Saw this comment by [Steven Gann] on https://hackaday.com/2016/12/09/diodes-the-switch-you-never-knew-you-had/
>The original Dalek voice effect was created using a ring modulator and a tape recording of a 60Hz sine wave as the carrier. Lower frequencies can give you a Cyberman, and higher frequencies can give you the newer Daleks.
In my updated version, DDS is used to generate the sine wave carrier frequency which is for modulate the audio. The firmware multiplies the input audio waveform with the carrier to produce Amplitude Modulation (AM) for the new voice.
I'll be using PWM as a DAC for audio output. The (PWM) timer overflow interrupt service routine is used for updating the phase accumulator for the DDS, waveform multiplication and PWM output. A low pass filter is used to filter out the PWM switching frequency to protect the speaker.
I'll try to use more accessible parts that you might already have rather than my usual preferred parts. The requirements are low enough for the old ATMega8 and probably even the ATTiny part with some modifications.
Normally, I do not recommend the lousy LM358 for audio use and low voltage designs. It this design, I have accounted for the drawbacks.
- the limited output range of Vcc-1.5V for 5V still covers for [0V, 2.56V] range I am using.
- Cross-over distortions - not driving significant loads. Distortions is not an issue for this application.
- It is very cheap and easily available.
I am not the only one that thinks LM358/LM324 are bad: