Category Archives: Vocoder

Limiter / Compresssor

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Limiter / Compressor front view

Limiter / Compressor front view

To handle the great dynamic range of the Shakuhachi I needed a compressor for my Shakuhachi 2 Synth project. Because a limiter is not that different I added this feature as well. This comes in handy with my Vocoder project also. The structure used here is derived from “Small Signal Audio Design” by Douglas Self p682ff. The audio signal did not flow through a VCA as in many other implementations. Instead the compression or limitation is done by subtracting the audio signal at the output summing node according to the control voltage derived from the audio signal.

Specs and features
• Switch compress or limit
• Switch Compression/Limit rate 50% or 90%
• Compression/Limit rate adjustable 0–max
• Runs on +/-15V and +/-12V (with minor resistor changes)
• Power consumption below 15mA each rail

The documentation for download can be found in my website.

Limiter / Compressor schematic 01

Limiter / Compressor schematic 01

The audio signal flows unaffected through IC1A/B. When the compressor – limiter kicks in the inverted signal is added (=subtracted) at the summing node of IC1A. The signal level to subtract is regulated through a Sims VCA. The CV generation for the VCA is pretty standard. Linear for the compressor and exponential for the limiter.

Limiter / Compressor schematic 02

Limiter / Compressor schematic 02

Precision full wave rectifier with filter to generate the control voltage for the VCA from the audio signal.

Limiter / Compressor populated PCB

Limiter / Compressor populated PCB

Limiter / Compressor back view

Limiter / Compressor back view

NGF Vocoder Project: Voiced – unvoiced detection

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NGF Vocoder : Voiced - unvoiced detection PCB

NGF Vocoder : Voiced – unvoiced detection PCB

Here is the voiced – unvoiced detection module for my Vocoder project. It is made off a high pass and a low pass filter. The outputs of the filters are compared. The comparator switches the output signal between carrier and noise.

The documentation for download can be found in my website.

NGF Vocoder project: Voiced - unvoiced detection schematic 01

NGF Vocoder project: Voiced – unvoiced detection schematic 01

The speech input signal derived from the speech input module initially reaches the buffer IC1A and is then split into two signals, each passing a filter. The high pass filter is constructed around IC3C and IC3D and the low pass around IC3A and IC3B. Their peak values are at 2500Hz and 600Hz respectively. The high pass filter is a Sallen Key filter, highly damped, gain around 5.6 dB. The low pass filter is constructed accordingly. Both filter sections have a slope of 24dB per octave to obtain a good separation. They are each followed by a precision full wave rectifier (IC4C, IC4D and IC4A, IC4B) and by a 12dB per octave smoothing filter. The rectified and calibrated filter output signals are now feed to a comparator IC1B. The voiced – unvoiced decision is taken by the comparator according to the signal levels. The comparator output steers the switch IC5 which selects if the carrier or the noise signal is send to the output.

NGF Vocoder project: Voiced - unvoiced detection schematic 02

NGF Vocoder project: Voiced – unvoiced detection schematic 02

The carrier and the noise input are both fed to OTA’s. The carrier input is buffered with IC9A, the noise input with IC9B. The noise input is switchable between internal and external source. The OTA’s are turned on or off according to the signal from switch IC5 (schematic page 01).

NGF Vocoder Project: Pink and white noise source

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NGF Vocoder: Pink and white noise source PCB

NGF Vocoder: Pink and white noise source PCB

Here is the noise module for my Vocoder project. This noise generator is not only useful for the vocoder but also for various other audio and acoustic measurements. The output can be switched between white and pink noise. The pseudo random noise is produced with the aid of a 31 bit shift register. Zero inhibit technique is used. This design follows closely the original from the Elektor magazine.

The documentation for download can be found in my website.

NGF Vocoder: Pink and white noise source schematic

NGF Vocoder: Pink and white noise source schematic

IC1A and IC1B together form the clock generator. Diodes D1..D31 combined with IC1D provide the zero inhibit. As soon as the “000…0” state occurs, a “1” is entered in the shift register by way of IC2A. Gate IC2B makes sure outputs 28 and 31 of the shift register are EXOR back coupled. IC1C is the buffer before the filter which can be switched to pink and white noise. The white noise filter is a low pass filter of about 23kHz with 6dB slope. The pink noise has to be slightly more amplified than the white. TR1 is used to equalise the output voltage for pink and white noise.

NGF Vocoder Project: Voiced/Unvoiced detection

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For those who are waiting. I decided to build the voiced/unvoiced detection with separate filters and not to use the control voltage outputs from the NGF Vocoder analyzer. This decision was made because I wanted to stay modular. So the voiced/unvoiced detection can be used for other patches as well. The filters are tested working. The switching circuitry is not yet tested. The PCB’s are ordered. The noise source should be ready within two ore three weeks. Until then here are the draft schematics for the voiced/unvoiced detection:

NGF Vocoder Project: Voiced/unvoiced detection. Schematic draft

NGF Vocoder Project: Voiced/unvoiced detection. Schematic draft

NGF Vocoder Project: Voiced/Unvoiced detection. Scheamtic draft

NGF Vocoder Project: Voiced/Unvoiced detection. Scheamtic draft

NGF Vocoder Project: Update

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The dedicated NGF Vocoder Project website is now online. It can be found here. The NGF Vocoder Analyzer Site is updated. It can be found here with updated schematics and downloads for the documentation.

NGF Vocoder

NGF Vocoder

Vocoder Analyzer: level of individual filters and summarized output

Vocoder Analyzer: level of individual filters and summarized output

Vocoder: PSU

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Vocoder PSU Front view

Vocoder PSU Front view

For the Vocoder PSU I didn’t make a new PSU. I just took what I have at hand. So this is a very short post. I just took three of my  ultra low ripple PSU with LM350 and put them in a frame. Two are daisy chained for the +/- 15V. The third is for the 6V which is needed for the display unit.

Vocoder PSU rear view

Vocoder PSU rear view

Vocoder PSU close up

Vocoder PSU close up

Vocoder: Speech Input

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Vocoder speech input faceplate

Vocoder speech input faceplate

Here is the speech input module for my Vocoder project. It consists of a microphone preamp, a line input, tone control, two level indicators and a limiter. The amplification of the microphone preamp is adjustable with one resistor (R8), to fit your needs. A level indicator is added to prevent overdrive and achieve a good SNR for your input signal. The line input and the microphone signal are mixed together with OpAmp IC6A. The amplification of the following OpAmp IC6B is determined by R39 and the variable resistance of the VTL5C3. The limiter stage is optional, you can leave it out with no problems. If used, adjust the values to your needs.

Vocoder speech input schematic microphone preamp

Vocoder speech input: schematic microphone preamp

This stages are followed by the tone control. You can adjust bass, middle and treble over a wide range. The output buffer drives the signal for the analyzer module. The signal level here is indicated by a level indicator.

Vocoder speech input: line in, limiter and tone control

Vocoder speech input: line in, limiter and tone control

Vocoder speech input: PCB stuffed

Vocoder speech input: PCB stuffed

Vocoder speech input module inside

Vocoder speech input: module inside

Vocoder: Synthesizer Filters

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Vocoder: Synthesizer band pass filter.

Vocoder: Synthesizer band pass filter.

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 this two. High pass / band pass filter and band pass / low pass filter. I updated my website.

Vocoder: Synthesizer schematic band pass filter.

Vocoder: Synthesizer schematic band pass filter.

Vocoder: Synthesizer schematic low pass / band pass

Vocoder: Synthesizer schematic low pass / band pass

Vocoder: Synthesizer schematic high pass / band pass

Vocoder: Synthesizer schematic high pass / band pass

Vocoder: Synthesizer band pass component values

Vocoder: Synthesizer band pass component values

Vocoder: Synthesizer filters PCB all stuffed

Vocoder: Synthesizer filters PCB all stuffed

Vocoder: Synthesizer VCA’s

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Vocoder: Syntesize Part VCA

Vocoder: 24Channels Synthesize Part VCA

Here are the VCA’s of the synthesize part from my Vocoder project. I followed the same modular principle as in the other parts of the Vocoder. 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. The filters are pluggable as in the analyze part. You have the opportunity to swap filters around and test different filter topologies. The schematic given here contains only two channels. The complete documentation will show up on my website when everything is finished. Next to come: The filters for the synthesize part.

Vocoder: Synthesize Part PCB 8 Channels stuffed.

Vocoder: Synthesize Part PCB 8 Channels stuffed.

Schematic shows only two channels. Complete documentation on my website asap.

Vocoder: Synthesize Part Schematic VCA

Vocoder: Synthesize Part Schematic VCA