| 6 octave CV-module for analog synthesisers | |||
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MIDI 2 CV 1kHz |
Block Diagram  This design shows one analog path of the circuit: The digital PDM output of the FPGA is filtered with a low pass filter to obtain the analog voltage. The PDM-signal runs at full FPGA-speed of 40MHz and creates 20MHz-like pulses. This way 20.000 pulses of resolution are available for the focussed 1kHz signal bandwidth. After filtering this shows up with an accuracy of 45dB ... 50dB THDN for a typical signal change of 500 Hz in maximum. A configurable amplifier with an OPAMP amplifies by ratio 3 taking an offset into acount. With this setting an analog output voltage in between 0,15 and 6,15 V can be achieved. This makes ist possible to tune and also calibrate the synthesizer in software. 4 channels can be realised with one single OPAMP of type LM324. Frequency Table The subsequent table shows the required voltages to create the music frequencies. Here more than 6 octaves are covered.  Here linear relationship is used only. For a logarithmic control option a larger scaling of the opamp amplification had to be implemented. Standard CV-commands as triggers can be applied any point of time. The circuit has full DC-capability. A Spartan 3E had been used to integrate the functions for 16 outputs. Data Entry has been done in plain VHDL. Read about the former Version. |
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| 48 kHzbr>Audio DSP | |||
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© Jürgen Schuhmacher 2007