Application Notes—Analog Optical Isolators
A which inverts the negative half of the signal. The rectifier charges
Gating and Muting
2
C used for RELEASE TIME control and drives the base of transistor
2
Background noise becomes very objectionable when a signal level in a
program is low. Noise is any unwanted sound and may be due to tape
hiss or amplifier hum. These noises can be eliminated by selective use
of gating and muting, that is, turning the amplifier on when the signal
level is high and off when it is low. This technique can also remove or
reduce unwanted echo, print through, presence or any other distracting
signal during portions of a program which are normally silent. The
gating circuit must be completely transparent to the listener, having a
smooth, rapid operation with no signal distortion.
Q , the LED driver. The threshold voltage is a sum of the forward drop
1
of the rectifying diodes, the voltage drop across R , V or Q and V
F
6
BE
1
of the LED. This voltage is 2.5 – 3.0V and when referred to the input
gives a threshold of 2.5 – 3.0 mV at the amplifier.
The circuit can be set up for a specified threshold voltage. Release
time is usually determined empirically. A typical set up procedure uses
an audio signal containing spoken dialog. Initially, the THRESHOLD
adjustment is set to the maximum and the RELEASE is set to the
minimum. The program is turned on and the THRESHOLD is
decreased until the audio starts coming through, but sounds chopped
up. The chopping occurs because the circuit is too fast on release. The
RELEASE is increased until the audio is smoothed out and sounds
normal. Setting of the two controls needs to be made carefully. A
threshold set too high cuts off the quieter sounds, while a setting which
is too low allows more of the noise to come through. Short release time
causes more chopping of the audio and can create some distortion at
the lower frequencies. Long release time keeps the gate open too long
allowing noise to come through after the signal is gone. Adjustments
should be made incrementally and worked between the two controls
until the best sound is achieved.
A practical gating circuit having these features is shown in Figure 3.
The circuit has five basic sections: the threshold adjustment, a high AC
gain stage, full-wave rectifier, LED driver and an electrically controlled
voltage divider. When the signal is below the threshold level, the
voltage divider consisting of the AOI and R has maximum
10
attenuation. When the signal exceeds the threshold, the voltage divider
allows the signal to pass through.
The circuit operation is as follows. The THRESHOLD potentiometer
applies a portion of the signal to the high gain AC amplifier consisting
of op-amp A , resistors R and R and capacitor C . The amplified
1
2
3
1
signal is full-wave rectified by diodes D and D together with op-amp
1
2
Figure 3. Audio Sound Gate
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PERKINELMER [ PERKINELMER OPTOELECTRONICS ]
