Apparatus for reproducing a multichannel record disc

A multichannel record disc reproducing apparatus has a pickup cartridge for picking up signals from a multichannel record disc. A multiplexed signal was recorded on the disc by first multiplexing direct wave signals and angle-modulated wave signals which are recorded on opposite groove walls. Filters separate the picked up multiplexed signal into the direct wave signal and the angle-modulated wave signal. A demodulates circuit for demodulating the angle-modulated wave signal, thus separated. A matrix circuit separates the signals to obtain a plurality of separate channels from the direct wave signals and the demodulated signals. A circuit in a stage preceding the demodulator removes substantially all components of the higher harmonics at the carrier wave center frequency, which carrier was used for angle-modulating the original signal.

BACKGROUND OF THE INVENTION 
The present invention relates generally to apparatus for reproducing 
multichannel record discs, and more particularly to apparatus for 
demodulating and reproducing an angle-modulated wave signal picked up from 
a multichannel record disc. The higher harmonic components are removed 
from the carrier wave center frequency, of the angle-modulated wave 
signal, contained within the picked up signal. 
In general, a recording apparatus multiplexes a direct wave signal and an 
angle-modulated wave signal. The resulting multichannel signal is cut on a 
record disc. The cutter head used for making this recording has a 
relatively good crosstalk characteristic between the left and right 
channels, up to about the upper frequency limit of the angle-modulated 
signal. For example, a carrier wave center frequency, of the 
angle-modulated wave signal, may be 30 KHz and a transmission frequency 
band width may be 20 KHz to 45 KHz. The cutter speed and other variable 
are so selected that the cross-talk characteristic will be good up to 
about 45 KHz. 
However, this crosstalk characteristic of the cutter head itself is poor. 
In general, it does not respond well in a frequency band higher than the 
band upper limit frequency, for example, in a frequency band higher than 
50 KHz. For this reason, there often is a crosstalk between the left and 
right channels. When the higher harmonic components have frequencies which 
are two, three or more times the center frequency of the carrier wave the 
angle-modulated signal of one channel may also become recorded in the 
other channel. That is, in the cited frequency range, components of 
frequencies such as 60 KHz and 90 KHz are also recorded. 
On the other hand, there are pickup cartridges for use in the apparatus 
which have reproducing characteristics which can reproduce signals of 
frequency bands up to about 90 KHz. When reproduction is carried out with 
such a pickup cartridge, even the unwanted frequency components such as 60 
KHz and 90 KHz, are picked up. 
In the reproducing apparatus, the multiplexed signal of the direct wave 
signal and the angle-modulated wave signal is picked up and supplied, 
respectively, to a low-pass filter and a band-pass filter. There, it is 
separated into the direct wave signal and the angle-modulated wave signal. 
The angle-modulated wave signal, thus separated, is supplied to a 
demodulating circuit, where it is demodulated. 
In this case, it is necessary for the band-pass filter to have a 
characteristic which passes angle-modulated wave signals in the 20 KHz to 
45 KHz band and to separate them from direct wave signals having 
frequencies lower than 15 KHz. For this reason, the filtering frequency 
characteristic has a dip in the vicinity of 18 KHz. In order to obtain a 
good group delay characteristic, it has a dip in the vicinity of 50 KHz to 
52 KHz (or 50 KHz), in a symmetrical relationship to the first dip, also 
in the high-frequency range. Accordingly, the filtering frequency 
characteristic of this band-pass filter has side lobes at frequency 
positions such as 15 KHz, 60 KHz, and 90 KHz. 
The unwanted frequency components of 60 KHz, 90 KHz, etc., produced by the 
crosstalk were recorded and, therefore, are contained in the picked up 
signal. These signals are attenuated to a certain level by the side lobe 
characteristic of the band-pass filter. They are passed and supplied to 
the demodulating circuit. At present, however, a phase locked loop (PLL) 
is generally used for demodulating the signals picked up from a 
multichannel record disc reproducing apparatus. Since this PLL is capable 
of detecting these frequency components 60 KHz, 90 KHz, etc., they 
influence the other channels as crosstalk. Consequently, when such 
frequency components of 60 KHz, 90 KHz, etc., are supplied to the 
demodulating circuit, beats are generated between these frequency 
components and the carrier wave center frequency of 30 KHz of the 
angle-modulated signal. A distortion, due to interference, is produced in 
the demodulated output, whereby an abnormal sound is generated. 
SUMMARY OF THE INVENTION 
Accordingly, it is a general object of the present invention to provide a 
novel and useful multichannel record disc reproducing apparatus in which 
the above described difficulties have been overcome. 
Another and specific object of the invention is to provide a multichannel 
record disc reproducing apparatus for removing the higher harmonic 
frequency components of the carrier wave center frequency of the 
angle-modulated wave signal, from a signal picked up from a multichannel 
record disc. Then, demodulation is carried out. In the apparatus according 
to the invention, a reproduced signal can be satisfactorily obtained 
without the generation of abnormal sound due to interference distortion 
even when unwanted frequency components are recorded by crosstalk in the 
recording apparatus. 
Still another object of the invention is to provide a multichannel disc 
reproducing apparatus in which a band elimination filter, dip filter, or a 
trap circuit is provided in the demodulator for removing the higher 
harmonic components of the carrier wave center frequency of the 
angle-modulated wave signal. 
A further object of the invention is to provide a multichannel disc 
reproducing apparatus in which a band elimination filter, a dip filter, or 
a trap circuit is provided within the pickup cartridge or within an 
adapter connected to the record player for removing the higher harmonic 
components of the carrier wave center frequency of the angle-modulated 
wave signal.

DETAILED DESCRIPTION 
FIGS. 1 through 5 show a first embodiment of the invention. In FIG. 1, a 
multiplexed signal, comprising a direct wave sum signal and an 
angle-modulated difference signal, for each pair of two channels, is 
recorded on each side wall of the sound groove of a four-channel record 
disc 10, as is well-known. A total of four channels are thus recorded. A 
pickup cartridge 11 picks up one pair of multiplexed signals comprising 
the direct wave sum signal and the angle-modulated wave difference signal, 
from the left wall of the grooves of the disc 10. The picked up signal is 
fed to an equalizer 12 in a demodulator 23, having an RIAA (Recording 
Industry Association of America) turnover characteristic. 
The resulting signal is fed from the equalizer 12 to a low-pass filter 13 
for eliminating the angle-modulated wave component and for deriving only 
the direct wave sum signal component. The direct wave sum signal is fed to 
a matrix circuit 15, via an equalizer 14 having the RIAA roll-off 
characteristic. 
The output of the equalizer 12 is partly fed to a band-pass filter 16 (or 
high-pass filter) having a passband in the approximate range of 20 KHz to 
45 KHz. An angle-modulated wave difference signal is derived from this 
filter. For demodulation, the angle-modulated wave difference signal is 
fed to a demodulation circuit 18, via a band elimination filter or trap 
circuit or dip filter 17, which will be described in detail hereinafter. 
The demodulated output from the demodulation circuit 18 is supplied to a 
low-pass filter 19. There the unwanted components are eliminated from the 
demodulated output. The output is fed from the low-pass filter 19 to the 
matrix circuit 15 via (in succession) an FM/PM equalizer 20 and an 
automatic noise reduction system (ANRS) circuit 21 comprising an expandor. 
The characteristic of the expandor compensates for the characteristic of a 
compressor (not shown) in the recording system. 
The matrix circuit 15 combines the direct wave sum signal from the 
equalizer 14 and the demodulated difference signal from the ANRS circuit 
21. From output terminals 22a and 22b are derived, for instance, the left 
front (the first channel) and the left rear (the second channel) signals, 
respectively. 
FIG. 1 shows only the circuit system for processing the first and second 
channel signals (the signals recorded on the left wall of the grooves of 
the disc 10). Exactly the same type of system is duplicated for the right 
front (the third) and the right rear (the fourth) channel. A detailed 
illustration and description of this right system are omitted herein. 
Here, the dip filter 17 constitutes an essential part of the system of the 
present invention. The dip filter 17 will now be described. 
As mentioned hereinbefore and as indicated in FIG. 2, the band-pass filter 
16 possesses its original band-pass filtering characteristic of passing 
signals in the band of 20 KHz to 45 KHz. In addition, it has side lobes 
with centers at 15 KHz, 60 KHz, 90 KHz, etc. The frequency components of 
60 KHz and 90 KHz are two, three, or more times the carrier center 
frequency of 30 KHz of the angle-modulated wave. In the recording 
apparatus, these components are present because there is crosstalk between 
the left and right channels. These components are also passed by the side 
lobe characteristic. 
Accordingly, in the present embodiment of the invention, the dip filter 17 
has the characteristic indicated in FIG. 3, with a dip at 60 KHz. Filter 
17 is provided in front of the demodulating circuit 18. By this provision, 
the unwanted frequency component of 60 KHz, which has passed through the 
band-pass filter 16, is greatly attenuated by the dip filter 17, and is 
almost completely blocked from the demodulating circuit 18. Therefore, 
interference distortion is elminated from the demodulated output. This 
distortion is due to a modulation of the angle-modulated wave signal by 
the frequency of the beats generated between the carrier wave signals of 
60 KHz and 30 KHz. 
The frequency component of 90 KHz is greatly attenuated in the band-pass 
filter 16, as is apparent from the characteristic indicated in FIG. 2. The 
desired results can be achieved in actual practice by providing only the 
dip filter 17 of 60 KHz, without providing another dip filter of 90 KHz. 
Depending on the necessity, however, a dip filter having a dip at 90 KHz 
may be further provided in addition to the dip filter 17 of 60 KHz. 
In the present embodiment of the invention, the dip filter 17 is provided 
between the band-pass filter 16 and the demodulating circuit 18. It may 
also be provided in a stage in front of the band-pass filer 16. 
The frequency eliminating band of the dip filter 17 may be a frequency band 
having a width of the order in .+-. 5 percent to .+-. 10 percent of the 
dip center frequency (60 KHz in the present embodiment of the invention). 
A specific example of a circuit for obtaining a characteristic which is 
substantially the same as that of the dip filter 17 will now be described, 
with reference to FIG. 4. A circuit comprising a resistor 31, a capacitor 
32, and a coil 33 passes signals of the frequency 60 KHz. This circuit is 
connected in the negative feedback circuit between terminal pins 7 and 
6 of an amplifier 30, in the form of an integrated circuit. Accordingly, 
when a signal is supplied from the band-pass filter 16 through an input 
terminal 34, its frequency component of 60 KHz is attenuated in the 
negative feedback amplifier 30. The resulting output signal is led out an 
output terminal 35 and supplied to the demodulating circuit 18. 
One example of a circuit for the dip filter 17, as illustrated in FIG. 5, 
comprises a resistor 36, a capacitor 37, a coil 38, and a resistor 39, 
connected as shown. The resistor 39 is the resistance value of the coil 
38. One example of specific constants for these components in this dip 
filter is as follows. 
Resistances -- 
Resistor 36: 6.8 K.OMEGA.; Resistor 39: 100.OMEGA. 
Capacitance -- 
Capacitor 37: 680 PF 
Inductance -- 
Coil 38: 10 mH 
next, a pickup cartridge in the second embodiment of the invention will be 
described with reference to FIGS. 6, 7, and 8. This pickup cartridge 40 
has a stylus assembly 41 comprising a stylus, a cantilever spring, and 
other parts, and a transducer 43 installed within the casing 42. The 
pickup cartridge 40 converts mechanical vibrations into an electrical 
signal. Terminal pins 44 are connected to the transducer 43 and extend out 
of the casing 42. A coil 45 and a capacitor 46 are both enclosed within 
the casing 42. 
In the present embodiment of the pickup cartridge, the coil 45 and the 
capacitor 46 are mutually connected in series. This series combination is 
further connected in parallel to a pickup coil 47, as shown in the 
equivalent circuit of the diagram in FIG. 7. The coil 45 and the capacitor 
46 constitute a trap circuit and have a characteristic with a dip at 60 
KHz. 
If this trap circuit does not exist, the original pickup frequency 
characteristic is as indicated by full line in FIG. 8. However, the pickup 
cartridge of the present embodiment has a built-in trap circuit comprising 
the coil 45 and the capacitor 46. The frequency characteristic thereof 
becomes as indicated by broken line in FIG. 8 at 60 KHz and the vicinity 
thereof. 
A picked up signal from which the 60 KHz frequency component has been 
removed is led out through the terminal pins 44 of the pickup cartridge 40 
and supplied to a demodulator of known type. This known demodulator 
corresponds to the demodulator 23 illustrated in FIG. 1 without the dip 
filter 17. 
The component of a frequency equal to twice the carrier wave center 
frequency is removed from the picked up signal in the present embodiment 
of the invention. This is an effectiveness equivalent to that of the first 
embodiment of the invention. 
The second embodiment of the invention may be modified as illustrated by 
FIG. 9. In this circuit, a coil 45a and a capacitor 46a constituting a 
trap circuit are mutually connected in parallel. This parallel combination 
is connected in series to the pickup coil 47. The frequency characteristic 
of this trap circuit also imparts a dip at 60 KHz. 
A third embodiment of the invention will now be described in conjunction 
with FIG. 10. An adapter 50 is connected by a plug and jack 51 through a 
cord 54 to a record player including a pickup cartridge 11 and by a plug 
and jack 52 through a cord 55 to a demodulator 53. This demodulator 53 is 
a known type, which is equivalent to the demodulator shown in FIG. 1, 
without the dip filter 17. 
The adapter 50 has a casing 56 within which is installed a parallel 
connected combination of a coil 57 and a capacitor 58, connected to the 
cords 54 and 55. The coil 57 and the capacitor 58 constitute a trap 
circuit having a dip at 60 KHz. Accordingly, the adapter 50 removes a 60 
KHz frequency component from a signal picked up and reproduced by the 
pickup cartridge 11. The resulting signal is then supplied to the 
demodulator 53. 
In the present embodiment of the invention, it is sufficient merely to 
interpose and connect the adapter 50 between the record player and the 
demodulator. There is no necessity of using a special demodulator or a 
special pickup cartridge, as in the preceding first and second embodiments 
of the invention. Accordingly, the present embodiment of the invention is 
readily applicable to a multichannel record disc reproducing apparatus in 
which a conventional record player and a conventional demodulator are 
employed thereby to readily modify and convert the apparatus to achieve 
the objects of this invention. 
Further, this invention is not limited to these embodiments. Variations and 
modifications may be made without departing from the scope and spirit of 
the invention.