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VTL5C1 Datasheet(PDF) 66 Page - PerkinElmer Optoelectronics |
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VTL5C1 Datasheet(HTML) 66 Page - PerkinElmer Optoelectronics |
66 / 76 page ![]() 61 Application Notes—Analog Optical Isolators Gating and Muting 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. 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 R10 has maximum 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 A1, resistors R2 and R3 and capacitor C1. The amplified signal is full-wave rectified by diodes D1 and D2 together with op-amp A2 which inverts the negative half of the signal. The rectifier charges C2 used for RELEASE TIME control and drives the base of transistor Q1, the LED driver. The threshold voltage is a sum of the forward drop of the rectifying diodes, the voltage drop across R6, VBE or Q1 and VF 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. Figure 3. Audio Sound Gate |
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