JPS6110373Y2 - - Google Patents

Description

[Detailed explanation of the idea]

The present invention relates to an FM stereo signal demodulation circuit for pilot tone FM stereo broadcasting. In the conventional FM stereo signal demodulation circuit,
Pilot signal (19KHz) in its output audio signal
In order to prevent residual contamination, a high-cut filter for removing pilot signals is required at the subsequent stage of the circuit. However, the cutoff lower limit frequency of this filter is about 16 to 18KHz, and the tail of its cutoff characteristics extends to around 15KHz.
As a result, this frequency was at the upper limit of the audio frequency range of FM broadcasting, and as a result, the high frequency characteristics of the output audio signal were compromised. In addition, the adverse effects of the phase shift caused by this filter were significant and inconvenient. An object of the present invention is to provide an FM stereo signal demodulation circuit that eliminates the conventional disadvantages as described above. Hereinafter, one embodiment of the present invention will be described based on the drawings. In the figure, IN is an input terminal into which the FM stereo composite signal D(t) output from the front-stage detection circuit (not shown) is input, and SG is the input terminal for inputting the composite signal D.
Switching signal generation that outputs four asymmetric square wave switching signals a, b, c, and d, each having a phase difference of π/2 radians based on the phase of the subcarrier (38KHz) of (t) and having the same frequency as the subcarrier. circuit, S1, S2, S3, S
4 is a switching circuit that separately switches the composite signal D(t) according to the switching signals a, b, c, and d, C1 to C4 are capacitors, A1 to A4 are buffer amplifier circuits, PC is a pilot signal removal circuit, and SUB1 to C4 are capacitors. SUB3 is a subtraction circuit, PS1 is a phase advance circuit that advances the phase of its input signal, PS2 is a phase delay circuit that delays the phase of its input signal, LC1 and LC2 are level adjustment circuits, and L-
OUT indicates the left signal output end, and R-OUT indicates the right signal output end. In the above configuration, first, the composite signal D
(t), D(t) = (L+R)+(L-R) sinω _ct + psin
ω _c /2t (1) where L+R: main signal LR: sub signal ω _c : subcarrier angular frequency ω _c /2: pilot signal angular frequency p: pilot signal level. Next, the four switching signals a, b, c, and d output from the switching signal generation circuit SG will be expressed by mathematical expressions. These are signals that are shifted by π/2 with respect to the phase of the subcarrier, so if the phase difference with the subcarrier is a...0b...π/2c...πd...-π/2, then a=s ₀ +s ₁ sinω _ct +s ₂ sin2ω _ct +… …(2) b=s ₀ −s ₁ cosω _ct −s ₂ sin2ω _ct +… …(3) c=s ₀ −s ₁ sinω _ct +s ₂ sin2ω _ct +… …( 4) Fourier expansion is performed as follows: d=s ₀ +s ₁ cosω _ct −s ₂ sin2ω _ct +… (5). When the composite signal D(t) is switched in the switching circuits S1 to S4 using the switching signals a to d shown in equations (2) to (5), respectively,
The buffer amplifier circuits A1 to A4 output signals f ₁ to A4, respectively.
f ₄ is output. Note that since the pilot signal angular frequency ω _c /2 is considered here, the third terms of the signals a to d are taken for calculation. f ₂ =b×D(t) =s ₀ (L+R)+s ₁ /2psinω _c /2t+s ₀ psin
ω _c /2t… (7) f ₄ =d×D(t) =s ₀ (L+R)−s ₁ /2psinω _c /2t+s ₀ psin
ω _c /2t… (9) However, φ=tan ⁻¹ s ₁ /2s ₀ . Among these signals f ₁ to f ₄ , first, signal f ₂ and signal f ₄
The subtraction circuit SUB in the pilot signal removal circuit PC
After adding the subtraction operation by 3, the signal f ₅ is obtained. f ₅ =f ₂ −f ₄ =s ₁ psinω _c /2t (10) This signal f ₅ is a signal consisting of the pilot signal itself. Therefore, the phase of the signal _f5 is changed to the phase advance circuit.
PS1 advances it by ψ, and the level is adjusted by level adjustment circuit LC1.

[Formula] After multiplying this, the subtraction circuit SUB1 outputs the signal f ₁
If you add it in reverse phase, you will get a signal f ₆ . f ₆ = s ₀ (L + R) + s ₁ /2 (L - R) (11) Similarly, the phase of the signal f ₅ is delayed by ψ by the phase delay circuit PS2, and the level is adjusted by the level adjustment circuit LC2.
by

[Formula] Multiply this and add it to the signal f ₃ in the opposite phase in the subtraction circuit SUB2 to obtain the signal f ₇ . f ₇ = s ₀ (L + R) - s ₁ /2 (L - R)...(12) In other words, as you can see from the signal f ₆ and signal f ₇ , this becomes pure left and right signals. The pilot signal has now been completely erased. As described above in detail, the FM according to the present invention
According to the stereo signal demodulation circuit, the pilot signal can be effectively removed without using a filter, and therefore the deterioration of frequency characteristics caused by using a filter (in this case, the deterioration of high frequency characteristics especially around 15KHz) can be eliminated. It is possible to completely prevent the simultaneous occurrence of harmful phenomena such as phase shift, and due to its configuration, even if the input pilot signal level increases, there is no possibility that the pilot signal or subcarrier will leak into the output signal. The circuit of the present invention has extremely excellent features, such as having no defects at all.

[Brief explanation of the drawing]

The drawing is a circuit block diagram showing an embodiment of the FM stereo demodulation circuit according to the present invention. IN...Input terminal, SG...Switching signal generation circuit, S1-S4...Switching circuit, C1-C4
...Capacitor, A1-A4...Buffer amplifier circuit,
PC...Pilot signal removal circuit, SUB1 to SUB3
...subtraction circuit, PS1...phase advance circuit, PS2...delay phase circuit, LC1, LC2...level adjustment circuit, L-OUT
...Left signal output terminal, R-OUT...Right signal output terminal.

Claims (1)

[Claims for Utility Model Registration] First and second signals whose phase difference with the subcarrier of the FM stereo composite signal is 0°, 90°, 180°, and 270°, respectively, and whose frequency is the same as that of the subcarrier. , a switching signal generation circuit that outputs third and fourth asymmetric square wave switching signals, and a switching signal generation circuit that individually switches the FM stereo composite signal by each of the first, second, third and fourth switching signals. a first subtraction circuit that subtracts the outputs of the second and fourth switching circuits, and a first subtraction circuit that separately phase-shifts the outputs of the first subtraction circuits. , a second phase shift circuit, first and second level adjustment circuits that separately adjust the output levels of the first and second phase shift circuits, and a first level adjustment circuit that adjusts the output levels of the first and third switching circuits. , second and third subtracting the output of the second level adjustment circuit, respectively.
1. An FM stereo signal demodulation circuit comprising: a subtraction circuit; and a pilot signal cancellation circuit having a subtraction circuit.