This paper describes a method of time constant change by gain control and its application for equalizers and automatic level controllers (ALC) for AC biased audio recording circuit.
Bilinear characteristics are used for these equalizers. The proposed method utilizes the fact that the pole is controlled by feedback gain and the null by feedforward gain. In ALC, the gain control per operating period is changed in attack and recovery condition, so that a different time constant can be obtained.
In the case of a bipolar IC for VTR, 10 time constants were used as the external components. However, the experimental results verify a significant reduction of these time constants to 2. That is, it was confirmed that this method has an equivalent performance to the above example: bandwidth is 10.5 kHz for SP (Standard Play Mode) and 5 kHz for EP (Extended Play Mode) with the recovery time being 1.8 s. Moreover, the attack time could be shortened to 1/20, namely 0.3 ms.
The proposed method is suitable for a system LSIs with a BiCMOS process.
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