Automatic gain control circuit

Details Automatic gain control circuit Automatic gain control circuit

1999-05-26

2003-01-28

Hsia, Sherrie (Department: 2614)

Television

Image signal processing circuitry specific to television

Gray scale transformation

C348S733000

Reexamination Certificate

active

06512554

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an automatic gain control circuit capable of receiving two broadcast signals of different systems as in a television signal of a digital system and a television signal of an analog system, for example.
2. Description of the Related Art
A prior art automatic gain control circuit will be explained in accordance with
FIG. 2. A
tuner
21
is a tuner capable of receiving both broadcast signals: a television signal (hereinafter called “analog broadcast signal”) of an analog system and a television signal (hereinafter called “digital broadcast signal”) of a digital system. The tuner
21
has an intermediate frequency signal output terminal (hereinafter called “IF signal output terminal”)
21
a
and an automatic gain control voltage input terminal (hereinafter called “AGC voltage input terminal”)
21
b
and is electrically connected to an antenna
22
. Further, the tuner
21
has a gain variable amplifier
21
c
, a mixer (not shown) for converting an input broadcast signal into an intermediate frequency signal (hereinafter called “IF signal”), etc. The gain variable amplifier
21
c
serves as an amplifier capable of varying amplification according to the magnitude of an automatic gain control voltage (hereinafter called “AGC voltage”) inputted from the AGC voltage input terminal
21
b
. When the AGC voltage is high, the amplifier serves so as to increase the amplification.
Further, the analog broadcast signal or digital broadcast signal inputted from the antenna
22
is amplified by the gain variable amplifier
21
c
according to the magnitude of the AGC voltage and converted into the intermediate frequency signal by the mixer, which in turn is outputted from the IF signal output terminal
21
a.
A high-frequency switch
23
is electrically connected to the IF signal output terminal
21
a
. The high-frequency switch
23
has output terminals
23
a
and
23
b
and an input terminal
23
c
and electrically connects the input terminal
23
c
to either the output terminal
23
a
or
23
b
according to the type of each received broadcast. Further, when the tuner
21
receives the analog broadcast signal therein, the high-frequency switch
23
serves so as to electrically connect the input terminal
23
c
to the output terminal
23
a
. When the tuner
21
receives the digital broadcast signal therein, the high-frequency switch
23
serves so as to electrically connect the input terminal
23
c
to the output terminal
23
b
. The high-frequency switch
23
can be constructed by means of a switch diode, for example.
A detector
24
for outputting an AGC voltage according to the level of the IF signal when the IF signal is inputted thereto, and a demodulator
25
for outputting a voice or speech signal and a video signal from the IF signal of the analog broadcast signal are electrically connected to the output terminal
23
a
. Thus, the IF signal is to be inputted to both the detector
24
and the demodulator
25
.
Here, the detector
24
outputs the AGC voltage to be outputted therefrom as high as practicable when the input IF signal is low in level. On the other hand, when the input IF signal is high in level, the detector
24
outputs the AGC voltage to be outputted therefrom as low as practicable. Further, the AGC voltage outputted from the detector
24
is to be inputted to the AGC voltage input terminal
21
b
of the tuner
21
through an automatic gain control voltage selector switch (hereinafter called “AGC voltage selector switch”)
26
.
Similarly, a detector
27
for outputting an AGC voltage when the IF signal is inputted thereto, and a demodulator
28
for outputting a voice or speech signal and a video signal from the IF signal of the digital broadcast signal are electrically connected to the output terminal
23
b
. Thus, the IF signal is to be inputted to both the detector
27
and the demodulator
28
.
Here, the detector
27
outputs the AGC voltage to be outputted therefrom as high as practicable when the level of the input IF signal is low, whereas when the level of the input IF signal is high, the detector
27
outputs the AGC voltage to be outputted therefrom as low as practicable. Further, the AGC voltage outputted from the detector
27
is to be inputted to the AGC voltage input terminal
21
b
of the tuner
21
through the AGC voltage selector switch
26
.
The AGC voltage selector switch
26
performs switching in interlock with the high-frequency switch
23
so that the AGC voltage outputted from the detector
24
or
27
is inputted to the gain variable amplifier
21
c
of the tuner
21
. The AGC voltage selector switch
26
has input terminals
26
a
and
26
b
and an output terminal
26
c
and electrically connects the output terminal
26
c
to either the input terminal
26
a
or
26
b
according to each received broadcast. When the tuner
21
receives the analog broadcast signal therein, the AGC voltage selector switch
26
electrically connects the output terminal
26
c
to the input terminal
26
a
. When the tuner
21
receives the digital broadcast signal therein, the AGC voltage selector switch
26
electrically connects the output terminal
26
c
to the switching terminal
26
b
. Since it is difficult to construct the AGC voltage selector switch
26
by a switch diode, for example, a relay or the like is used for the AGC voltage selector switch
26
.
Since the AGC voltage outputted from the detector
24
increases when the level of the analog broadcast signal is lowered upon receiving the analog broadcast signal, for example, the amplification of the gain variable amplifier
21
c
of the tuner
21
is made high and hence the level of the IF signal to be outputted is set so as to be kept constant, whereby suitable speech and video signals are obtained.
In the same manner as described above even when the digital broadcast signal is received, since the AGC voltage outputted from the detector
27
increases when the level of the digital broadcast signal is lowered, the amplification of the gain variable amplifier
21
c
of the tuner
21
is made high and hence the level of the IF signal to be outputted is set so as to be held constant, whereby suitable speech and video signals are obtained.
Since, however, the prior art is constructed so that both the analog broadcast signal and the digital broadcast signal can be received by one tuner, it is necessary to perform the switching between the high-frequency switch
23
and the AGC voltage selector switch
26
in interlock with each other according to each received signal.
Therefore, the high-frequency switch
23
can be easily constructed by means of the switch diode. Since, however, the AGC voltage selector switch
26
is switched over by using mechanical switch means such as the relay or the like, a problem arises in that, for example, a rise in the cost thereof occurs and its wide installation location is needed.
SUMMARY OF THE INVENTION
With the foregoing problems in view, it is accordingly an object of the present invention to allow switching between AGC voltages owing to a simpler configuration without using the mechanical switch means for the AGC voltage selector switch
26
and without any restriction on its installation location and at low cost.
According to one aspect of this invention, for achieving the above object, there is provided an automatic gain control circuit, comprising: a tuner capable of receiving broadcast signals of different systems and for amplifying an intermediate frequency signal according to an increase or decrease in automatic gain control voltage when the automatic gain control voltage is inputted thereto and outputting the same therefrom; a first detector for outputting a first control voltage increased as the level of the intermediate frequency signal increases, when the intermediate frequency signal is inputted thereto; a second detector for outputting a second control voltage maximal when the intermediate frequency signal is not inputted thereto and lowered as the level of the intermediate freque

Affiliated with

Also associated with

Rating

Automatic gain control circuit does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Automatic gain control circuit, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Automatic gain control circuit will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3072637