This repository collects together all materials (CAD Drawings, diagrams, firmware source code) related to the NEMUS Harpsichord Interface project.
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| Model Keyboard |
| Jack | Centre distance from backboard (mm) |
|---|---|
| 1 | 10.25 |
| 2 | 25.1 |
| 3 | 38.5 |
Gives a rough jack pitch of 14.4875 mm with an offset of 10.25 mm.
Pitch of the back row of jacks was determined by measuring from the open edge of the model to the front edge of the jack. Each jack had its width measured providing the values in the table below.
| Jack | Front edge dist (mm) | Width (mm) | Centre dist (mm) |
|---|---|---|---|
| 4 | 59.85 |
4.25 |
61.975 |
| 5 | 45.25 |
4.30 |
47.400 |
| 6 | 31.20 |
4.60 |
33.500 |
Give a rough pitch of 14.2375 mm with an offset of 33.5 mm from the front edge
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| Top view of the 49-key keyboard |
The full scale version of the interface has 49 keys and 2 jack rows. The jacks are generally around 4.65 mm.
There is not much variation in the pitch of the jacks. On average the centre-to-centre is 14.02 mm. Likely error between jacks would compound across the full length. This should be compensated for by the separate pcbs which have groups of 7 sensors.
Space between the jacks is 9.75 mm, meaning central pcbs will have to have less than half this distance (4.875mm) in order to slot together.
The jacks have a height of 91.65mm. Front jacks have a travel of about 8.5mm and back jacks 9.0mm between key release and full key press.
With a pitch of 14.02 mm a theoretical limit to the width of a 7-sensor PCB would be 98.2 mm, but with margin required from traces this is not possible. The upper limit is 112.22 mm but with variation during fabrication it is unwise to push the tolerance to the nearest 1/10 mm. A 1.5 mm tolerance brings the limit to about 99.7 mm to 110.7 mm
Face boards mount the QRE1113s perpendicular to the board so that there is no need to bend the through hole mounting legs during assembly.
There are a couple of designs, at at present what is most important is the hole mounts at (18.00mm, 10.00mm) and (18.00mm, 81.40mm)
The QRE1113 reflects from a tag with a gradient printed.
The should be 40 mm in length to avoid potential catching on the jack hole. The active area of the gradient need only be the length of the jack travel
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| Cross-section of full sized harpsichord interface |
The electronics uses the following components:
- Arduino Nano 33 BLE
- QRE1113
- MB85RS64 SPI Non-Volatile FRAM
- [Rotary Encoder w/ tactile switch]
- CD4051BE multiplexer
- WS2812B RGB LEDs
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| Model keyboard and QRE1113s attached to Arduino Nano 33 IoT |
Displacement of the jacks is measure using QRE1113 IR LED / Transistor pair. The section covers some general observations and helpful information about these sensors.
Distance between pins is 1.8mm meaning the space from the centre is 0.9mm. CAD placement should offset by 0.9mm to compensate.
| short leg pitch | 1.80mm |
| long leg pitch | 3.80mm |
QRE1113s are used with alongside adhesive strips with a black and white gradient printed on it, which are attached to each jack.
The optimal distance between the strip and the sensor is around 6mm with a voltage reference of 1.65V provided by Arduino SAMD boards
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| Jacks with gradient strip attached |
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| QRE1113 Dimension. Not that pin 1 should always have a chamfer on the corner to help identify it. | QRE1113 Pinout |
Multiplexing is achieved using a CD4051BE, chosen for the configuratoin of:
- 14 Boards
- 7 sensors each
- 7 analog channels
- 8 channel multiplexer
Microcontroller is the Arduino Nano 33 BLE which has 8 ADC channels A0 - A7. For 8 ADC channels an 8-channel multiplexer will allow for 8 boards to address 8 sensors, though only 7 channels are used.
Configuration:
- Use 2 micro controllers, one for front, one for back
- each a separate MIDI device
- cascade front and back
- two groups of optical sensors
- 16 optical sensors
- 2 sets of 7 for signals
- 2 extra for interfacing with the others
- 2 extra digital pins used
- 16 optical sensors
- another bank of multiplexers
- 7 multiplexers for groups of seven boards
- 3 more digital pins for control
Each sensor board needs 8 pins:
- GND
- 5V
- 3V
- Mux Address A
- Mux Address B
- Mux Address C
- LED Data
- Signal
The interface is intended to be operated from a 5V 1A power supply.
The rotary encoder, FRAM and RGB LEDs draw from the Arduino Nano's 3.3v regulated power line.
The power supply requirements are then dictated by the remaining components, which use the power directly:
| Min Voltage | Max Voltage | |
|---|---|---|
| Nano 33 BLE | 4.5 | 21.0 |
| MUX | 5.0 | 18.0 |
| QRE1113 | 0.0 | 30.0 |
The CD4051BE MUX dictates the power range of 5V to 18V. The system has only been tested using 5V, so behaviouyr will likely change.
Below are additional MIDI specifications retrieve the most information from the controller.
Currently this is being written and subject to change
There is one channel per key as per Standard MPE specifications.
There are two note ons with associated velocities and note offs
A note on message channel, pitch value, velocity.
Velocity demarcates wether the note on message is related to the first jack or the second jack.
Given the signal obtained from the jack tracking does not provide the resolution in velocity to utilize the full 7 bit set of velocity values.
The First jack uses the velocity values from 0 to 63 and the second jack from 64 to 127.
Note off messages use the same specification.
Therefore the physical model can relate the note off message to it's respective note on message.
example:
| Jack1 | Jack2 | |
|---|---|---|
channel |
1 | 1 |
pitch |
56 | 56 |
velocity |
0-63 | 64-127 |
NOTE: There is a secondary use case where the controller is used with a digital instrument that expects a standard MPE signal. In that case there may be problem. However, it is not guaranteed to not work. Behavior in this case is undefined.
The same is true of the plug-in, which will be written to expect the above format of note messages. As a result, the plug-in will need two modes of operation. A standard MPE for using with a standard MPE keyboard requiring a single note on/note off per key. The second mode designed specifically for the harpsichord keyboard interface. This gives control of the excitation of both of the strings.
Channel Pressure (MIDI Status event nybble 0xD) is sent as one stream, taken from the front jack tracking signal.