Hammond T-500 Electromechanical Synthesizer

 My evaluation board has arrived. I'm new to the Analog Devices evaluation ecosystem, and this is evidenced by my purchase of an evaluation board that is meant to be used with their demo platform.  The platform costs about $300 on top of the $120 I paid for the ADC eval board. I wish they'd do a razor/razorblade scheme here, but instead I will just put off the $300 investment and make do for now.  I'm really impressed with the quality of both this board and the its documentation. There are like four external power supply options, two connector types for each input, about two dozen solder point jumpers for various options, some filters, all kinds of shit. Fortunately you're not locked into the AD demo platform, and there is a barrel jack for power (selectable by moving a 0 Ohm resistor jumper) and a pin header to plug in your SPI and other control/data pins. I will be taking this route until I feel rich enough to drop $300 for the platform.  This is probably wayyyy to...

This blog is a space for me to store project notes, photos, data, and ideas. Some ideas will be silly, many of the notes will be poorly organized. Hopefully it will improve with practice. My goal here is to allow for as wide a range of sounds as possible with a spinet TWG. I intend to build a system that offers significant per-pitch tone control with FPGA-based digital signal processing. Overall system DSP should enable a wide range of effects and further tone-shaping capability.  I have removed most of the extraneous portion of this organ. The cabinet is being repurposed as a table for a small milling machine, the drum machine is off to the side for a later project (that's for you, Taylor), and the rest of the horseshit VLSI circuit boards and wiring is in the damn trash. Someone salvaged the two keyboards, good for them, and I'm going to keep the two transformers and spring reverb tank. All that's left are the motor, the TWG, and the vibrato scanner (scanner may get trash...

The Hammond organ sound owes its popularity to almost exactly one thing: its presence in Black churches. Gospel music became popular in the US in the 1930s, and churches that couldn't afford a pipe organ had to make do with the next best thing, the Hammond. Gospel organ players would play the Hammond in church on Sunday, after playing in a club Saturday night. As above, so below. The music played in the clubs became rock and roll, and the next thing you know the budget pipe organ sound was in Procol Harum songs.  Laurens Hammond had intended to create a replacement for the pipe organ, and any characteristic of the sound that deviated from this goal was a flaw to be eradicated. Many of these flaws became integral parts of the beloved Hammond sound, but our friend Laurens didn't care. He hated the Leslie, he hated key click, he hated crosstalk. Even worse, in his obsession with eliminating these imperfections, he added circuits that not only failed to entirely eliminate key click...

 The web-literate reader can easily find the objective answer to this question. I will therefore provide one of many subjective answers, not entirely unique, but interesting enough to me that I feel like writing it down. There are three legitimate, grown-up Hammond organ models. They are the B3, A3, and, C3. There are minor variants of those three with different numbers, usually signifying a different cabinet material. They are similar enough and all old enough that differences in sound between individual units can be larger than those between models. The B3 is the most mobile of the three, and is thus the most popular, and epitomizes the Hammond sound. If you hear that someone plays an organ, and they don't sigh after you ask them what type, this is what they play. These models may be purchased regularly in various states of repair for $2k to 6k. The Leslie rotary speaker sound is nearly inseparable from the Hammond sound. More information to follow. The second tier of Hammond org...

Non-traditional harmonics schemes for the organ could be interesting. Without looking at the tone generator charts, I wonder if a square wave could be approximated in the lower register? What sort of interesting additive synthesis is possible with 88ish pitches?  Could pitches be shifted digitally to make (nearly) arbitrary waveforms a la the Yamaha DX7? Picture a DX7 algorithm scheme with Hammond tones. There is a lot of flexibility in this potential system. Also, don't forget using subtractive synthesis on top of the final additive signal. It would be nearly trivial to add a voltage controlled ASDR and filter section. Some of this could be closely approximated digitally as well, I'm certain that Korg or Moog filter clone source code is available. Keyboard tracking, aftertouch, and velocity sensitivity could be added as well, and each control signal could be routed (via a matrix) to any digital (or analog) parameter.  This is getting ambitious, plenty of fun available. 

Naked Tonewheel Generator Fused & Switched IEC Power Input T-500 Filter Circuit Diagrams Look, It's Safe!  

Initial Project Outline Phase 1 - Testing Analog Devices AD7771 8-channel ADC with Teensy 4.1 Microcontroller Analysis of TWG (Tonewheel Generator) output signals Pre-ADC conditioning SPI control of ADC ADC signal output to Teensy via SPI Digital mixer on Teensy (note selection & amplitude control) Analog output from Teensy (mostly for fun) Matlab analysis of audio signals Determine feasibility of final mixing & stereo/mono effects on Teensy Phase 2 - Testing Alhambra II FPGA as Digital Signal Processor Determine multi-channel capacity of FPGA Basic DSP on FPGA (LP, BP & HP filters) Digital mixer on FPGA Advanced DSP on FPGA (effects, multiple filters, see DAFX book) Determine feasibility of final mixing & stereo/mono effects on FPGA Phase 3 - Design of 88-Channel ADC Board Likely to include 11 each 8-channel ADC chips 11 FPGAs? How many channels can one FPGA handle?  Phase 4 - MIDI control via Teensy Note-on messages Effects selection and control Phase 5 - Analog O...