The OHMCODER is a 16 channel fully analog vocoder, housed in a 3U 19"
enclosure with a full patch bay and feed-through functionality.
This project is inspired by this instructables project by user planetone.
In vocoders, a program and carrier signal are filtered into a number of frequency bands. The envelope of the program signal bands then modulates the carrier signal bands. Finally, the per-band contributions are summed back together to get the vocoded output.
The core of the design thus made up of
- Jack and XLR Input
- Patch Bay
- Per-Band Gain Control with LED Indicators
- Program or Carrier Band Feed-Through and/or Mixing
- Stereo and Mono Jack Output
We used the same second order multiple-feedback infinite-gain active bandpass filters as in the instructibles project. TL074 quad opamps are used.
Notice, that the nice thing about these filters is, that we only need to change the capacitor values to get different center frequencies, as seen in the SPICE simulation below.
| Band | f_low [Hz] | f_high [Hz] | C [nF] |
|---|---|---|---|
| 01 | 65 | 105 | 0.91 |
| 02 | 90 | 150 | 1.3 |
| 03 | 125 | 200 | 1.8 |
| 04 | 175 | 285 | 2.4 |
| 05 | 255 | 400 | 3.3 |
| 06 | 345 | 555 | 4.7 |
| 07 | 465 | 755 | 6.8 |
| 08 | 635 | 1050 | 9.1 |
| 09 | 920 | 1455 | 13 |
| 10 | 1220 | 1960 | 18 |
| 11 | 1765 | 2880 | 24 |
| 12 | 2500 | 4075 | 33 |
| 13 | 3515 | 5605 | 47 |
| 14 | 4640 | 7550 | 68 |
| 15 | 6375 | 10375 | 91 |
| 16 | 9200 | 15000 | 130 |
This is where we depart from planetone's design. We use a low-pass filtered precision rectifier signal to follow the program envelope.
The majority of the spectral energy of speech amplitude modulation lies below about 15Hz, i. e. the mean time between syllables is about 1/15Hz (see speech_modulation.ipynb for an example). The filter is chosen such that we follow this amplitude modulation.
We use VCAs based on the LM13700 quad operational transconductance amplifier, making use of its linearizing diodes and the integrated buffer circuits.
The program envelope controls the amplifier gain via a voltage to current converter circuit.
