XM8 as a vocoder

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

Vocoder analysis/synthesis boards arrived

I picked up the vocoder analysis/synthesis boards yesterday. They look good as always, though I noticed some differences. Nothing to be alarmed about but I get why they are a bit cheaper than other boards:

- The silk screen is a bit misplaced on some boards
- The holes aren't always dead center on the pads
- One of the boards isn't completely flat, i.e. the fibre glass is slightly bent.

These are just very tiny deviations and well within what is acceptable, they just aren't perfect.

I'm very excited to get started on the build, to see if I've managed to get the circuit and layout right :)

ETI Vocoder voltage to current converter

The VCAs in the  ETI vocoder use a voltage to current converter that is slightly different than what I'm used to. This post tries to understand what is going on. It may be wrong, so do not use the findings without verifying them first.

The transistor in the vocoder has its base tied to ground, collector connected to the control pin of the LM13600 and the emitter to the output of an opamp (and a trim pot) through a resistor.

Unlike other converters I've seen, it does not have the transistor inside the feedback loop of the op amp.

I suspect that the converter does not rely on the \(\beta\) or Hfe of the transistor, it only has to be high enough. I believe that what is important is the relationship between \(V_b\), \(V_e\) and the output voltage of the op amp.

I've chosen the VCA in the internal excitation circuit as my reference when analysing the circuit.

R34 is a 10k resistor, you can see these in most all designs using the LM13600 OTA. It's most likely there to protect the lm13600 from self destructing - the maximum control current of an LM13600 is 2mA. See my post on the Xonik VCA for an explanation of its value.

So how do we calculate the collector current (which is what controls the OTA, Iabc)?

Here is a page that explains how a transistor may be used as a constant current source:

http://www.radio-electronics.com/info/circuits/transistor/active-constant-current-source.php

It says that

\(I_{load} = \frac{\beta \cdot V_e}{(\beta + 1) \cdot R_e}\)

If \(\beta\), the transistor gain, is large, \(\frac{\beta}{\beta + 1}\) is approximately 1 (and thus \(I_{load} = I_c = I_e\)). Also, \(V_e\) is always one diode drop below \(V_b\) when the transistor is on (\(V_e = V_b - 0.6V\)), which means that the formula above may be simplified to

\(I_{load} = \frac{V_b - 0.6V}{Re}\)

In our case, \(I_c = I_{load}\) and \(R_e = R32\) (=22k).

What the page above fails to mention is that the bottom of \(R_e\) must be connected to ground, or at very least that \(V_e\) is the voltage across \(R_e\).

This final detail is of importance to us, because in our case the voltage at the op amp end of \(R_e\) (R32) is what varies, not the voltage at the base. The base is stuck at 0V/GND. Thus, the voltage across the resistor is not \(V_e\), it is \(V_{out} - V_e\), where \(V_{out}\) is the output voltage of the opamp (IC8b).

We also have to take into account that the vocoder uses a PNP transistor, not an NPN as in the constant current source above. while the emitter of an NPN transistor is 0.6V below the base, the emitter of a PNP transistor is 0.6V above the base. Thus, for a PNP transistor:

\(V_e = V_b + 0.6V\)

Now we can calculate the emitter current = current through \(R_e\), which will also be approximately the collector current if \(\beta\) is large:

\(I_e = \frac{V_{out} - V_e}{R_e} = \frac{V_{out} - V_b -0.6V}{R_e} = \frac{V_{out} - 0V -0.6V}{22k}\)
\(I_e = \frac{V_{out} - 0.6V}{22k}\)

What does this mean

Well, first of all, Ie is independent of the transistor \(\beta\), which means that we can replace the BC212L transistor with something else without having to look too hard for a perfect match.

Second, we can calculate the maximum current through the base. Knowing that

\(I_e = (\beta + 1) \cdot I_b\)

we get that

\(I_b = \frac{\frac{V_{out} - 0.6V}{22k}}{\beta + 1}\)

The vocoder uses a +/-12V power supply. Thus, the maximum output value of the opamp is +/-12V (in practice it will be a bit lower than this, and the vocoder may even be designed to use an even lower control voltage, but the absolute theoretical maximum is +/-12V).

The transistor only conducts if the emitter is more positive than the base. Thus, only \(V_{out}\) values > 0.6V will turn on the transistor. As the maximum \(V_{out}\) is 12V, the maximum \(I_b\) can be found as

\(I_e = \frac{12V - 0.6V}{22k} = \frac{11.4V}{22k} = 0.52mA\)

\(I_b = \frac{0.52mA}{\beta + 1}\)

Thus, when selecting the transistor to use, we need to make sure that it has a maximum \(I_b\) higher than this when entering its \(\beta\) in the formula above.

Also, we can see that \(I_e = I_c = 0.52mA\) is well within the 2mA maximum for the LM13600.

Trimming

As for the trimmer potentiometer PR1 and its associated resistor R33, I assume they add a constant current that trims the 0-point of the VCA.

The potentiometer acts as a voltage divider, so the maximum current through R33 will be:

positive:

\(\frac{12V - 0.6V}{470k} = 0.024mA\)

negative:

\(\frac{-12V -0.6V}{470k} = -0.027mA\)

A final note

I suspect that the diode D5 in the op amp feedback loop is there for this reason:

When the opamp positive input is at 0V, the opamp output must be at 0.6V (one diode drop above to keep the negative output at 0V. This means that the opamp output always stays 0.6V above its positive input, which cancels out the -0.6V from \(V_e\) in the calculations above.

The voltage to current conversion formula will then be

\(I_e = \frac{V_{out} - 0.6V}{R_e}\)
\(I_e = \frac{V_{in} + 0.6V - 0.6V}{R_e}\)
\(I_e = \frac{V_{in}}{R_e}\)

where \(V_{in}\) is the input voltage at the positive opamp input terminal, i.e. the control voltage.

I tried breadboarding the circuit to confirm my suspicions.

R31, D5 and IC8b form what is called a simple precision rectifier, see this wikipedia post: https://en.wikipedia.org/wiki/Precision_rectifier. As we tap the output at the opamp output instead of at the negative input, we should see the 0.6V offset.

My measurements clearly showed that this is indeed true. The positive input and the output of IC8b follow each other closely, with a difference of 0.6V.

Input at the bottom, output on top. The difference is almost exactly 0.6V

Also, to confirm that this is indeed the precision rectifier from the wikipedia article, once the input goes below 0V, the output immediately drops to the negative rail - no surprises there. As the input is connected to the previous rectifier however, this doesn't really matter, the CV will never be negative.

The output saturates to the negative rail once the input is less than 0V

As for R31, removing it makes the circuit act like a normal buffer. Increasing the value from 3.9k to 12k has no effect at all. Replacing the diode with a wire also changes the circuit back to a buffer.


Some transistor rules:
The load should always be on the collector side
\(V_b = V_e + 0.6V\) for NPN
\(V_b = V_e -0.6V\) for PNP
The \(V_b\) to \(V_e\) relationship stays constant, the currents are what change.

If \(\beta\) is large, \(V_c = V_e\). Good designs do not rely on \(\beta\) as is varies widely.

ETI Vocoder article errors - internal excitation

There is a mismatch between the schematics and the parts list for the internal excitation module.

Schematics says C11 is 22nF, parts list says 220nF.

C11 forms a low pass filter together with R39. Together with the 100k resistor on each channel of the analysis card - the whole setup is called an inverting amplifier filter or active inverting op amp low pass filter.

According to wikipedia:

https://en.wikipedia.org/wiki/Low-pass_filter

the cutoff frequency is 1 / 2 * pi * R*C

where R is the resistor in the feedback loop of the op amp.

For 22nF this is 1 /  6.28 * 47000 * 22 * 10^-9 = 154Hz
For 220nF it is 1 / 6.28 * 47000 * 220 * 10^-9 = 15.4Hz

In comparison, C12 and R40 gives:

1 / 6.28 * 10000 * 1 * 10^-6 = 1 / 0,01 = 15.9Hz.

The output from the analysis boards is the value of the envelope follower. If the volume of a frequency band is at its max continously, the envelope follower will be a DC voltage with a similar max value.

The two circuit parts sum up channels 1-9 (for the <2kHz input) and channel 13 and 14 (for the >4kHz input). For the sums to be comparable, the max sum of each parts must be the same.

R39 and R40 are selected to achieve this:
The gain of <2kHz is -10k/100k = -0.1
The gain of >4kHz is -47k/100k = -0.47

Multiplied by the number of channels on each input we get a 'maximum' sum for each input:
<2kHz: 9 channels * -0.1 = -0.9
>4kHz: 2 channels *-0.47 = 0.94

These are similar enough to be compared.

The low-pass filter is there to make sure the Vocoder only reacts to slow changes in envelope amplitudes. It is reasonable to assume that they should be roughly the same. The capacitor has been matched with the feedback resistor (R39, R40), which in turn was selected to give correct gain as described above.

The closest matching alternatives are then 15.4Hz and 15.9Hz. Thus, the correct value for C11 is 220nF.

Transistors in the ETI Vocoder

Many of the transistors in the ETI vocoder are either hard to find or very expensive (or both). I am trying to figure out if alternatives are available.

For the internal excitation board, two BC182L (NPN) and two BC212L (PNP) are used.

Q1 is definitely replaceable, probably with a 2N3904. It only acts as a logical inverter together with R20 and R21. It is similar to the one found here:

http://www.cs.unca.edu/~brock/classes/Fall2012/csci255/labs/lab05.html

Not sure if we have to change the resistor values, but probably not.

Q2 is a voltage to current converter. I am not entirely sure how it is meant to work so I cannot say for sure right now what parameters to look for. The same converter circuit can be found in the analysis/synthesis section, so if one is solved, both are solved. I think looking at the exponential converter theory I've written earlier may solve some of it - the way it works is that the base is tied to ground while the emitter is connected to the output of IC8b which probably follows the envelope follower. D5 is similar to what I have in my Xonik VCA. R33 and PR1 probably adds a constant current to trim offness or similar.

Q3 and Q4 control two switches. Again, I believe they are considered binary - the output of the comparator is either high or low and the switches are also controlled by high or low voltages. We should definitely try 2N3904 and 2N2906 here.

Update: I've confirmed that Q2 is not depending on Hfe and may thus be replaced by a difference transistor (see my post on the ETI voltage to current converter). The BC557 has the same pin-out (CBE, PNP) and at least the 557B has a Hfe in the same region.

As for the BC182L, it seems that its pin out (ECB, NPN) is harder to find. However, the BC182 is still available from Farnell and RS Components (though not mouser). If the circuit board is redesigned, using a different transistor will be problem free.

BTW: Here is a nice comparison page for different transistors: http://www.edutek.ltd.uk/Transistors_NPN.html

As for the BF244 N-channel JFET, I have yet to find a replacement. It could possibly be replaced with a J112 (which also has interchangeable source and drain). I would have to look at the exact function of the transistor but I suspect that it is only used as a switch. The difference between J111/112/113 is the gate-source cutoff voltage, the zero-gate voltage drain current and the drain-source on resistance. The cutoff voltage for the BF244 varies widely, more than between the various J variants, as does the zero gate voltage drain between the versions (A,B,C) of the BF244. The vocoder article does not specify a particular version of the BF244 so this may not be very important either.

The ETI Vocoder

I can't remember if I've posted about this earlier, but for quite some time now I've planned on building the ETI Vocoder. I was approached by a guy who wanted me to help him etch the circuit boards, and we spent at least a year planning for this, touching up the original drawings and adding/moving connectors etc.

In the end though, after buying presensitized PCBs and doing a few test etchings, I decided not to do this at home anyway. I simply have too few time slots long enough to do the setup, etching and cleanup.

What I ended up doing instead was redoing the boards and send them off to be professionally produced. I have shrunk all the boards as much as possible (while still using through hole components). I've split the internal excitation board into two separate pcbs so that all pots are soldered to a board.

In addition, I've combined the LP/BP/HP versions of the voice board into a single circuit. This means that you have to add some solder bridges and that some parts are left out for some of the boards, so populating them is slightly harder. However, it also means that I was able to make dual channel boards that, when repeated 7 times, turns into the full voice board. Using a service like dirtyPCBs, 7 similar smaller cards are much cheaper than one large one.

In retrospect I've realized that I could possibly simplify the circuits a bit, since the left channel on a card will never be HP and the right never LP (or the other way around, I can't remember).

I've also managed to jam all the non voice boards (except one of the internal excitation parts) into two 10 x 10 cm pcbs, keeping the cost really low.