Gain control is an important part of audio but needs special treatment when you introduce reconfigurability.
Automatic gain control (AGC) circuits turn up in many applications and there are a variety of ways to implement them. But a drawback with many designs is that they cannot be reconfigured. Reconfigurability is important in audio and active noise control (ANC) applications, because such systems can be installed into various environments. A super-market and an aircraft cabin have different kinds of acoustic environments. In acoustic noise-control applications the acoustic environment defines signal amplification and also other parameters of the noise controller.
One method for implementing AGC circuits is to use compression. By compression it is possible to modify the dynamic range of the signal, for example to limit the amplitude of the signal at a specified level. Such an operation is needed, when the dynamic range of a signal could exceed the dynamic range of an amplifier stage, filter or any other electronic device. This is important, because if the dynamic range of the electronic device is exceeded, non-linear distortion will be generated and loss of information will occur.
An AGC circuit can be implemented by making the compression ratio depend on the input signal amplitude. Depending on configuration, an AGC circuit can, for example, amplify weak signals or limit the signal level at a specified level.
From an electronics designer's point of view an AGC circuit is basically an amplifier with variable gain. In addition, the gain of the amplifier depends in a certain way on the input signal level. In the simplest case, the gain is kept constant when the signal level is below a threshold level. When the signal level exceeds the threshold level, the gain is adjusted in such a way that the signal level is constant. This can be accomplished by making the gain inversely proportional to the input signal level. This kind of a simple control algorithm produces a very sharp corner between linear region and compression region. If softer transition is needed, a control algorithm according to the equation in Fig 1 can be used, producing the lower line.
A reconfigurable AGC circuit can also be used to implement other kinds of signal level adjustment algorithms. Fig 2 presents the operation of one such case. At low signal levels the gain is constant. When the external control signal level exceeds the lower rotation point, the gain is adjusted in such a way that the output voltage increases as the external control signal increases. To prevent saturation and clipping, the gain is kept constant when the external control signal exceeds higher rotation point. This kind of a system can be used to adjust signal level as a function of background noise for example in cars and supermarkets.
Because the adaptive amplifier must be reconfigurable, off-the-shelf integrated circuits are not suitable. Instead, the control algorithm was implemented by software running on an Atmel 8bit microcontroller. With the adaptive amplifier the signal level can be adjusted in real time as needed in various applications. The device is designed to be connected into a mother device, such as an ANC system. This kind of device can be used in conjugation with an active noise control system with fixed filter to prevent saturation of the system.
Fig 3 shows a block diagram of the device. The signal path runs from INPUT2 to OUTPUT through two buffer amplifiers and a digital potentiometer. The signal level detector generates a control voltage Vcontrol, which is proportional to peak value of its input signal. The microcontroller reads the control voltage and controls the digital potentiometer.
To minimise phase shift and to avoid stability problems, the signal path is DC-coupled. The digital potentiometer is controlled by the microcontroller using the SPI bus. The signal level detector measures the peak value of its input signal and generates a slowly varying voltage Vcontrol, which is proportional to the peak value of its input signal. The signal level detector consists of an active rectifier and a peak level detector. Either the input signal or an external control signal can be connected to the level detector. The microcontroller converts the control voltage into digital form and calculates appropriate control words for the digital potentiometer.
The signal-level control algorithm runs in a microcontroller. The program code reads the value of analogue-to-digital conversion, and based on it, calculates the control word to be sent to the digital potentiometer. Two different control algorithm were implemented, and there is a dedicated operation mode for each. First, the ANC mode prevents clipping and distortion of signal by limiting signal amplitude. The ANC mode is designed to be used in ANC systems. Secondly, the VOL mode increases amplification as the external control signal increases. The VOL mode can be used for example to adjust signal volume as a function of background noise. It is possible to control the threshold levels of the control algorithm and also the release times of the signal level detectors by adjusting software parameters.
As well as using more formal tests, the device was also tested in practice by listening to FM radio broadcast with the adaptive amplifier in the signal path. The adaptive amplifier was controlling the signal level as a function of background noise. The output signal level increased as background noise level increased, as expected. The operation of the device was stable, and it didn't produce any additional sounds. No signs of instability were observed during the tests. In general, the operation of the device was considered unobtrusive.
The developed device is planned to be used in an ANC system comprising electrostatic audio transducers and feedback control electronics produced by Panphonics. The exact impulse response and high directivity of the transducers are beneficial in ANC usage. Their flat shape enables attaching two transducer panels together as an actuator and a sensor to form a less than 1cm-thick active sound-insulation layer. Without adaptive gain the feedback loop would make the system saturate at high level noise causing distortion and other unwanted noise products. The adaptation needs to have minimal reaction time to prevent erroneous function also when impulsive noise is present.
The developed prototype is highly flexible as it can be configured by software for various applications. Further ideas include software improvement, extending the operation range of the signal level detector, and integrating the active rectifier into the microcontroller.