Vocoder Synthesizer www.haraldswerk.de

Motivation

Here is the synthesize part from my Vocoder project. I follow the same modular principle as in the other parts of the Vocoder project. Each VCA PCB contains 8 VCA channels. So you can start with 8 channels and advance to 16 or 24 channels. The VCA's are designed plain forward around the LM13700 as commonly used. The output voltage for each VCA is adjustable to compensate different gain from the filters. You can invert the input of every other VCA with a jumper. This can lead to better speech intelligibility. The filters are pluggable as in the analyze part. You have the opportunity to swap filters around and test different filter topologies. Each filter PCB contains two filters. Both are BP filters with two exceptions. The first filter in the Vocoder synthesize part must be a high pass filter and the last a low pass filter. I designed two extra filter PCB for this two. HP/BP filter and BP/LP filter.

Specs and features:

up to 24 filters
audio input 10Vpp
CV input 0..5V DC
audio output 10Vpp
variable number of filters
exchangeable plug in filters
+/- 15V PSU. Runs on +/- 12V as well (With minor changes in some part values)
Schematic VCA
Schematic input output
Schematic filters
Pictures

Implementation

Schematics VCA

Vocoder Synthesizer schematic VCA 1 — Vocoder Synthesizer schematic VCA p. 1

Vocoder Synthesizer schematic VCA 2 — Vocoder Synthesizer schematic VCA p. 2

Description VCA

The PCB contains 8 identical VCA's. The implementation is plain forward. The LM13700 is used with the linearization diodes to achieve a better signal to noise ratio. The OTA is linear controlled. The input of very other VCA can be changed to invert the output signal. This can lead to better speech intelligibility. On page one of the schematic you can see an input buffer and the output summing OpAmp for all 8 channels.

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Vocoder Synthesizer schematic input output — Vocoder Synthesizer schematic input - output

Description input output

The input - output schematic is used to attenuate the input signal to a level that the filters can handle. The signal is distributed to 3 the VCA PCB which gives a total of 24 channels. And put back together with the output OpAmp. I included a gross level detection to control the input level. The power distribution for the VCA PCB are done here as well.

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Vocoder Synthesizer schematic BP — Vocoder Synthesizer schematic band pass

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Vocoder Synthesizer schematic BP HP — Vocoder Synthesizer schematic band pass - high pass

Vocoder Synthesizer schematic BP LP — Vocoder Synthesizer schematic low pass - band pass

Description Filters

As said before the filters are pluggable. You can realize your own filters with your preferred topology. Here i implemented the filters which are used in the Jürgen Haible Living Vocoder. Each filter PCB contains two filters. Both are band pass filters with two exceptions. The first filter in the Vocoder synthesize part must be a high pass filter and the last a low pass filter. I designed two extra filter PCB for those two. High pass / band pass filter and band pass / low pass filter.

Vocoder Synthesizer BP Values — Vocoder Synthesizer values for band pass filters

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Calibration

Text t.b.d.

Special parts

You have to use 1% capacitors in the bandpass filters. Cheapest way to get them is selecting by hand. Buy a couple and measure. There is no need for absolute match. Relative match within the filter is sufficient. Keep in mind that you have to match the capacitor in the analyzer and the synthesizer part of the Vocoder per channel as well.

All resistors 1% metal film