I have thought about the posibility of using the XM8 as a vocoder for a long time, but had decided to not go ahead with it due to the added complexity.
However, while designing the voice cards I realised that by just adding tiny modifications, I could at least leave the door open to adding a vocoder daughterboard later. Let me explain.
Vocoder architecture
A vocoder has some central components. First of all, it takes two inputs:
- one (usually a voice) that is analysed for frequency content by dividing the input into frequency bands using bandpass filters. The amplitude of each band is captured using an envelope follower.
- another one (usually an instrument sound rich in harmonics) which is divided into the same frequency bands as the first one, and where the amplitude is controlled by the amplitude of the first input.
The effect is that the second input "mimics" the first, making the instrument "talk".
In addition to the inputs, a noise source is used, and amplitude is sampled (analog sample and hold) at intervals set by a variable clock.
Here is the block schematics of the classic ETI Vocoder DIY kit:
It has three main parts - analysis and synthesis at the top, and sound generation at the bottom. The two oscillators are used in place of the second input.
How to implement this in the XM8
Now, we would need to implement the whole analysis, sample and hold and voiced/unvoiced detection on the daughterboard. The output from the board would be 16 CVs that can control VCAs.
The second input will be either
- External audio - if we want to use a different source for our "instrument"
- Output from the low pass filter on voice card 1 - that way we can use everything on the voice card except the state variable filter as a mono synth.
We would then feed the second input as External input to all voice cards. By sending the input directly to the SVF - this is already part of the architecture - we can use the SVF in place of the second block of band pass filters in the schema above. The cutoff frequency for each filter is fixed and can be calibrated (part of the existing design) digitally. As an added bonus, the cutoff frequency may be moved to change the voice pitch (or even invert or reorder the bands).
There are two additional important things in the ETI vocoder design:
- the first and last frequency bands are made using low pass and high pass filtering respectively. Fortunately, this is supported by the SVF.
- every second band is phase inverted. This is also supported by our architecture.
Changes needed to the voice cards
- Add a CV input going to the SVF.
- Add an output from the low pass filter (pre VCA), this will be used on the first voice card to use as the second input.
Changes needed to the main/input board
- Add a switch between external input and voice card 1 LPF output (this is a good idea anyway)
- Add a switch between this input and a third input used for vocoder noise source. This would be controlled by the voiced/unvoiced detector on the vocoder analysis board.
- CV outputs going to the vocoder board to control voice input volume etc.
- Chaining between synths - either we need a separate analysis board on synth two or we need 8 VCA inputs. We need a separate Ext audio out, tapped after ext audio mux, to send voice card 1/noise to second synth.
A four channel mux may be used, if we let the vocoder analysis board control A1 and input noise to input 3 and 4, the mainboard may control A0 to switch between external input and voice card 1 LPF. A1 would then effectively override the two inputs (and should have a pulldown resistor so that it is disabled if no vocoder daughterboard is fitted.
Usage: LPF filter VCA must be turned down on all voices, and Ext in turned up and connected to SVF for all. Voice card 1 may be used as input to the other sources.
As one XM8 probably ends up having only 8 voices, we need to chain two to get a full 16 band vocoder. Here is one way of doing that:
We need to have a single analysis board to be able to detect voiced/unvoiced properly. Bus chaining requires level matching between the synths, so ext bus input should probably have VCAs.
Things to test
I am unsure of the steepness of the bandpass in the ETI vocoder. The SVF has a 12dB bandpass.
Update: According to the Deliyannis bpf docs, one filter has 12dB cutoff, so two must have 24dB. That means that we may have to run the audio through both the LPF and the SVF (as HPF).
Deliyannis band pass filter
While reading about the human voice filter bank in the VP-330 (on the Oakleysound site) i realised that the ETI bandpass filter is called a Deliyannis bandpass filter or resonator, a calculator can be found here:
https://www.changpuak.ch/electronics/Deliyannis_Bandpass.php
http://earmark.net/gesr/opamp/bpf.htm
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