digitalVolume

Description: This circuit is designed to digitally adjust the volume of an analog audio signal. It employs a single AVR microcontroller to generate a pulse width modulated (PWM) signal for control. The innovation of the circuit lies in the utilization of a transconductance amplifier (OTA). Although these amplifiers are readily available, they are infrequently used in homemade circuits, potentially due to a lack of awareness. The microcontroller's primary function is to produce a PWM signal with the desired duty cycle, which can be substituted with any circuit capable of generating PWM signals. With appropriate modifications to the provided example code, it is possible to adjust the PWM signal's duty cycle wirelessly, such as using an infrared remote control similar to those used for television operation. It is important to note that the 100nF capacitor located to the right of the AVR microcontroller should be soldered as close to the microcontroller's power supply pins (pins 20 and 10) as possible.

The circuit is based on a transconductance amplifier, with the input signal limited to a maximum of 5 volts peak-to-peak. To utilize the linear range of the input-output characteristic of the CA3080, the signal is attenuated using a voltage divider. A reference DC voltage of Vcc/2 (2.5 volts) is added as a bias voltage. Consequently, the input signal at pin 2 of the CA3080 oscillates around 2.5 volts, with a peak-to-peak variation of 100 mV (ranging from 2.450 to 2.550 volts). The transconductance (gm) of the amplifier can be adjusted through an external bias current (IB), according to the equation gm = IB/2Vt, where Vt represents the thermal voltage (approximately 25 mV at room temperature).

The objective is to control the output sound volume, necessitating the conversion of the output current from the CA3080 to a voltage. This can be accomplished using a 20kΩ output resistor (Vo = Io Rout). In this analysis, V2 is set to 0, and V1 equals Vin, leading to an equation that reflects a phase difference of 180 degrees. This phase shift is inconsequential, as human hearing integrates sound signals and is not sensitive to phase differences.

Incorporating the transconductance equation reveals that the output voltage is proportional to both the input voltage and the bias current. A voltage-to-current converter can be added to the circuit to make the output voltage proportional to the bias voltage instead, which is achieved using the LM358 operational amplifier. In this configuration, the bias current is directly proportional to the input voltage of the converter (IB = Vcon/R). Thus, the output voltage Vo becomes proportional to the input signal Vin, modulated by an external constant voltage Vcon.

By varying Vcon (from 0 to 5V), the bias current and gain of the circuit can be adjusted (gain = Vcon Rout/2Rvt). This adjustment directly influences the output sound signal's volume. A potentiometer can be employed to alter the constant bias voltage Vcon. For digital volume control, a digital-to-analog converter (DAC) or PWM can be utilized, with PWM being simpler to implement.

In PWM, the duty cycle of the generated pulses is controlled by the microcontroller. The duty cycle can be calculated using the equation DC(%) = tH/T * 100%, where tH is the duration of the high voltage pulse (5 volts) and T is the total period (T = tH + tL, with tL being the low voltage pulse duration). By employing a low-pass filter, higher frequencies can be suppressed, allowing only the DC component to pass through. The resulting Vcon can be expressed as Vcon = DC(%) Vcc/100. For a 50% duty cycle, Vcon equals 2.5 volts; at 100% duty cycle, Vcon is equal to Vcc (5 volts), and at 0% duty cycle, Vcon is 0 volts.

The final equations governing the circuit's operation ensure that the volume adjustment is both effective and responsive to the input control signal.This circuit is designed to digitally increase/decrease the volume of an analog audio signal. It uses a single AVR microcontroller to produce the controlling pulse width modulated (PWM) signal. However, the circuit`s innovation is the use of the transconductance amplifier (OTA). Although you can find these amplifiers easily, they are rarely used i n home made circuits. Perhaps because most people ignore their existence. The only thing the microcontroller does, is producing a PWM signal with the desired duty cycle. You can replace it with another circuit capable of producing PWM signals. If you are creative enough, you can modify the provided example code, so as to be possible to change the duty cycle, of the PWM signal, wirelessly (for example with an IR remote control, like the ones you use to control the TV). Attention: The 100nF capacitor at the right of the AVR microcontroller must be soldered as close to the power supply pins of the microcontroller as possible (meaning close to the pins 20 and 10).

The circuit is based on a transconductance amplifier. The input signal must have a maximum of 5Volts peak to peak voltage. In order to take advantage of the linear portion of the input - output characteristic of the CA3080, we suppress the signal with a voltage divider. We also add a reference dc at Vcc/2 (=2. 5 Volts. Bias voltage). After that, the signal at the input of the CA3080 (pin 2), varies around 2. 5Volts with a maximum of 100mV peak to peak voltage (meaning from 2. 450 to 2. 550 Volts). gm is the amplifier`s transconductance and it can be adjusted by an external bias current IB, according to the equation gm = IB/2Vt, for an area of 4 - 5 decades.

Vt is the thermal voltage Vt = KT/q. At room temperature equals to 25 mV. Our goal is change the sound volume. So we must convert the output current of CA3080 to voltage. We can do that by using the 20K output resistor (Vo = Io Rout). Notice that V2 = 0 and V1 = Vin (ac analysis. We ground the bias voltage). The equation now becomes: The minus sing shows a phase difference of 180o. We don`t have to change the phase because our ears will never hear the difference (our ears are simply integrating the sound signal, incapable of hearing phase differences). By adding the transconductanceequation we now have: As we can see, output the voltage is proportional to the input voltage and to the bias current.

By adding a voltage to current converter to the circuit, we can make the output voltage proportional to a the bias voltage instead. This is what LM358 does. The bias current is now proportional to the voltage at the input of the converter (IB = Vcon/R). So: Now we have exactly what we wanted on the first place. An output voltage Vo, proportional to the input signal Vin, controlled by an external constant voltage Vcon.

If we change Vcon (from 0 to 5V), we change the bias current and the gain of the equation (gain = Vcon Rout/2Rvt). When we change the gain, we change the volume of the output sound signal. We can change the constant bias voltage Vcon by using a potentiometer. In our case we want to digitally change the sound volume. So we can use a DAC (digital to analog converter) like the one in the article R/2R ladder DAC, or PWM (Pulse Width Modulation).

PWM is simpler to implement. With pulse width modulation (PWM) we control the duty cycle of the pulses. The pulses are generated by the microcontroller. Duty cycle is calculated according to equation DC(%) = tH/T 100%, tH is the time where the pulse is at high voltage (5 volts) and T is the period (T = tH + tL, with tL the time where the pulse is at low voltage = 0 Volt). If we use a low pass filter, we can suppress the higher frequencies and keep only the dc. Vcon will then equal to: Vcon = DC(%) Vcc/100. When duty cycle is 50% then Vcon will be 2. 5 Volts. With 100% duty cycle, Vcon equals to Vcc (=5Volt) and with 0%, Vcon = 0Volt. The final equation is: In order to