JPS6281889A - Frequency band inversion circuit - Google Patents

Abstract

PURPOSE:To obtain a frequency band inversion circuit without a carrier leakage and the leakage of an input sound signal by applying a D/A conversion and a complementary conversion on the input sound signal after it is band-limited, and performing the inversion operation of a frequency band. CONSTITUTION:By adding a sound signal from an input terminal 101 and, a band exceeding over a sampling frequency is eliminated with an LPF to prevent a reflected distortion. The signal is added to a 90 deg. phase shifter 103 and the signals having a phase difference of 90 deg. are outputted from output terminals 104 and 105. These two signals are added to A/D converters 110 and 111 through sample/hold circuits 107 and 108, and furthermore, to complementary conversion circuits 115 and 116. And by adding the outputs of these two complementary conversion circuits 115 and 116 at an adder 117 and passing it through a D/A converter 118 and an LPF119, the sound signal from which a high pass distortion and a noise are eliminated and the frequency band of which is inverted, can be obtained at an output terminal 120.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a frequency band inversion circuit for concealing audio in broadcasting and the like.

Conventional technology When performing frequency inversion as a method of audio concealment in general broadcasting, etc., the audio is multiplied by a carrier signal that is the inversion axis using an analog method, and then the SB , and the method of obtaining the multiplication result, the audio signal phase-shifted by 90 degrees, and the carrier by 90 degrees.
The phase shift method adds the products of the 0' phase shifts to obtain the LSB (Radio Equipment (Volume 1), edited by the Radio Operator Education Association).

(September 19, 1972), Kindai Kagakusha, P323-P
325).

Problems to be Solved by the Invention As mentioned above, two methods have been described as conventional techniques.
The latter method improves the leakage of VSB components due to the non-ideal LPF, which is a drawback of the former method, but due to the special purpose of frequency inversion, the input and output bands completely overlap. , the input audio signal leaks along with the carrier leakage to the frequency inverted output. In particular, the main components of the audio signal are in a relatively low frequency range, and this frequency inverted output is converted to a high frequency range. Due to the relatively high audibility of the audio in the range and the selective ability to cut out noise, there was a problem in that the original input audio signal leaking to the output was emphasized.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a frequency band inversion circuit that is free from carrier leakage and input audio signal leakage.

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention band-limits the input audio signal and uses a 90' phase shifter to separate the input audio signal from the reference phase audio signal by using a 90' phase shifter.
Obtain audio signals with a phase of 0°, perform analog-to-digital conversion on each, and perform complement conversion on each digital value obtained from this using the frequency timing of the axis of frequency band inversion and a timing having a phase of 90° with respect to the above timing. , the sum of these values is taken, digital-to-analog conversion is performed, and this output is passed through an LPF to perform a frequency band inversion operation of the audio signal.

Effects Since the present invention uses the above-described configuration to perform the product and sum of signals as digital values, leakage of input audio signals and leakage of carriers can be eliminated.

Embodiment FIG. 1 is a schematic block diagram showing an embodiment of the frequency band inversion circuit of the present invention. FIG. 2 is a timing chart of main parts of the circuit shown in FIG. 1. The operation of the frequency band inversion circuit shown in FIG. 1 will be described below with reference to the timing chart in FIG. 2. Add an audio signal from the input terminal 101,
In order to perform sample hold and D-conversion in the subsequent stage, the band above the sampling frequency is removed by the LPF 102 to prevent aliasing distortion. 900 phase shifter 1
03 outputs signals within the audio band between its output terminals 104 and 105 with a phase of 90°. The output of the output terminal 106 is 90 degrees behind the output of the output terminal 104 in phase. The output at output terminal 104 is sampled and held by a sample and hold circuit 107 controlled by a control signal (the sample and hold pulse in FIG. 2) from a sampling frequency source 106. When the control signal is at a low level, it is sampled, and when it is at a high level, it is halted. As shown in Figure 2B, the pull/hold circuit 1
When the oa is in the hold state, the timing circuit 109id
Conver,), 7 frames/Totoshite, Mudoko/Bata 11
0, analog-to-digital conversion is performed, and the data (DI, D2.D3°D4...
...) is output. Similarly, the output at output terminal 105 is sampled and held by a sample and hold circuit 108 controlled by a control signal (human sample and hold pulse in FIG. 2) from a sampling frequency source 106. The MCU converter 111 is controlled by a conversion command (FIG. 2B) output from the timing circuit 109, performs analog-to-digital conversion, and performs analog-to-digital conversion.
The data (Fl, F2.F3°F4.
...) is output. The complement conversion circuit (c'rc circuit) 116 converts the data from the MCU converter 110 as shown in FIG. 2E by the high level of the control signal (FIG. 2D) output from the control terminal 113 of the timing circuit 112 Convert the complement to . The control signal is a signal having a frequency that is the axis of frequency band inversion of the input audio signal. The complement conversion circuit 116 performs complement conversion on the data from the MCU converter 111 as shown in FIG. 2H in response to the high level of the control signal (FIG. 2G) output from the control terminal 114 of the timing circuit 112. The control signal is a signal with a phase delay of 90° with respect to the control signal output from the control terminal 113. The adder 117 includes the complement conversion circuit 11
6 and the output data of the complement conversion circuit 116 are added, and as shown in Figure 2, the combined data is input to the DA converter 118, and after digital-to-analog conversion, the LP
High frequency distortion and noise are removed by F119 and output terminal 12
It is possible to obtain an audio signal whose frequency band is inverted from 0.

Further explanation will be given using FIG. 3, which is a spectrum waveform diagram of the main part. Here, the input terminal 101 has a band of OHz to 15k.
Input the Hz audio signal. This is shown in Figure 3a. By converting this from analog to digital using a sample hold 107° 108 and a digital converter 110, 111, a band of 45 kHz to 7 kHz is obtained as shown in Figure 3.
The resulting image is converted to 6kHz. Here, the output of the MuD converter 11 has a phase lag of 90' with respect to the output of the MuD converter 10. Assuming that the former is cos θ and the latter is sin θ, then as explained above, if the CTC circuits 116 and 116 perform complement conversion at a timing with a phase difference of 900, then if the timing frequency is 8, then the cos θ and sin θ are obtained. for each c
It is the product of os a and sin a.

Therefore, cosθ×cosa=−(cos(θ−a)−4−co
g(θ11))・・・・・・(1) sinθ×5ina
=-(cos(θ-a)-5in(θ+&))...
...(2) From the above equation, the output of the CTC circuit 115 is
As shown in Figure C, the output of the CTC circuit 116 is also
This results in a spectrum distribution as shown in Figure d. In Figure 3d, the band 15k) lZ ~ 30 kHz, 60 kHz ~
The amplitude at 90kl-17 is negative in the above-mentioned (
2) Indicates the component of the second term on the right side of the equation. Therefore, the above (1
) and (2), we get (') + (2) = cos (θ-a)
--=(3) From equation (3), the output of the adder 11 is, as shown in FIG. This results in a spectrum distribution in a band of 30 kHz to eokl-1z that causes distortion. Therefore, the above 30kHz to 60kHz
By removing the harmonic distortion in the band by the LPF 119, a frequency band inverted output of the input audio signal can be obtained as shown in FIG. 3f.

Effects of the Invention As described above, according to the present invention, it is possible to obtain a high-quality frequency band inversion output without carrier leakage or input audio signal leakage, and it is extremely useful in practice.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a frequency band inverting circuit according to an embodiment of the present invention, FIG. 2 is a timing chart of the main parts of the circuit, and FIG. 3 is a spectrum waveform diagram of the main parts of the circuit. jol...Input terminal, 1o2...Low pass filter, 103...90° phase shifter, 107,1
08.../Pull-hold circuit, 106...
... Sampling frequency source, 109 ... Timing circuit, 110, 111 ... AD converter, 112 ... Timing circuit, tf3, 11
4... Control terminal, 115, 118...
Complement conversion circuit (CTC circuit), 1j7...Adder, 118...DA co/parter, 119...
...Low-pass filter, 120...Output terminal. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure/θ3 Figure 2 Figure 3

Claims (1)

[Claims] A band-limited audio signal is obtained by a 90° phase shifter to obtain an audio signal at a reference phase and an audio signal at a phase of 90° with respect to the reference phase, and convert them from analog to digital, respectively.
The digital values obtained from this are respectively 90 degrees from the frequency timing of the frequency band inversion axis and the above timing.
It is characterized by performing complement conversion according to the timing with the phase of Frequency band inversion circuit.