posts/piano: rough draft

posts/piano.md

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+# reviving a digital piano with new brains
+
+2024-05-11
+
+One day, I was playing my Roland HP-1500 digital piano,
+which is an incredibly old model.
+It suddenly started making weird electrical noises, and then it died.
+I opened the piano up, and looked at the circuit board,
+but my efforts to figure out what went wrong were ultimately futile.
+
+At this point, I had a thought: maybe I could build a brand new circuit for the piano,
+replacing the broken original board.
+After all, how hard could it be?
+I had just learned the basics of electronics, and this definitely seemed like a good learning experience.
+
+That was a few months ago.
+Recently, I finished implementing this project, which I named geode-piano.
+Here is a quick demo of it (excuse the poor microphone quality):
+
+<video width="640" height="360" controls>
+  <source src="/public/img/piano/demo.mp4" type="video/mp4">
+Your browser does not support the video tag.
+</video>
+
+This project is powered by a single Raspberry Pi Pico, which runs firmware written in Rust.
+Source code and build instructions are available on the [project repository](https://github.com/dogeystamp/geode-piano).
+
+It took quite a while to get to this point, and so this blog post will document the process of designing and implementing geode-piano.
+
+## how a digital piano works
+
+First, before even designing anything, I did a bit of research on what was going on inside a digital piano.
+This helps understand how feasible the project is and how complicated it will be.
+
+### velocity detection
+
+A digital piano is, electronically, just a bunch of buttons that trigger sound when pressed.
+There is, however, a slight nuance to this.
+A button switch has only two states, on and off.
+On a piano, hitting a key really hard makes a loud note, and softly pressing it makes a soft note.
+From the perspective of our hardware, a button press is just a button press ­
+there is no information about intensity.
+
+To measure the intensity of key-presses, some engineer decided that instead of every key having one switch,
+they should have two switches.
+These switches are placed so that they trip one after the other during a keypress.
+By measuring the time between the switches' activations, the digital piano can estimate the intensity of a press.
+A fast press is a hard press, and a slow press is a soft press.
+This system works well, and is present in most digital pianos.
+
+### key matrix
+
+Because of the need for velocity detection, we expect that a typical 88-key piano will have 176 switches in total.
+This is a problem:
+usually, a microcontroller (the little computer inside the circuit that controls everything)
+only has a few dozen input pins.
+How are we going to connect all those switches to it?
+
+If you are familiar with mechanical keyboard design, you'll have probably heard about a "key matrix".
+This is what powers keyboards, both the typing and music-playing kind.
+Essentially, a key matrix helps cram all those 176 key switches onto a microcontroller with way less input pins.
+For example, look at this key matrix:
+
+```
+     column
+     1 2 3 4
+row  
+     │ │ │ │
+  1 ─┼─┼─┼─┼
+  2 ─┼─┼─┼─┼
+  3 ─┼─┼─┼─┼
+
+```
+
+These lines represent wires.
+The columns can be powered on and off,
+and the row wires on the left are input wires.
+Each intersection in this grid has a switch.
+
+- When a switch is off, power flows only vertically.
+- When a switch is on, power is also able to flow from the column into the row. Power can not flow from row to column, though.
+
+> Note: this diagram and explanation are both simplified, so [click here](http://www.openmusiclabs.com/learning/digital/input-matrix-scanning/) for a more detailled explanation.
+> In practice, diodes are used to ensure power doesn't flow the wrong way.
+
+Let's say we turn on two switches in the column 1, and also put power through it:
+
+```
+     column
+     1 2 3 4
+row  
+     ┃ │ │ │
+  1 ━╋━┿━┿━┿
+  2 ─╂─┼─┼─┼
+  3 ━╋━┿━┿━┿
+
+```
+
+Power can now flow into both row 1 and row 3, where it can be detected.
+Based on this, we can deduce that the switches pressed were `C1R1` and `C1R3`.
+
+Now, turn off the power in column 1, and then test column 2 in the same way:
+
+```
+     column
+     1 2 3 4
+row  
+     │ ┃ │ │
+  1 ─┼─╂─┼─┼
+  2 ━┿━╋━┿━┿
+  3 ─┼─╂─┼─┼
+
+```
+
+Here, only row 2 detects power, so we can deduce that only `C2R2` is pressed in this row.
+
+At this moment, the switches in column 1 could still be pressed,
+but we only check one column at a time.
+Because power can not jump from a row back into a column, we only get results from the specific column that we are testing.
+Now, we can test the 2 other columns in the same way.
+
+By testing all columns rapidly, we will detect all the switches that are pressed down at any moment.
+Using this matrix, we need 8 pins, while an equivalent non-matrix circuit would need 16 pins.
+However, we sacrifice a bit of speed to save these pins because it takes time to scan each column.
+
+### under the hood
+
+So that's how a digital piano works, theoretically.
+What does that look like, under the hood?
+As it turns out, the matrix is accessible through ribbon cables (or _flat flexible cable_, or FFC).
+
+The contacts on these cables correspond to the columns and rows of the key matrix.
+Usually, you'll find one or multiple ribbon cables with one end plugged into the main board of the digital piano,
+and the other ends leading inside the piano key mechanism.
+Mine had two cables with respectively 18 and 22 pins.
+
+A good practice when making projects like these is to ensure that it _is_ possible before doing anything.
+To test that the ribbon cables were properly connected, I tested them with multimeter probes:
+
+![A flat flexible cable with alligator clips on the contacts.](/public/img/piano/ffc-test.jpg)
+
+After getting the polarity right, pressing some keys did trip the continuity test (which beeps if there is an electric connection between the probes).
+Therefore, making my own digital piano circuit was feasible.
+
+## project design
+
+