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

Teensy 4.0 bottom pins - finding a suitable header and connector

If I'm to use the Teensy 4.0 as my voice controller MCU, I need to use the 10 additional pins on the back to get to the second SPI.

This means I either have to solder pins to the 10 pads, or use a breakout board with castellated connections like https://forum.pjrc.com/threads/57672-Another-Teensy-4-0-Breakout-Board?highlight=Teensy+4.0+rear+pins

If I am to solder pins, I need to make sure that I can find a matching mating connector. As the SMD pin header is higher, the mating connector needs to be lower. Also, the pins need to be short enough to fit the connector.

I measured the pre-soldered pins on the Teensy, the bottom plastic is 2.54mm high, the pin is 5mm.

Normally, mating connectors for the single rows are 8.5mm high, with some saying that the pins may be 6.6mm long.

I am going to try out the following:

Pins (from Farnell):

Samtec TSM-105-04-L-DV

These have a 0.15" (3.81mm) height before the pins, or 1.27mm more than the ones on the Teensy. The pins are 3.05mm, which is a bit on the short side.

Update: Samtec TSM-105-01-F-DV has a pin height of 5.84 which may be more suitable.

GCT BG050-10A-0-0450-0737-0350-L-D

These have a height of 3.5mm before the pins, but the pins are 4.5mm. That looks very promising. Unfortunately, ordering these in Norway incurs a $13 fee because they are only in stock in the US.

F = 3.5mm, D=4.5mm

Female connectors (from Farnell):

Amphenol 76342-305LF

This is a 7mm high receptacle. combined with the GCT connector it would give a total height of 10.5mm, as opposed to the 11.04mm of the side connectors, leaving a 0.5mm gap. This is probably fine. Combined with the Samtec one the gap is reduced to 0.2mm, but the short pins (3mm) may not be long enough. The max pin length is not stated but from the "normal" connector we can guess around 4.9mm.

AMP 215307-5

Similarly, a 7mm high receptacle. Max pin length is not stated for this one either, but it says that the connection point is 2.2mm into the connector which is a tad too long for the 3mm Samtec pins.

From JLCPCB:

I managed to find a couple of 7mm headers from JLCPCB as well:

Liansheng FH-00097

UPDATE: This is a 20p part and won't fit

part C2829902, though only 8 are in stock right now:

Wcon 2171-210SG0CUNT3

UPDATE: This is a 20p part and won't fit

part C721788 - unfortunately, the mating position is  3.57mm down which again may be too far for the pins.

I will order the Farnell ones shortly to test.

Update:  Results from Farnell

14 pin socket:

Part 1593466 / 2212-14SG-85 

May pull pins 0.8mm out and still have good contact. Tested using pins that protude about 5.66mm and has 2.54mm plastic at the bottom

When pressed together, pins and header measure 10.87mm. Header is 8.42.

10 way SMD

Pins: 3551099 / Samtec TSM-105-01-F-DV (NB: F, not L). Plastic + bent pins measure 3.86mm. The pins measure 5.68mm so the total height is 9.66mm

Header: 1098050 / Amphenol 76342-305LF. Measures 6.97mm. Pins may be pulled out at least 1mm and still have good fit. Height with pins is 10.85mm

Header: 3419216 / AMP 215307-5. Measures 6.96mm. Pins may be pulled out at least 2mm and still have good fit. Height with pins is 10.84mm. Has a very nice, tight fit

Conclusion: The combination of either of the headers with the SMD pins will work, and the height is damn near spot on when used with the 14 pin socket and standard pins!

I still have the GCT BG050-10A-0-0450-0737-0350-L-D on order and will add the results when they get here, but it seems the Samtec ones work nicely.

Update 2:

Pins: 1798977 / GCT BG050-10A-0-0450-0737-0350-L-D. Plastic + bent pins measure 3.65mm. 

Combined with header the total is 10.62. The pins are at least 1mm shorter than the Samtec ones. They still fit quite well even when pulled back 0.2mm

Pins: 3585115 / Samtec TSM-105-04-L-DV. Pins above plastic is 3mm, which is too short to be comfortable. I would not recommend this.

Final conclusion:

The clear winner is the Samtec TSM-105-01-F-DV pins with either of the two headers. Since I've bought enough of the GCT pins already, and they work fairly well, I will go with those. The price is also between half and 1/3 of the Samtec, so if you're in the US they are probably a lot cheaper. Since I had to pay extra they are more or less the same. Had I not bought them I would have gone with the Samtec.

Both SHOULD work with the LCPCB headers too.

UPDATE: Suggested female headers are 20p and wont work. This one may work:

 

JLCPCB C2682198, https://jlcpcb.com/partdetail/2776421-X5521FV_2x05_C70D301000/C2682198