Circuit for detecting existence of an information signal

A circuit for detecting existence of an information signal comprising an input circuit provided for supply with the information signal, a clamp circuit for clamping a signal obtained from the input circuit to a predetermined fixed DC level, a level detector for detecting the amplitude level of an output of the clamp circuit and for producing a DC output with its level corresponding to the detected amplitude level, and a level comparator for comparing the DC output from the level detector with a predetermined reference voltage to produce an output signal assuming a first level when the DC output is higher than the reference voltage and a second level when the DC output is lower than the reference voltage. The first level or second level of the output signal from the level comparator indicates that the information signal is supplied to the input circuit.

BACKGROUND OF THE INVENTION 
This invention relates generally to circuits for detecting existence of a 
given electric signal, and more particularly, is directed to improvements 
in a circuit for detecting existence of an information signal which is 
suitable for use in an apparatus for reproducing an information signal in 
the information signal recording and/or reproducing system such as the 
digital audio disc system. 
In a digital audio disc system a digital audio disc which is a rotatable 
disc shaped recorded medium on which a digital audio signal is recorded as 
an information signal is provided. Such a digital audio disc is applied to 
a reproducing apparatus so that the digital audio signal recorded on the 
digital audio disc is reproduced. In such a known system, a reproduced 
audio signal of high quality can be obtained easily, and various different 
types of such a system have been already proposed. One of them is the 
optical digital audio disc system wherein a laser light beam, for example, 
is utilized to record and/or read out the digital audio signal. 
In production of the digital audio disc, that is, recording of the digital 
audio signal, in the optical digital audio disc system, first an audio 
signal to be recorded is converted into a digital signal, for example, a 
pulse code modulation (PCM) signal modulated in a predetermined pulse 
code, and a laser beam modulated in intensity by this PCM signal is 
impinged on a recording medium provided appropriately to form a large 
number of pits or bumps on the recording medium in response to the 
variations in "1" and "0" of the PCM signal, so that a record master is 
produced. Then a plurality of duplicated digital audio discs are pressed 
by the record master. 
In a manufacturing process of such a record master, the pits formed on the 
record master are provided in a different manner with the same PCM signal 
as a result of the difference in the power of the laser beam or other 
conditions in relation to the laser beam, and therefore it would also 
result that the digital audio discs duplicated from different record 
masters have different pit formations depending on the respective record 
masters. For example, digital audio discs having the different respective 
ratios in length of a pit to a portion next to the pit at the place where 
the PCM signal recorded thereat has the same pulse width for both "1" and 
"0" would be produced. When one of such digital audio discs is applied to 
a reproducing apparatus which has an optical head for reading out the 
recorded PCM signal from the disc by means of scanning the pits by the 
laser beam therefrom, it will result that the signal read out from the 
disc has its portions corresponding to for "1" and "0" with different time 
lengths, respectively, at the place on the disc where the PCM signal 
recorded thereat has the same pulse width for both "1" and "0". This 
phenomenon is called "asymmetry". In case the asymmetry arises, it is 
easily caused that a reproduced PCM signal obtained by wave-shaping the 
signal read out from the disc into a pulse train becomes different from 
the recorded PCM signal. This would result in the audio signal not being 
reproduced properly. 
Accordingly, to avoid the problem mentioned above, in the reproducing 
apparatus in the optical digital audio disc system, a wave shaping circuit 
for reforming the signal read out from the digital audio disc, that is, a 
reproduced information signal into a pulse train, is provided with a 
countermeasure to the asymmetry so that the reproduced PCM signal can be 
properly obtained under the situation of the asymmetry. In such a 
reproducing apparatus, the wave shaping circuit which is operative to 
correct the errors induced in the reproduced information signal by the 
asymmetry and reform the reproduced information signal into the properly 
reproduced PCM signal is provided. However, when the reproduced 
information signal is not obtained when the disc is not rotated or the 
disc is not applied to the reproducing apparatus, the wave shaping circuit 
generates a spurious pulse signal as if it were generating to reproduced 
PCM signal, and the spurious pulse signal causes trouble in the 
reproducing apparatus such that it makes a buzz noise sound and click 
noise or to make a control mechanism therein operate improperly. 
To avoid such a problem, it is effective to distinguish between a situation 
wherein the reproduced information signal is obtained and a situation 
wherein the reproduced information signal is not obtained, and make the 
reproducing apparatus inoperative and further suitably protect suitably 
the control mechanism in the reproducing apparatus when the reproduced 
information signal is not obtained. 
For the purpose of making the above distinction, a circuit for detecting 
existence of the reproduced information signal has been provided in the 
reproducing apparatus. The previously proposed detecting circuit employed 
in the reproducing apparatus utilizes the fact that the reproduced 
information signal obtained from the disc has the frequency characteristic 
which has a peak at a particular frequency when the disc rotates at a 
proper rotating speed for detecting existence of the reproduced 
information signal. In such a detecting circuit, a frequency component 
having the particular frequency is extracted from the signal obtained at 
the output of the optical head and the level of the frequency component is 
measured, and as a result, it is concluded that the reproduced information 
signal is obtained when the level of the frequency component exceeds a 
predetermined reference level. 
However, with the previously proposed detecting circuit mentioned above, it 
is very difficult to determine appropriately the predetermined reference 
level of the frequency component having the particular frequency, and in 
addition the distinction can be made only when the disc rotates at the 
proper rotating speed. Further, the previously proposed detecting circuit 
requires many parts and the circuit structure is very complicated. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved circuit for 
detecting existence of an information signal which avoids the above 
mentioned defect and disadvantage inherent in the previously proposed 
detecting circuit. 
Another object of the present invention is to provide an improved circuit 
for detecting existence of an information signal suitable for use in a 
reproducing apparatus in the optical digital audio disc system. 
Other objects, features and advantages of the present invention will be 
apparent from the following description taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
First, to assist understanding of the embodiment, an example of a 
reproducing apparatus in the optical digital audio disc system, to which a 
circuit for detecting existence of an information signal according to the 
present invention can be applied, and a previously proposed circuit for 
detecting existence of an information signal, will be explained with 
reference to FIGS. 1 to 4. 
In the reproducing apparatus in the optical digital audio disc system, a 
part of which is shown in FIG. 1, a signal reading laser light 1 from a 
digital audio disc which is rotating is received by a photo-detector 2 and 
subjected to the photo-electrical conversion therein. In case the signal 
reading laser light 1 is coming from a recording track portion of the 
digital audio disc, where a PCM signal is recorded, a reproduced 
information signal S.sub.1 having a waveform like a sine curve and a 
negative DC level, for example, as shown in FIG. 2A, is obtained by the 
photo-electrical conversion in the photo-detector 2. The reproduced 
information signal S.sub.1 is amplified by an amplifier circuit 3 and 
supplied to a wave shaping circuit 4. At the wave shaping circuit 4, the 
reproduced information signal S.sub.1 is reformed into a rectangular pulse 
train to produce a reproduced PCM signal S.sub.2 as shown in FIG. 2B at an 
output terminal 5. The wave shaping circuit 4 consists of a comparator 6 
and a reference voltage generator 7, and the reproduced information signal 
S.sub.1 and a reference voltage Vr from the reference voltage generator 7 
are supplied to positive and negative input terminals of the comparator 6. 
At the output of the comparator 6, an output signal of a predetermined 
voltage appears when the level of the reproduced information signal 
S.sub.1 coincides with or exceeds the reference voltage Vr, so that the 
rectangular pulse train assuming "1" or "0" in response to the reproduced 
information signal S.sub.1 is obtained. The reference voltage generator 7 
produces such a reference voltage Vr that the portions representing "1" 
and "0", respectively, of the output of the comparator 6, which correspond 
to the portions representing "1" and "0", respectively, of the PCM signal 
recorded on the digital audio disc which are successive and have the same 
time length, such as portions representing a frame synchronous signal, 
respectively, are given the same time length. For example, in the case of 
the reproduced information signal S.sub.1 as shown in FIG. 2A, the 
reference voltage Vr assumes a level -v.sub.r shown in FIG. 2A. 
With such a wave shaping circuit 4, even if the asymmetry arises, the 
reproduced information signal S.sub.1 is compared to be reformed with the 
reference voltage Vr having the level determined so as to correct the 
errors induced in the reproduced information signal S.sub.1 by the 
asymmetry at the comparator 6, and therefore the proper reproduced PCM 
signal is obtained. 
However, in the reproducing apparatus as mentioned above, when the 
reproduced information signal S.sub.1 is not obtained such that the 
digital audio disc is not rotated or is not applied to the reproducing 
apparatus, a spurious signal S.sub.1 ' formed of a noise component having 
a predetermined DC level, as shown in FIG. 3A, appears at the output end 
of the amplifier circuit 3 and is supplied to the wave shaping circuit 4. 
In this case, the comparator 6 and the reference voltage generator 7 
forming the wave shaping circuit 4 processes the spurious signal S.sub.1 ' 
in the same manner as the reproduced information signal S.sub.1, and the 
reference voltage generator 7 produces the reference voltage Vr having a 
level -v.sub.r ' shown in FIG. 3A. The comparator 6 produces an output 
signal of the predetermined voltage when the level of the spurious signal 
S.sub.1 ' coincides with or exceeds the reference voltage Vr having the 
level -v.sub.r ' so as to form a spurious pulse signal S.sub.2 ' as shown 
in FIG. 3B. The spurious pulse signal S.sub.2 ' appearing at the output 
terminal 5 is supplied to a decoder (not shown) in the same manner as the 
reproduced PCM signal. Since the spurious pulse signal S.sub.2 ' 
represents the noise and is coded at random, the coding errors of the 
spurious pulse signal S.sub.2 ' are not corrected by the error correction 
in the decoder. Consequently, a buzz noise sound or a click noise sound is 
produced by the spurious pulse signal S.sub.2 ' and the control mechanism 
in the reproducing apparatus operates improperly as a result of the 
spurious pulse signal S.sub.2 '. 
The above mentioned problem caused by the spurious pulse signal S.sub.2 ' 
results from the fact that the reference voltage generator 7 which is 
contained in the wave shaping circuit 4 as the countermeasure against the 
asymmetry generates the reference voltage Vr having the level determined 
in response to the level of the input signal to the wave shaping circuit 
4, and supplies it to the comparator 6. This problem is caused only when 
the reproduced information signal S.sub.1 is not obtained. Therefore, it 
is possible to avoid the trouble mentioned above if the situation wherein 
the reproduced information signal S.sub.1 is not obtained is detected and 
the reproducing apparatus is made inoperative when the reproduced 
information signal S.sub.1 is not obtained. 
For the purpose of detecting the situation wherein the reproduced 
information signal S.sub.1 is not obtained, such a detecting circuit as 
shown in FIG. 4 has been proposed previously. This detecting circuit 
comprises a band pass filter 8 which is connected to the output end of the 
amplified circuit 3 to extract a signal component having a particular 
frequency f.sub.0 from the output signal of the amplifier circuit 3. A 
level detector 9 is connected to the output end of the band pass filter 8 
and an output terminal 10 is attached to the level detector. This 
detecting circuit detects existence of the reproduced information signal 
by means of utilizing the fact that the reproduced information signal 
obtained from the disc has the frequency characteristic which has a peak 
at a particular frequency, for example, about 400 Hz when the disc rotates 
at a proper rotating speed. In the circuit, the signal component having 
the particular frequency f.sub.0 is extracted by the band pass filter 8 
and is supplied to the level detector 9 so that the level of the signal 
component having the particular frequency f.sub.0 is measured. As a result 
of the measure at the level detector 9, in case the level of the signal 
component having the particular frequency f.sub.0 is lower than a 
predetermined reference level, an output signal is obtained at the output 
terminal 10, for example. That is, it is concluded that the reproduced 
information signal is obtained when the level of the extracted signal 
component having the particular frequency f.sub.0 coincides with or 
exceeds the predetermined reference level, and on the other hand, it is 
concluded that the reproduced information signal is not obtained when the 
level of the extracting signal component having the particular frequency 
f.sub.0 is lower than the predetermined reference level and the detected 
output is produced at the output terminal 10. 
However, this previously proposed detecting circuit has the defect and 
disadvantage aforementioned. 
Now, one circuit embodiment of circuit for detecting existence of an 
information signal according to the present invention will be explained 
with reference to FIGS. 5, 6A-6D and 7A-7D. 
FIG. 5 shows the embodiment employed in the reproducing apparatus in the 
optical digital audio disc system for detecting existence of the 
reproduced information signal from the digital audio disc. In FIG. 5, the 
photo-detector 2 for receiving the signal reading laser light 1, the 
amplifier circuit 3, the wave shaping circuit 4 and the output terminal 5 
constitute the part of the reproducing apparatus in the optical digital 
audio disc system, as explained with reference to FIG. 1. The amplifier 
circuit 3 forms an information signal input portion of the circuit 
according to the present invention. A DC stopping circuit 11 is connected 
to the output end of the amplified circuit 3, and at the output end of the 
DC stopping circuit 11, a clamp circuit 12, a level detector 13 and a 
level comparator 14 are connected in series. An output terminal 15 is 
provided at the output end of the level comparator 14. Incidentally, one 
of the input terminals of the level comparator 14 is connected to a 
terminal 16 to which a reference voltage is supplied. 
Next, the operation of the embodiment constituted as above mentioned will 
be explained. When a reproduced information signal is obtained from the 
digital audio disc, an amplified reproduced information signal S.sub.10 
having a predetermined DC level and an amplitude a, as shown in FIG. 6A, 
is obtained at the output end of the amplifier circuit 3. This reproduced 
information signal S.sub.10 is supplied to the DC stopping circuit 11. The 
DC level of the reproduced information signal S.sub.10 is shifted to zero, 
as shown in FIG. 6B, at the DC stopping circuit. The signal S.sub.10 with 
the DC level shifted to zero is supplied to the clamp circuit 12. The 
clamp circuit 12 is operative to clamp an input signal supplied thereto to 
a predetermined fixed DC level, for example a negative level -Vc, and at 
the output end of the clamp circuit 12, the signal S.sub.10 with its 
bottom clamped to the level -Vc as shown in FIG. 6C, is obtained to be 
supplied to the level detecting circuit 13. The level detector 13 is 
formed pratically as a low pass filter or an integrating circuit and a DC 
output v.sub.O of the average level or the peak level of the signal 
S.sub.10 clamped to the level -Vc, as shown in FIG. 6D, is obtained at the 
output end of the level detector 13. This DC output v.sub.O is supplied to 
one of the input terminals, for example a positive input terminal, of the 
level comparator 14 and compared with the reference voltage supplied to 
the terminal 16 which is connected to the other of the input terminals, 
for example a negative input terminal, of the level comparator 14. 
On the other hand, when the reproduced information signal is not obtained, 
a spurious signal S.sub.10 ' having a noise component and a predetermined 
negative DC level, as shown in FIG. 7A, appears at the output end of the 
amplifier circuit 3. The noise component contains a noise element caused 
by vibrations of the disc or an optical head which emits a laser light 
toward the disc and guides the signal reading laser light 1 from the disc 
to the photo-detector 2 and another noise element caused in the amplifier 
circuit 3. The amplitude of this noise component is very small in 
comparison with the amplitude a of the reproduced information signal 
S.sub.10 but is capable of being reformed into a pulse train at the wave 
shaping circuit 4. The DC level of the spurious signal S.sub.10 ' is 
shifted to zero, as shown in FIG. 7B, at the DC stopping circuit 11, and 
thereafter the spurious signal S.sub.10 ' with the DC level shifted to 
zero is supplied to the clamp circuit 12. At the clamp circuit 12, the 
bottom of the spurious signal S.sub.10 ' is clamped to the level -Vc, as 
shown in FIG. 7C. The spurious signal S.sub.10 ' with its bottom clamped 
to the level -Vc is supplied to the level detector 13 and a DC output 
v.sub.O ' of the average level or the peak level of the spurious signal 
S.sub.10 ' clamped to the level -Vc, as shown in FIG. 7D, is obtained at 
the output of the level detector 13 to be supplied to one of the input 
terminals of the level comparator 14. In this case, since the amplitude of 
the spurious signal S.sub.10 ' is so small in comparison with the 
amplitude of the reproduced information signal S.sub.10, the level of the 
DC output v.sub.O ' becomes very low compared with the level of the DC 
output v.sub.O mentioned above. Besides, since both the signal S.sub.10 
and the spurious signal S.sub.10 ' are clamped at their bottoms to the 
level -Vc provided in common, then their average levels or peak levels are 
derived therefrom as the DC outputs v.sub.O and v.sub.O ', respectively. A 
difference between the DC outputs V.sub.O and v.sub.O ' becomes large and 
emphatically represents the difference between the amplitudes of the 
signal S.sub.10 and the spurious signal S.sub.10 '. 
The reference voltage supplied to the other of the input terminals of the 
level comparator 14 from the terminal 16 is given a level Vb between the 
DC outputs v.sub.O and v.sub.O ', as shown in FIGS. 6D and 7D. The level 
comparator 14 operates to produce an output when the DC output v.sub.O 
which is higher than the level Vb of the reference voltage is supplied to 
the one input terminal thereof and not to produce any output when the DC 
output v.sub.O ' which is lower than the level Vb of the reference voltage 
is supplied to the one input terminal thereof; or to produce a positive 
output in the former case and produce a negative output in the latter 
case, so that a detected output is provided from which it can be learned 
that either the DC output v.sub.O is supplied or the DC output v.sub.O ' 
is supplied, that is, whether the reproduced information signal S.sub.10 
is obtained or not. This information is obtained at the output terminal 
15. 
In such a manner as mentioned above, the detection of the existence of the 
reproduced information signal S.sub.10 is performed, and in the case of 
the example shown in FIG. 5, the reproducing apparatus is made inoperative 
and the appropriate protection for the control mechanism in the 
reproducing apparatus is conducted by the detected output obtained at the 
output terminal 15 when the reproduced information signal S.sub.10 is not 
obtained. Incidentally, in case that the circuit according to the present 
invention is applied to the reproducing apparatus in the optical digital 
audio disc system as described above, since the variations in the 
amplitude of the reproduced information signal S.sub.10 are relatively 
small, the veriations in the DC output v.sub.O obtained at the level 
detecting circuit 13 are also small and therefore there is no difficulty 
in determining appropriately the level Vb of the reference voltage at the 
level comparator 14. 
As explained above, in accordance with the present invention, the 
information signal or the spurious signal which is obtained when the 
information signal is not obtained is clamped to the predetermined fixed 
level. Then the DC output having the level corresponding to the amplitude 
of the reproduced information signal or the spurious signal clamped to the 
predetermined fixed DC level, and the detected output, is produced by 
means of comparing the DC output with the reference voltage. Accordingly, 
the detection of existence of the information signal is surely achieved 
with high sensitivity and reliability. Besides, since each circuit portion 
of the circuit according to the present invention is not required to 
perform any complicated operation, the whole circuit structure can be very 
simplified. Further, in case the circuit according to the present 
invention is employed in the reproducing apparatus in the optical digital 
audio disc system, even if the digital audio disc is not rotated at the 
proper rotating speed, the situation wherein the reproduced information 
signal is obtained from the disc can be detected. 
Although various minor changes and modifications might be suggested by 
those skilled in the art, it will be understood that I wish to include 
within the claims of the patent warranted hereon all such changes and 
modifications as reasonably come within my contribution to the art.