JPS5925440B2 - 4 channel phonograph record decoder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION J The present invention relates to a decoder for four-channel phonograph records.

In a true four-channel phonograph disc record, the standard stereo sound channel contains four signals. Looking at one side of the sound groove, standard modulations of 50H2 to J15KH2 are used for A+B information and A-
The B signal is frequency modulated to a carrier frequency on the order of 30 KH2 and carried on the one side of the sound groove. The other side of the sound groove contains channels C+D, with C-D being carried in the same manner as before. In conventional decoders, it was necessary to use many tuning circuits and tuning rate detectors or discriminators to extract the A and B signals from the record.

Therefore, in this prior art, the adjustment was very complicated and there was a limit to the separation between the front and rear. In the present invention, a phase-locked loop (hereinafter abbreviated as PLL) circuit is used, thereby. The need for a tuned circuit for decoding is completely eliminated. The technique of the present invention provides a simple and inexpensive method for deriving the AB signal. Further, there is no need to adjust the circuit, and standard components can be used.
Furthermore, according to the circuit of the present invention, front and rear information is sufficiently separated, and the linearity of the demodulated output is very high.
Therefore, there is an advantage that the distortion of the output signal is small. PLL
Although the circuit is an effective decoding device for four-channel records, it is most effective when used with phonograph records, as it encounters several problems not encountered when receiving regular FM radio. It is desirable to modify the PLL circuit so as to obtain a beneficial effect.

In a normal FM radio system, the deviation (DeviatiOn) is small compared to the transmission frequency. For example, if the deviation is 75 KHz, the deviation is approximately Only 10%.

However, in phonograph records, the carrier frequency is usually 30KHz, and for this carrier
An excursion in KHz is given. Therefore, the deviation in a typical phonograph record system is approximately 50%, which also presents several problems. Conventional PLL circuits designed for relatively narrow frequency bands do not perform well when excursions are high.

Voltage controlled oscillators sometimes lose lock and then take extensive corrective steps in an attempt to regain lock. This problem can be solved by reducing the circuit's tracking bandwidth and still use a standard PLL circuit, but the demodulated output will then have a lower frequency response and a lower output level. There are drawbacks that make it considerably worse. Such a situation also occurs with records that are damaged or dusty such that the carrier is temporarily lost. According to improvements in the present invention, the problems inherent in these standard PLL tracking methods are solved by dividing the loop into two feedback sections, one feedback section which is limited to a DC feedback circuit and the other feedback section which is an AC bandpass feedback circuit. Solved by dividing. In this method of the invention, the locking and tracking ranges of the AC and DC loops are each independently controlled. The gain of the DC loop controls the lock range of the PLL; the greater the gain, the wider the lock range. This DC loop is actually a low pass filter with a cut-off frequency of about 40 Hz, so that it only affects the lock range of the PLL. The bandpass of the AC loop controls the tracking range and recovered demodulated audio bandwidth. This AC loop has a frequency of approximately 50Hz to 15KHz.
A bandpass filter with a response, thus controlling only the tracking range and recovery audio bandwidth. The advantage of the circuit of the present invention is that it provides a memory function in the DC loop so that the voltage-controlled oscillator can be used in a wide It prevents you from taking steps.

Referring now to FIG. 1, the composite signal from one side of the sound groove enters the apparatus through line 1 from the pickup.

Of course, the pick-up produces two separate composite signals, but only one of them will be discussed herein, since both signals are processed by exactly the same means. For example, the signal from line 1 is for the left channel, and this signal contains A+B information and frequency modulated A-B information in the frequency range of 50Hz to 15KHz.
The signal includes a 30KHz carrier signal. A portion of the input signal enters an input amplifier 3, from which the signal (essentially only the A+B signal) is sent to a delay line 4 to correct time errors, and then to a separate adjustment network 5. and is applied through de-emphasis circuitry 7 to a first input of matrix 9.

The operation of this matrix 9 will be described later.
The other part of the signal from line 1 is fed to a high pass amplifier 11 where the low frequency components are removed and a 30 KHz carrier signal is amplified.

A limiter is also provided here to remove unnecessary residual AM from the signal. The signal is passed through this high-pass amplifier 1
1 to a phase detector 13, the output of which is fed through a low-pass filter 15 to a voltage controlled oscillator (CO) 17 with a feedback loop to said phase detector.

The free running frequency of CO17 is approximately equal to the carrier frequency. The output from the low-pass Weilme 15 is a DC error voltage,
This voltage is used to make VCO17 track the input signal. The error voltage is the audio output from the desired FM carrier that occurs when the carrier is deflected, and the PLL tracks this error signal and generates an audio output at the output of the low-pass filter. This audio output is an A-B signal and is taken out through line 21. High pass amplifier 11
A portion of the signal from is fed through a quadrature detector 23 to an indicator 25 indicating that a four channel record is being played.

Quadrature detector 23 uses its input signal to sense the lock on CO1 7 and generates a DC output that activates indicator 25 and muting gate 27. This signal is then applied through carrier de-emphasis circuitry 29 to the second input of matrix 9. If active audio compression is used when recording a phonograph record, an active audio expansion is used between the muting circuit 27 and the matrix 9. In the matrix 9, the DC A+B signal from the input amplifier and the A-B signal from the error detector are combined into two channels, i.e., the amplifier 31
An A channel signal is amplified by the amplifier 37 and supplied to the speaker 33, and a B channel signal is amplified by the amplifier 37 and supplied to the speaker 35.

That is, the composite signal on line 1 is split into two fundamental components such that the signal input 1 yields channels A and B. It is clear that the present apparatus is equipped with two sets of circuits as described above, so that the C and D signals for the signals on the other side of the sound groove can be generated in the same manner. The isolation adjustment circuit 5 may be a simple attenuator and is adjusted to properly select the ratio of A+B and AB to obtain the best isolation.

The muting circuit 27 is A when there is no carrier.
The -B output is suppressed so that only the A+B signal is supplied to the matrix so that both outputs 33 and 35 contain A+B information. In the circuit shown in block diagram form in FIG. 2, voltage controlled oscillator 50 has two return paths, an AC return line 52 and a DC return line 54.

Although these feedback lines are shown separated, ie, with parallel paths, for convenience of illustration, they may be parallel or series in an actual circuit, and in the embodiment shown in detail in FIG. 3, the two paths are It's actually in series. Carrier is line 5
6 to a phase comparator 58 in the usual manner, where the CO5 supplied through line 60
The output from O is compared to the phase of the carrier provided through line 56 and if there is a difference, an error voltage is generated on line 62. The lead wire 62 is connected to a bandpass filter 64 forming an AC loop, and the output from the filter is supplied to the VCO 5O through the AC feedback line 52 as described above. This AC error voltage also constitutes the desired audio output taken from line 66. The output from line 62 also passes through a low pass filter 68, the output from which constitutes a DC feedback voltage, and the output from line 54.
is supplied to VCO5O through. That is, the regulated DC voltage controls the lock range of the PLL and prevents wide excursions such as those obtained when demodulating unusually high percentage excursion signals using standard PLL circuits. At the same time, the AC loop normally functions to control the tracking range and serve as a precise error voltage to generate the desired demodulated audio. FIG. 3 shows a circuit for actually carrying out the invention.

This circuit has four main components: a voltage controlled oscillator 70. It includes a comparator or phase detection circuit 72, an AC feedback circuit 74, and a DC feedback circuit 76. The voltage controlled oscillator or CO7O consists primarily of two Schmitt triggers 78 and 80, which act as free-running multivibrators whose free-running frequency is determined by resistor 79 and capacitor 90.

This free-running frequency is approximately 30 KHz in a typical phonograph record decoder. As will be detailed below, the error voltage from phase detector 72 is supplied from line 82 through transistors 84 and 85 to triggers 78 and 80, and this voltage determines the charging time of capacitor 90, and therefore V
Determine the actual frequency of CO. The frequency of the voltage controlled oscillator is fed to a phase detector for phase comparison purposes as in a conventional PLL. In the illustrated embodiment, the unbalanced voltage with respect to ground is taken out through line 94, as will become clear later.
4 and 95 can also be taken out as a balanced or differential voltage. The phase detector in this embodiment uses a pair of transistors 102 and 104 that form a balance circuit and an actual phase comparator circuit.

As shown, the CO frequency is applied to the bases of the transistor pairs through line 94, and the incoming frequency is applied to the emitters of these transistor pairs from lines 96 and 98. When the loop is locked, there is 90° between the voltages coming into the emitters and bases of these comparison transistors.
There is a zero phase difference, so that no output is produced at the collectors of these transistors, and an output voltage is produced when the loop is unlocked. transistor pair 102
and the collectors of transistors 101 and 1
03 and forming an active filter, the bases of these amplifiers being interconnected through filter circuits 74 and 76.
The outputs from these amplifying transistors are taken from their respective collectors through lines 106 and 108, brought to a single-ended voltage at transistors 118 and 120, and then provided through line 82 as an error signal as previously described. This error signal is fed back to VCO 7O and taken out on line 92 as an audio output. The response characteristics of the amplifier or active filter constituted by the transistor pair 101 and 103 are determined by the response shaping circuit or line 1 between the transistors.
05 and 107 to filters 74 and 76.

Although these filters are physically connected in series with each other, they actually act independently of each other and are substantially parallel. Filter 76 is a relatively large capacitor 109
, preferably a 4mfd capacitor. This filter causes the amplifier to act as a DC filter with a first cut-off point of approximately 4 Hz and a second cut-off point of 40 Hzf)O to 40 Hz. As will become clear later, filter 7
4 has no effect on such low frequencies, and therefore capacitor 109 effectively controls the lock range of the PLL, thus requiring a high lock range in other words for circuits with high percentage excursions. To enable the use of a PLL circuit in such a circuit. Capacitor 109 acts as a memory element in the return loop, so that even if the carrier is temporarily lost, the CO will not seek the correct frequency over a wide range of travel, but will instead It stores the last frequency that was previously applied and continues to supply an appropriate error voltage to continue oscillating the CO at the last frequency until the carrier signal is restored.

This capacitor 109 functions as a memory circuit as follows. When the PLL is locked, capacitor 109
voltage is equalized and therefore capacitor 109 is connected to line 1
It has high impedance with respect to 05 and 107.

However, if the PLL loses its input signal and attempts to return to its free-running frequency, the voltage on the capacitor changes rapidly. When this sudden change occurs, capacitor 109
becomes a low impedance and prevents the voltage on the capacitor from changing rapidly, creating an artificial correction voltage on line 82 until such time as the input signal is restored and the lube requests lock again. A large capacitor has a smoothing effect only at relatively low frequencies, controlling the lock range and acting as a storage element, but has no effect on the relatively high frequency journey of the oscillator tracking the input signal. Not affected.

Thus, the AC loop independently acts to control the tracking range to provide accurate error voltages to produce the desired audio output. The AC portion 74 of the shaping loop consists of a relatively low value resistor 113 and a capacitor 111 in parallel with it.

Preferably the value of resistor 113 is 1000 ohms and the value of capacitor 111 is 0.01 mfd. In such a circuit, filter 74 has a cutoff point of approximately 15 KHz, passing all normal audio frequencies and blocking higher frequency components that would interfere with VCO operation. As mentioned above, the two filters are physically in series, but operate independently of each other.

That is, at low frequencies, the resistance of resistor 113 is so small that it is not a limiting factor for the passage of low frequencies, and the high capacitance capacitor 109 determines the characteristics of the filter at low frequencies. On the other hand, at high frequencies, the resistance of capacitor 109 is so small that it has no effect on the passage of high frequencies;
Only filter 74 is activated. In the wiring diagram of FIG. 2, two filters are shown in parallel, which form the feedback path, ie the voltage control between the phase comparator and the CO.

Since these filters are elements that control the operation of the amplifier in the phase comparison circuit shown in Figure 3, and therefore control the amount of error voltage, these filters can be effectively inserted into the path of the error voltage. DC and AC feedback are independently controlled by the characteristics of these filters. It is often necessary to provide an indicator to indicate whether a multi-channel record is being played and if the input signal is FM.
It is desirable to mute the circuit when it does not have a carrier input.

Outputs for operating the mutating and indicating circuits are taken from lines 115 and 116. Although a practical record decoder circuit is shown herein, it will be apparent to those skilled in the art that various modifications may be made without departing from the spirit of the invention. Dew.

For example, in the present invention, the phase comparator is provided with an unbalanced output from the CO through line 94, while the incoming frequency is provided as a balanced input. However, these circuits may be balanced or unbalanced, for example from the VCO to lines 94 and 95.
to provide a balanced output to the phase comparator; conversely, bias one of lines 96 and 98 and provide the voltage to be compared through the other line to provide an unbalanced input to the phase comparator. You can also do it like this.

[Brief explanation of the drawing]

Figure 1 shows the basic PLL used for demodulating phonograph records.
FIG. 2 is a block diagram showing a circuit using a double feedback loop, and FIG. 3 is a connection diagram showing a practical circuit of the present invention. 3...Input amplifier, 4...Delay line, 5
... Separation adjustment circuit, 7 ... De-emphasis circuit, 9 ... Matrix, 11 ...
・-High-pass amplifier, 13... Phase detector, 1
5...Low pass filter, 17...Voltage controlled oscillator (VCO), 23...Quadrature phase detector, 25...Indicator, 27... ... Muting circuit, 29 ... Carrier de-emphasis circuit, 31 ... Amplifier, 33, 35 ...
...Speaker, 37...Amplifier, 50...
...Voltage controlled oscillator, 58... Phase comparator,
64...Band pass filter forming an AC loop, 68...Low pass filter forming a DC loop, 70...Voltage controlled oscillator, 72...
... Phase comparator, 74 ... AC loop, 7
6...DC loop.

Claims (1)

[Claims] 1 Contains two components from each side of the sound groove of a gramophone record, that is, A+B information, and has a frequency of 50 Hz to 15 KHz.
4 of the form in which a composite signal is derived consisting of a first or audio frequency component having a frequency range of about 30 KHz and a modulating frequency component consisting of a carrier having a frequency of about 30 KHz and frequency modulated with the A-B audio signal. In a channel gramophone record decoder,
means for separating the second component from the composite signal and supplying the second component to a phase-locked loop including a phase comparator and a voltage-controlled oscillator, connected between the phase comparator and the voltage-controlled oscillator; and a first return path containing a low pass filter having a frequency range of approximately 0 to 40 Hz and a frequency range of approximately 50 Hz to 15 KHz.
a main feedback path comprising a second feedback path including a bandpass filter having a band of A decoder for a four-channel phonograph record, characterized in that it comprises means for supplying an audio component together with said A+B signal to matrix means, and means for obtaining a separated A signal and a separated B signal from said matrix means.