Motivation

Vocoder Synthesizer Back

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:

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


Vocoder Synthesizer schematic VCA 2

Vocoder Synthesizer schematic VCA p. 3


Vocoder Synthesizer schematic VCA 2

Vocoder Synthesizer schematic VCA p. 4

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 J├╝rgen 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

Download

Vocoder Synthesizer backplane documentation download

Vocoder Synthesizer band pass filter documentation download

Vocoder Synthesizer band pass - highpass filter documentation download

Vocoder Synthesizer band pass - lowpass filter documentation download

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Vocoder Synthesizer back view

Vocoder Synthesizer 24 Channels - inside


Vocoder Synthesizer VCA PCB stuffed

Vocoder Synthesizer VCA PCB stuffed


Vocoder Synthesizer BP PCB stuffed

Vocoder Synthesizer band pass PCB stuffed


Vocoder Synthesizer all filters

Vocoder Synthesizer 24 Channels - all filters


Vocoder Synthesizer topview

Vocoder Synthesizer 24 Channels - inside - top view


2017 February update

2016 July

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