multiplex signal demodulator according to the present invention, and FIG. 2 is a graph showing a frequency spectrum of a signal of each part of the embodiment. FIG. 3 is a block circuit diagram showing another embodiment of an audio multiplexed signal demodulator according to the present invention, FIG. 4 is a graph showing a frequency spectrum of a television audio multiplexed signal used in the present invention, and FIG. FIG. 6 is a block circuit diagram schematically showing a conventional television audio multiplex signal demodulator, FIG. 6 is a block circuit diagram showing a specific configuration of a conventional LPF and BPF, and FIG. 4 is a graph showing a frequency spectrum of a signal. 1 ... television audio multiplex signal input terminal 2 ... LPF (low-pass filter) 5 ... second audio signal detection circuit 10 ... BPF (band-pass filter) 31 ... phase equalizer 32, 33, 37, 38 , 43, 44… trap (trap filter) 34, 45… carrier signal input terminal 36, 41… BPF 42… subtractor 52… carrier regeneration circuit

Claims (1)

(57) [Claims] 1. A carrier signal of a second and a third frequency set higher than the first television audio signal band,
A demodulator for demodulating a television audio multiplex signal, which is modulated and frequency-multiplexed by second and third audio signals, respectively, wherein the audio multiplex signal is inputted and a band having the third carrier frequency as a center pass frequency. A pass filter; a subtractor for subtracting the output signal of the band-pass filter from the input audio multiplex signal; and an output signal from the subtracter multiplied by a signal of the second carrier frequency in which harmonics are suppressed. An apparatus for demodulating a television audio multiplex signal, comprising at least a synchronous detection circuit and a third audio signal circuit to which an output signal from the band-pass filter is supplied.