The goal of this project is to build a polyphonic multitimbral analog synthesizer with the SID chip, using a Commodore 64 and trying to fulfill three requirements:

1. It must be playable as an instrument with any MIDI keyboard, connected to the user port through an Arduino interface.
2. It must be as standard as possible: minimal soldering, neither drilling nor opening the computer case.
3. And as inexpensive as possible: only an old (working) Commodore 64, an Arduino board and a few of electronic components are needed. The most expensive device will be the SD2IEC drive you'll need to load the program, unless you are willing to type it manually.
If you are going to buy an SD2IEC drive, please make sure you get one that connects to some C64 port other than the user port, as the user port will be used for the interface described on this site.

New in v 3.0

● Vibrato
● Oscillators fine tuning
● Notes routed through the filter can restart or not the filter envelope.
● The LFOs for the pulse width and filter cutoff can now also oscillate with a square wave.
● Now it's possible to split the keyboard at any point (not necessarily at a C) just by pressing the desired key, instead of repeatdly pressing a button to shift the split point.
● There are fewer garbage characters on screen.
● You can optionally add three push buttons to the board to select the polyphony modes and keyboard split (otherwise, this can be directly done from your MIDI controller as in previous versions).

Disclaimer

My electronics knowledge is very elementary. I can't guarantee you won't fry a chip by building this project. I can only say I've spent many hours with wires and pins, both in Arduino and the C64 user port, and nothing went wrong. I read very carefully lots of documentation available in the net before trying anything, so those readings are my only advice if you want to avoid accidents. If you know more, I'd appreciate any safety tips you could provide to this section.
I had never soldered before this project and it took me some practice to make firm solder joints on the user port connector (German).

Please don't use any batteries to power the Arduino. In other projects I've used a 9V battery connected to the Vin pin, but I’m not sure if this could damage the C 64. Anyway, in this project the Arduino works fine with the 5V provided by the C64 user port. Alternatively, I unplugged the 5V and GND wires that connect Arduino and the C 64 and powered Arduino with a USB cable from the PC instead; this worked well too, although I tried it only for a few minutes.

Here or here you can see the Commodore 64 user port pins.

During the development I've used different setups with two Commodore 64 C (this model has the 8580 SID), an Arduino UNO and an Arduino Nano, and they all worked fine. I'm using Arduino Nano now due to a lack of space on my desk. I’ve never tried it with a Commodore 64 equipped with the 6581 SID, but it should work as well (feedback on this will be appreciated).
On the internet you can find MIDI in/out schematics, and how MIDI data is sent/received in Arduino. For this project only MIDI IN is required, which is a bit more complicated than MIDI OUT, but still pretty easy to build.

General
● Breadboard: 2 split power buses, 10 columns, and 63 rows (see the image below).
● Jumper wires in different lengths.
● Soldering iron
● User port connector (English)

For the MIDI circuit
● MIDI connector
● 220 Ohm 1/4watt resistor
● 1N4148 diode
● 10k Ohm 1/4 watt resistor
● 470 Ohm 1/4 watt resistor
● 6N138 optocoupler

Other parts
● 3 x LEDs. I've used green, but I assume other colors should work as well.

Alternative A (new) 3 x 220 Ohm resistors, one for each of the 3 LEDs.
Alternative B (old) 1 single resistor for all 3 LEDs: 1k Ohm resistor or a 470 Ohm resistor, it all depends on how much light you want the LEDs to emit.

The alternative A produces a more balanced light in the 3 LEDs, regardless of how many of them are lit, but might be a bit more complicated to build: I´ve soldered each 220 Ohm resistor to an LED but you can also wire the Arduino pins to free holes on the breadboard and then connect the LEDs from another hole in the same column as the wire to the GND power bus.

● 3 x pushbuttons (optional*)
● 3 x 10 K resistors (optional*)
* These optional parts are used to select keyboard modes and set the split point by pressing pushbuttons, but this can be done much more easily by sending control changes from your MIDI controller. They are only necessary if your MIDI instrument lacks proper buttons to send MIDI messages.

The Arduino sketch works with both circuits: with and without pushbuttons.

Below are the schematics and some pics.
Please note that in the schematics the 8 bits in Arduino are assigned from right to left (0 to 7) to the digital pins 2 to 9, while in the user port (viewed from outside) these 0 to 7 bits are assigned from left to right to the pins C to L.

The BASIC code is a spaghetti mess, and the user interface is not very user-friendly, but it works. You'll see some garbage characters I've been using as a kind of visual debug to pin down what's going on in the machine language routines. Someday I'll clean it up. The machine language code, however, is pretty optimized and clean.

Once the program is running, please be careful when entering data. Values outside the range 0-255 aren't checked for validation when you enter them and will cause an ILLEGAL QUANTITY ERROR. If that happens, you'll have to restart the program.

In any case, all of the SID's paramaters can be edited and you can save and load patches.The keyboard can be split in two parts, each of which can control one or two of the 3 SID voices. If not split, the entire keyboard controls all 3 voices (full polyphony). There are 6 different modes (see below).

To tune the SID, I'm using the PAL frequency, as shown in this table, where C-0 is 278. In line 1 of the BASIC program, this is divided by 1.0595 so that FI=262.39 (and multiplied again by it in the loop of lines 7-8 to get the frequency of the first 12 semitones). If you are using NTSC, you might need to set FI=252.95 in line 1, so that the frequency of C-0 is 262.

Polyphony modes and keyboard split
(MIDI CCs or push buttons)

There are three functions you can control using control changes from your MIDI controller or with push buttons from the Arduino circuit.

1. Cycles through the 6 modes: 1 > 2 > 3 > 4 > 5 > 6 > 1. This is CC #97 or the push button connected via the white wire.
2. Sets the point where the keyboard is split. After pressing this button, just press the desired split point note on your MIDI keyboard. This is CC #96 or the brown wire. Please make sure you press only one key on your instrument at a time; if you play more, the program might start to become unstable, keeping notes on when they should be off.
3. Swaps the left and right parts of the split keyboard. This is controlled by CC #66 or the blue wire.

If you want to use different control changes, you can search for "97", "96" and "66" in the Arduino sketch and change them to other values. Alternatively, you can assing the CCs to other buttons, sliders or knobs on your MIDI controller in real time (although sliders and knobs will be a bit tricky to tweak, requiring very precise finger motions). To reassign CCs, touch any control other than the three you are currently using. You'll see that two LEDs start to blink rapidly. While they are blinking, touch the three controllers you want to assing to CCs 97, 96 and 66, one after another. When all three are assigned, the LEDs will stop blinking. Check out a video here.

The 6 polyphony modes

1		3 independent voices/oscillators: Fully polyphonic, allowing you to play up to 3 notes at the same time across the entire keyboard.
2		Voice 1 is on one half of the keyboard, while voices 2 and 3 coupled on the other half: you can play one note on each half of the keyboard. On one half, voices 2 and 3 are triggered simultaneously when you press a key; you can detune the voices, synchronize, or ring modulate voice 3 with voice 2.
3		Voice 1 plays "pseudochords" on one half of the keyboard, while voices 2 and 3 are played independently on the other half. The pseudochords allow you to play up to four notes simultaneously on one part of the keyboard; those notes are actually played like an arpeggio, and when played fast enough, they sound almost like a chord (a well known effect you may have heard in many games).
4		Voice 1 plays "pseudochords" on one half of the keyboard, while voices 2 and 3 are coupled on the other half.
5		Voice 1 is on one half of the keyboard, while voices 2 and 3 are on the other half, but they are not coupled. This is similar to mode 1, but you can use different sounds in two separate parts of the keyboard (multitimbral). Alternatively, you can use the same timbre in both halves, but ensuring that the notes on one half never override the notes being played in the other half.
6		3 voices coupled. This is a monophonic synth with 3 oscillators. You can detune any of them, synchronize or ring modulate to each other.

Once you run the program, you will be prompted (Y/N) to read the data for the machine language code and the tables used by it. You should select "Yes" the first time. If for some reason you have to restart the program without turning the computer off, you can bypass this by selecting "No" to save some time. You might need to restart the program if you enter a wrong value while editing a patch or accidently press Run/Stop. The program is still in development and it's not very user-friendly in this regard.

Things that can't be done (yet) and bugs that I will fix in future versions:

Only the five central octaves can be played on the keyboard (C2 - C6). However, you can transpose the oscillators or shift your controller's keyboard one octave higher or lower, allowing you to reach C1 and C7.
The vibrato is not fully independent of the glide speed (see below for more details).
Although each voice has its own independent LFO to control the pulse width, currently all voices' pulse widths are routed through the same LFO.
No parameters can be controlled with the pitch wheel or the knobs on the MIDI controller.
No touch sensitivity (note velocity).
Some parameters (vibrato and square wave LFOs) can't be saved. When you load a saved patch, you will need to set them again manually.

----------------------------------------------------------------------

Once you RUN the program and all data have been read, the main menu will appear:

The numbers 1, 2 and 3 in the middle of the screen stand for the three voices.

The number highlighted in white indicates the voice currently selected, which can be changed by pressing the keys 1, 2 and 3. Some parameters can be edited only for one voice at a time, so you must first select the voice you want to edit. These parameters are PW ENV LOOP (option D), ADSR (option G) and the oscillator tuning, located at top right:

Using the cursor down/up key you can increase or decrease the pitch by 12 semitones (one octave) and with the right/left key adjust it 1 semitone more or less.

This main screen contains the only menu you'll use to edit all settings. At this point, you can perform an initial sound test: by pressing the space bar, you'll enter the play mode. You'll see some characters rapidly changing on screen, and if you press keys on your MIDI keyboard, you should be able to hear the notes. The default mode is polyphonic, allowing you to play up to three notes simultaneously. By pressing the Return key you'll return to the edit mode; any character should stop changing on the screen.

By pressing the keys A to M you can access the following options. Only B to J are visible in the menu.

A

Wave tables and arpeggios editor (see option K)

This is the most complex section, containing several parameters explained below. The three upper rows

54276 ($D404) Voice 1
54283 ($D40B) Voice 2
54290 ($D412) Voice 3

are the waveforms editor, which basically represents a sequence of four states in the registers 54276, 54283 and 54290. These SID registers determine the oscillators' waveforms, as well as the synchronization, the ring modulation effects, and the gate state. Each character represents a state of the register at a given time unit. All 8 bits can be switched on or off independently.

The numbers 1 to 8 correspond to the keys you need to press, but when I refer to bits, they are numbered from 0 to 7 and from right to left.

Function	Noise	Square	Saw tooth	Triangle	Test	Ring modulation	Synch.	Gate
Key	1	2	3	4	5	6	7	8
Bit	7	6	5	4	3	2	1	0

The first four states are swept in a loop from left to right during the ADS phase (when you press a key on the MIDI keyboard) and the last 4 states are swept in the same way during the R phase (when you release the key and the sound starts to fade out). By combining different waveforms within the same phase, distorted and metallic sounds can be generated.

The three lower rows

are the arpeggios editor.

B

Arpeggio speed

Sets the speed at which arpeggios are swept. Arpeggios can be set as fixed in the arpeggio editor or as pseudochords in real time, while playing several notes simultaneously (keyboard mode 3 and mode 4). Valid values range from 1 to 128.

C

Pulse width sweep speed,
square LFO on/off

This synthesizer uses two software-generated LFO to modulate the cutoff frequency and the pulse width of the square waveform, respectively. Although the SID can use the third oscillator's output to hardware-generate an LFO, I left out this built-in LFO, as it sacrifies the sound of voice 3.

Polytimb 64 actually features three independent software LFOs to modulate the pulse width (PW) of each of the three voices, but for the time being I'm using the same LFO for the square waveform of all voices.

Here you need to enter three values. The first one determines the speed of the ascending sweep through a defined PW range (see option D). The second value does the same for the descending sweep. And the third one sets whether the the LFO wave will be square (1) or a varietion of triangle or sawtooth (0).

Enter "1,1,0"	Enter "2,2,0"

Enter "1,2,0"	Enter "8,1,0"

Enter "1,1,1"	Enter "8,1,1"

When the LFO wave is square, the slash (/) between the values is displayed in reverse video. For instance, here are the PW RATE values:

D

Pulse width envelope/loop

This controls the LFO as well as a kind of primitive envelope for the pulse width (PW) by determining the starting, highest, and lowest PW levels (first, second and third digits, respectively). The fourth digit sets the initial direction of the PW sweep (1 = ascending, 0 = descending) and the fifth digit determines whether the PW stops sweeping once it reaches the lowest/highest point for the first time (0), if it stops after reaching a second lowest/highest point (1), or if it continues sweeping up and down indefinitely between the lowest and highest levels.

The range of the first three digits is between 0 and F. For the sake of simplicity, only the most significant byte of the PW can be selected by the user. However, the PW sweeps across the least significant byte (at the speed defined in option C).

In this settings, the values are entered using the keys 0 to 9 and A to F, shifting the selected digit left and right with the cursor keys. Pressing the Return key confirms the selected values, and pressing T copies the selected values to the PW envelopes of all voices.

Enter "5C210" The third digit, "2", is not relevant in this example.	Enter "5C211"

Enter "5C212"	Enter "AD302"

E

Filter cutoff sweep speed, square LFO on/off, filter resonance

This is the same as option C, but in this case it applies to the filter cutoff frequency. In addition to the three values explained in (C) you must enter here a fourth value between 0 and 15 for the filter resonance. For instance, "1,1,0,15" represents sweep speed up = 1, sweep speed down = 1, no square LFO, and 15 (maximum) resonance.

F

Filter cutoff envelope/loop

Same as option D, but it applies to modulating the filter cutoff frequency. Since the SID cutoff frequency is 11 bits long, the maximum value you can enter for the first three digits is 7 instead of F.

G

Volume ADSR

This is the amplitude envelope generator. Unlike options D and E, this is a hardware feature of the SID: registers 54277 (attack/sustain) and 54278 (sustain/release) for oscillator 1, registers 54285 and 54286 for oscillator 2, and registers 54291 and 54292 for oscillator 3. The values are entered by pressing the keys 0 to F; while the left and right cursor keys shift the selected digit, the Return key confirms the entered values and the T key copies them to all three voices.

There is a known bug in the SID chip that prevents the attack to start properly for certain ADSR values, resulting in no sound when you press a note key. More about this here.

H

Save patch

Enter a name (max. 16 characters) for the patch you want to save.

I

Load patch

Enter a name (max. 16 characters) for the patch you want to load.

J

Glide speed /
Vibrato

Here six values must be entered: the first group of three values is for glide (voice 1, 2 and 3) and the second group for the vibrato (voice 1, 2 and 3).

The glide values determine how quickly (1 = fastest, 255 slowest) a note's pitch increases or decreases when you press a key on the instrument. Three values separated by comma must be entered, one for each voice. Very slow glide speeds will produce audible "steps" along the pitch variation, but for most "normal" speeds you'll hear a continuous, smooth glide.
The range of values for the vibrato is between 1 and 32. I haven't tried any values outside this range, but I assume values beyond the range will cause strange sounds or a lockup.

To activate the glide effect, it has to be set to one of the four ON modes. With the keys F1 (voice 1), F3 (voice 2) and F5 (voice 3) you can sequentially select:

@	Glide off
A	Glide downwards: starting from an octave lower than the currently played note and gliding up to the actual note.
B	Glide upwards: starting from an octave higher than the currently played note and gliding down to the actual note.
C	Glide from the previously played note in one of the two sections of the keyboard (or just the entire keyboard if it's not split) to the currently played note.
D	Glide from the previously to the currently played note in the other section of the keyboard (provided the keyboard is split).

Basicaly, C and D modes allow for independent portamento effects in each part of the keyboard, when the keyboard is split. If the keyboard is split and you select the same mode (either C or D) for voices played in different parts, notes on different sections of the keyboard may be linked by portamento, resulting in unwanted or interesting effects.

In the current version, the speed/rate of vibrato is locked to the speed/rate of the glide (8 in the example above), while the amplitude/amount del vibrato (16) can be set on its own. This will be fixed in future versions, to allow complete independence between both types of modulation. For the time being, interesting effects can still be produced. For instance, set the glide mode to B for the three voices with the keys F1, F3 and F5, then press J and enter the values 8,8,8,16,16,16. If you use a triangle waveform in all voices, these values will produce a ghostly sound.

K

Wave tables speed

(see option A)

Enter a delay value for each voice, separated by commas, to determine how fast the sequence of waveforms is read in a loop (the higher the value, the slower the sequence). More about this here.
Please remember this option, as its "K" is not listed in the menu.

L

Enter POKEs manually

This is a kind of debug option. In future updates I will list here several memory addresses and values you can enter to adjust the program's behaviour. The format is: memory address,value.

M

Fine tuning

Enter the voice number (1, 2 or 3) followed by a comma and a value from 0 to 255, where 0 means no detuning at all. Detuning is more noticeable when using two or three oscillators in unison (modes 2, 4 and 6), which produces a "fat" sound. For instance, select the keyboard mode 6 (3 voices played as monophonic), press M and then enter "2,7" (detuning voice 2 by 7 units), then press M again and enter "3,14" (detuning voice 3 by 14 units). Play some notes to hear the difference: voice 1 is in tune, while voices 2 and 3 are slightly detuned.
You can also enter 4 as the first value, which assigns the same fine-tuning value to all voices. This is useful when you want to set all voices perfectly tuned again: press M and then enter "4,0".

N

Glide distance in
glide modes A and B
(see J key above)

Sets how many semitiones above or below the pressed key the glide will start from. The default value is 12. Three values, one for each voice, must be entered separated by commas, for instance: "24,24,24".
Please remember this option, as its "N" is not listed in the menu.

Waveforms / Arpeggios editor

Filter settings

Troubleshooting