Frequency discriminating circuit

A frequency discriminating circuit is provided with multiple comparators corresponding in number to the number of frequencies to be discriminated, and the signal input to each comparator is also connected to each remaining comparator so as to raise the reference levels of the remaining comparators.

BACKGROUND OF THE INVENTION 
This invention relates to a frequency discriminating circuit for 
discriminating a plurality of signals of different frequency. 
In a channel up/down control or a volume up/down control in a remote 
control type receiver, up and down control signals of different 
frequencies are provided by the control section, and the frequencies of 
the control signals are discriminated on the receiver side. For this 
purpose, a frequency discriminating circuit for discriminating the 
frequencies of the two signals is employed. 
FIG. 1 shows one example of a conventional frequency discriminating circuit 
which comprises capacitors C.sub.1 and C.sub.2 for cutting off the DC 
components of the input signals, active filters 1 and 2 having selectivity 
characteristics corresponding to the frequencies of the input signals, 
rectifier circuits 3 and 4 for rectifying the outputs of the filters, and 
output transistors Q.sub.1 and Q.sub.2. These circuit elements are 
provided separately according to the two input signals of different 
frequency, in this case. The input signals are detected by the filters 1 
and 2 corresponding to the frequencies thereof, and are then rectified by 
the rectifier circuits 3 and 4, to thereby drive the output transistors 
Q.sub.1 and Q.sub.2, respectively. However, when two signals at levels 
higher than certain values are inputted simultaneously, or when the 
difference in frequency between two input signals is small, or when the 
selectivity characteristics of the filters 1 and 2 are broad, then the two 
output transistors Q.sub.1 and Q.sub.2 may be turned on at the same time, 
which causes erroneous operation. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a frequency discriminating 
circuit in which such erroneous operation is positively prevented. 
The frequency discriminating circuit according to the invention has a 
plurality of comparators which are provided separately according to a 
plurality of signals of different frequency, the comparators providing 
outputs when the levels of the signals are higher than respective 
reference levels. In the frequency discriminating circuit, a signal input 
to one of the plurality of comparators is employed to control the 
reference levels of the other comparators.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 2 is a block diagram showing one example of a frequency discriminating 
circuit according to one embodiment of the invention. In FIG. 2, those 
components which have been previously described with reference to FIG. 1 
are accordingly designated by the same reference numerals or characters. 
The frequency discriminating circuit in FIG. 2 will be described with 
reference to the case where two signals having different frequencies 
f.sub.A and f.sub.B are discriminated, for simplification in description. 
An input signal IN is applied to active filters 5 and 6 through the DC 
component blocking capacitors C.sub.1 and C.sub.2. The tuning frequencies 
of the active filters 5 and 6 are f.sub.A and f.sub.B, respectively. The 
signals which have been selected and amplified by the active filters 5 and 
6 are rectified by the rectifier circuits 3 and 4 into DC voltages 
corresponding to the signal levels, respectively. The outputs of the 
rectifier circuits 3 and 4 are applied through resistors R.sub.1 and 
R.sub.2 to the inverting input terminals of comparators 7 and 8, 
respectively, and are applied through resistors R.sub.3 and R.sub.4 to the 
non-inverting input terminals of the comparators 8 and 7, respectively. 
A reference level setting circuit 9 is provided to set the reference levels 
which are compared with the levels of the input signals. The reference 
level setting circuit 9 thus applies predetermined reference voltages to 
the non-inverting input terminals of the comparators 7 and 8, 
respectively, through the resistors R.sub.5 and R.sub.6. 
When a signal having the frequency f.sub.A is applied to the circuit thus 
organized, the signal is amplified by the active filter 5 and is then 
converted into a DC voltage corresponding to the signal level by the 
rectifier circuit 3. The DC voltage is applied, as the inverting input, to 
the comparator. When the level of this input signal is higher than the 
reference level preset by the reference level setting circuit 9, the 
output of the comparator is changed from the high level to the low level, 
for instance, thus determining that the signal is effective. In this 
operation, the signal of the frequency f.sub.A is further applied to the 
active filter 6; however, the output level of the comparator 8 is not 
changed, if the selectivity characteristic of the filter is steep. 
However, if the difference between the freuqencies f.sub.A and f.sub.B of 
the two signals is small or if the selectivity characteristics of the 
filters are broad, then the output voltage of the rectifier circuit 4 is 
slightly increased, and the output level of the comparator 8 may be 
changed, thus causing an erroneous operation. In order to eliminate this 
difficulty, in the circuit according to the invention, the output 
terminals of the rectifier circuits 3 and 4 are connected through the 
resistors R.sub.3 and R.sub.4 to the non-inverting input terminals of the 
comparators 8 and 7, respectively, so that the reference levels are are 
controlled by the input signal. Owing to this arrangement, when the signal 
of frequency f.sub.A is input and the output level of the rectifier 
circuit 3 becomes higher than the reference level of the comparator 7, the 
output level of the comparator 7 is inverted, but the reference level of 
the comparator 8 is increased through the resistor R.sub.3. Accordingly, 
even if the output of the rectifier circuit 4 is slightly increased, the 
output of the comparator 8 is not inverted. In the case when the signal of 
frequency f.sub.B is inputted, the operation is opposite to that described 
above. The output of the comparator 8 is inverted, while the reference 
level of the comparator 7 is increased. Accordingly, both outputs of the 
comparators 7 and 8 are inverted only when two signals having the 
frequencies f.sub.A and f.sub.B are inputted with the same level at the 
same time. 
While the above embodiment has been described with reference to the case 
where two signals are discriminated, the invention is not limited thereto 
or thereby. That is, the technical concept of the invention can be applied 
to a circuit for discriminating more than two input signals, by changing 
the reference levels with the respective rectification output. 
As is apparent from the above description, the frequency discriminating 
circuit is free from erroneous operation even if the difference in 
frequency of a plurality signals is small or the filters have broad 
selectivity characteristics. Accordingly, not only can the filters be 
readily designed, but also, plural signal frequencies can be readily set 
up. The frequency discrimminating circuit of the invention is, 
accordingly, suitable for discriminating, for instance, channel up/down 
control signals or volume up/down control signals in remote control type 
receivers.