1. System Microphone Keyboard Output

2. Cross Synthesis: Two Implementations

3. What are the implementations and what do they do? 2 command-line programs Implemented in C/C++ To use them you type: –cross -g.4 -h.3 –voCode -g.4 -h.3 They run off-line, not real time. BUT...

4. How do they do it? Cross uses LPC techniques VoCode uses OLA - Frequency domain convolution with a time varying filter

5. LPC Review Used to transmit low bandwidth voice Send filter coefficients in lieu of samples But how do you find the coefficients? ???

6. LPC: Some Math

7. H(z) Follows the Spectral Envelope (Show Transparencies!)

8. Finding the filter coefficients Matrix is Toeplitz. Use the Levinson-Durbin Algorithm

9. LPC Cross-Synthesis System Diagram

10. OLA Implementation STFT of microphone and instrument signal frames Multiply one by the other. Microphone Signal Acts as a time-varying filter. !

11. OLA Math

12. More OLA Math

13. OLA With Time a Varying Filter Filter taps are band-limited by the window –You can’t allow them to vary too fast. –It sounds very bad. My system Parameters: –Frame Size: 256 –Hop Size: 128 –Interpolated Filter (‘Microphone’) over 70 frames. Time resolution only 200 ms.

14. OLA Cross Synthesis System Diagram

15. Performance of My OLA Approach My LPC implementation performs better –Time resolution 15.8 ms compared to 200 ms –Several times faster than real time compared to much slower than real time Suggestions for improvement –Faster FFT implementation –Warp Spectrum to get better LF resolution with shorter filter. –Multirate implementation

16. High Frequency Problem Voice energy mostly below 3.5 kHz Wide instrument spectra lopped off. Solution: –LPC: Add cross-over to front end, pass HF. –OLA: Same, but implemented in Frequency Domain This doesn’t quite work

17. Solution to HF Problem