Increasing the sensitivityįor simplicity the above program takes the top 8 bits of the 10-bit digital sample and ignores the bottom 2 bits. If you want to feed the output to an audio amplifier you should include a low-pass filter, consisting of a 1kΩ resistor and a 0.1♟ capacitor otherwise you risk overloading the amplifier:Ĭonnecting the Audio Pitch Shifter to an audio amplifier. In the above circuit the loudspeaker acts as a low-pass filter, so it's not affected by the 250kHz PWM carrier frequency. This Audio Pitch Shifter could be used as a voice-changer, or as a special effect in a synthesiser or audio processor. It's not much more complicated but gives a noticeable improvement in sound quality. The ADC is set up as follows: ADMUX = 2>1 The first part of the audio processing consists of sampling the audio input, using a pair of differential inputs with a gain of x20. The pitch shift happens in real time, without affecting the tempo of the music, so it's quite different from simply speeding up or slowing down playback of a sample. For example, reading at a 36kHz sample rate will shift the audio up an octave. However, by increasing the ReadPtr sample rate we can shift the pitch up, and by decreasing it we can shift the pitch down. If these samples were read at the same rate as the WritePtr samples the two audio streams would be identical. The next step is to read samples from the buffer using a second pointer, ReadPtr. Because we are sampling at approximately 18kHz the buffer represents about 14msec of audio. This makes it easy to implement a circular buffer we simply truncate WritePtr to a byte after incrementing it. In this application the buffer is 256 bytes long, which fits within the memory of the ATtiny85. How the Audio Pitch Shifter buffer works. When WritePtr reaches the end of the buffer it goes back to the first element, so the buffer is circular: Each sample is written to the buffer using a pointer WritePtr, which is then incremented to the next element in the buffer. The audio pitch shifter works by continuously sampling the input audio, digitizing it to 8-bit samples, and writing the samples to a buffer. The Audio Pitch Shifter circuit, based on an ATtiny85. Two pushbuttons cause interrupts which change the pitch shift up or down. The chip first amplifies the input by a factor of x20, digitizes it, stores the samples in a circular buffer under interrupt, reads samples back from the circular buffer under a separate timer interrupt, and finally converts the processed digital signal back to analogue, using high-speed PWM, and outputs it to a loudspeaker. You can increase or decrease the pitch shift using two pushbuttons:Īudio Pitch Shifter, based on an ATtiny85, allows you to increase or decrease the pitch of an audio input.įor example, you can plug in an mp3 music player or phone, play some music, and change the pitch of a male singer's voice to a female voice as it's playing.Īlthough the result is not hi-fi quality, it's an excellent demonstration of how much you can achieve with a single ATtiny85. But, with due diligence, Cecilia will bring smiles and good cheer to the most jaded musical mind.This article describes a simple device based on an ATtiny85 that takes an audio input, shifts its pitch up or down in real time, and plays it through a loudspeaker. Thus you cant complain if it hurts your feelings. It proposes dozens of complex and troubling sound-processing modules that invite the composer to revisit the notion of what constitutes spirited sonic matter.īeyond these highly desirable qualities, Cecilia is also known to cause mild skin rashes and turn small dogs into pickles.Ĭecilia is free. Cecilia is a complete sound production environment for the adventurous audio designer. It has seen a number of iterations since then, because, well, nothing quite does what it does as well as it does. Cecilia was first launched in 1996 to take advantage of the then emerging real-time audio capabilities of Csound. The legendary front-end and sound production platform for the Csound language is back in an fresh new set of duds. I find Cecilia great for accidentally creating amazing sounds.
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