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“Alien Voices"

“Alien Voices". Texas Instruments University Programme Teaching Materials. Alien Voices. Objectives. To generate “alien voices” using ring modulation. To implement ring modulation in C code.

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“Alien Voices"

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  1. “Alien Voices" Texas Instruments University Programme Teaching Materials

  2. Alien Voices

  3. Objectives • To generate “alien voices” using ring modulation. • To implement ring modulation in C code. • To speak into the microphone connected to the Texas Instruments TMS320C5505 USB Stick and hear the results on headphones/computer loudspeakers.

  4. Introduction • In many science fiction films and TV series, when an alien speaks it is given a very strange voice. • This is normally achieved using what is known as “ring modulation”. • This is one of the simplest forms of DSP processing and is fun to experiment with. • Click on the icon below to hear the effect.

  5. Ring Modulation Block Diagram • Multiply audio input by a sine wave.

  6. Waveforms of Ring Modulation

  7. Cosine Identities • The following identities exist to add or subtract cosines:

  8. Equation for Ring Modulation • To implement ring modulation, multiply together two sinusoidal waveforms cosA and cosB.

  9. Ring Modulation Frequencies

  10. Sum and Difference Frequencies • The output of ring modulation consists of: • Sum of two frequencies: • ½cos(A+B). • Difference between the two frequencies: • ½cos(A-B). • Note: there is no cosA or cosB term at the output.

  11. Ring Modulation 50Hz and 500 Hz • The output contains 450 Hz and 550 Hz.

  12. Applications of Modulation • Modulation is used in radio transmissions. • In the case of radio, the modulating frequency can be in kHz to GHz. • Ring modulation is similar to AM radio. • In radio transmissions, the sum and difference frequencies are referred to as sidebands.

  13. C Code Implementation

  14. C Code Implementation • To generate the sine waves, the C code uses the sine function in DSPLIB • The multiplication of the audio input by the sine wave is performed in the function ring_modulation().

  15. Introduction to Laboratory

  16. Practical Alien Voices • In the Laboratory you will build a C code implementation of alien voices • When you speak into the microphone you hear different ring modulation effects from the TMS320C5505 USB Stick.

  17. USB Stick Setup TMS320C5505 USB to PC Microphone Headphones

  18. Installing the Application • Use the code given in Application 8 Alien Voices • Follow the steps previously given in Chapter 1 to set up the new project.

  19. Create New CCS Project

  20. Files Used

  21. Console

  22. Experiments

  23. Change Modulation Frequencies • Change the frequency for each channel: 200Hz 500Hz

  24. Improving the Effect • To make the speech more intelligible, change amount of un-modulated input. Try mono_input/2

  25. Programming Challenge • The alien voices effect can be further modified using reverberation • Add some reverberation by copying the files reverberation.c and reverberation.h from the Application 3 Echo and Reverberation • Add the following to main.c: • #include “reverberation.h” • right_output = reverberation (left_output + mono_input/4);

  26. Questions • What is ring modulation? • Why does ring modulation produce sum and difference frequencies? • What happens if you apply ring modulation to music?

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