Can PIC microcontroller = TOS?

[Unkie Al]

The old Top Octave Synthesizer chips from 70s-80s vintage poly synths are pretty much a relic now, with remaining new-old stock approaching $40-50 per chip.

I have only a rudimentary knowledge of PIC microcontrollers, but it seems to me that one can be programmed to perform the functions necessary to generate a complete chromatic octave from a single frequency input (with division such as x/239, x/301 etc, where x=clock frequency). Follow that up with a pile of flipflops for further octave division, and its like having a PAIA Organtua dropped into your lap.

Has anyone explored the potential of using a PIC for this purpose? I have done numerous searches on the web but have come up with nothing. It's tempting, especially because the stability would be rock-solid, and a PIC would be $10 or less, compared to the cost of the afore-mentioned 25-year old TOS chip.

If you or someone you know is doing something along these lines, I'd be grateful to know more.

U.A.

[yusynth]

Oldcrow who is known for some of his reverse engineering of synth modules had a very old project (1997) with a PIC16C54 to emulate a top octave generator.

You can still find it here :http://chip.aeug.org/oct54all.asm and some other PIC projects for music :

http://chip.aeug.org/other.html

[Unkie Al]

I am in receipt of an email from Jarek Ziembicki - the developer of the AVRSYNTH described well on Laurie Biddolph's pages - and he suggests the use of an FPGA (field-programmable gate array) to design an alternative to a top-octave generator.

I am not an E.E., so until this morning had never heard of an FPGA. But I looked up a PDF of the Atmel line of FPGAs, and damn , the thing has some promise.

There goes at least a month of winter weekends shot trying to figure this thing out

AP

[DrJustice]

Hi Al!

to electro-music.com!

FPGA's are indeed awesome. Here's a couple of forum threads on the subject: one here, and another here.

Another device type that may be suited for this is a CPLD (Complex Programmable Logic device), which is effectively a smaller cousin of the FPGA. The main differentiating features are that CPLD's have a built in non volatile memory for the configuration data, they're usually very power efficient and they haven't got as many gates as FPGA's. We are starting to see the first FPGA's with built configuration memory now, and some are quite power efficient too, thus the boundary between CPLD and FPGA may be getting less clear in the future.

Other than that, I reckon a PIC or an SX or similar high speed microcontroller will do fine as a TOS.

DJ
--

[yerpa58]

have you checked:

http://chip.aeug.org/oct54all.asm

also good stuff on:

http://chip.aeug.org/other.html

[blue hell]

yerpa58

looks like good links.

[yusynth]

[blue hell]

Oops, sorry, those earlier ones look good as well

[yerpa58]

oops my bad. Has anybody tried the PIC version? I once took four of the old Mostek chips and wired them directly thru a 37-key keyboard. No other electronics, pressing a key just ran the sqare wave right from the TOS chip to a wire bus that went to an output jack for connection to an amplifier. A 555 timer clocked the TOS chips, and had a potentiometer for overall "tuning". Later on, I might have added a transformer to couple the over-hot output to an amp. Anyway, it sounded like a hurdy-gurdy, no envelope, no velocity info. The coolest sound I ever got out of it was pressing down all 37 keys at once and then thru a wah-wah pedal while turning the pot on the 555. It made a sound I describe as "chrome (chromatic) wind". Now that you have brought back the memories, I'm thinking of doing it again with the PIC.

[Sam_Zen]

I remember having a toy keyboard, which I opened of course, finding a one-bit PIC-processor.
It could indeed produce 'harmonic' notes.

[Tony Deff]

The PIC series has had a lot of promotional spin, such that if you announced you'd built an entire electronic organ into a PIC and just needed interfacing to a keyboard, stops and speakers, someone, somewhere, would believe it.

As I understand the problem, the timers in most of the PICs are pretty basic, designed for a simple 8-bit time-out. This means that there is a "latency" or timing over-head whereby time must be spent polling the timer to see if it has finished, else time must be spent reacting to an interrupt, toggling an output pin and resetting the timer. They say to you "By the way, did you know your timer has over-flowed?" Presumably, that's why they're not used in missions to Mars, where a little more precision is required.

Some micro-controllers (DS89C420, AT89S8252) have 16-bit auto-reload and a direct output which allows you to set-up the generation of a square-wave, then to go away and do something else.

With a PIC, timing over-heads must be carefully calculated and compensated for, else you will have pretty poor precision at the higher frequencies. Talking of frequencies, the timers have to be synchronised by effectively dividing the clock speed by 4, so you may think you're running at an impressive 16MHz.

To emulate the achievement of those old-fashioned TOG (TOS) chips, running at 4.25 MHz, you would need a clock-rate 8 times this, because you have to toggle the output twice to get a whole output cycle.

The programs that claim to generate the top-octave are open enough about it not actually being the octave required to get the 8th & 16th harmonics of the top keyboard octave, but it may not be obvious that the outputs will have poor accuracy and frequency jitter. The world works like this:

Suppose that we are trying to derive outputs that toggle every 508, 538, 570, 604, 640 (etc. etc.) clock cycles. We could theoretically toggle an output bit on the count of 508, then another output bit 30 counts later, then yet another output bit 32 counts later etc. etc.

Life is not that simple. You cannot do that next time around or you'll simply get 12 phase-shifted outputs. Sooner or later, we will be required to toggle two or more outputs within one or two clock cycles, or perhaps simultaneously. The first co-incidence will be after 508×538 clocks, then you get all sorts of pseudo-random permutations; 508×570, 538×570, 570×640×761

The PIC program that attempts this impossibility merely responds to a time-out as best it can and determines that several more pins are overdue for toggling. Pulse-width jitter.

The PIC16F877 has one 16-bit timer/counter and two PWM (pulse-width modulation) modules.
This offers some interesting possibilities, in that by controlling pulse-width we get some even harmonics and thus may not need as high a frequency as with square-waves.

Application Notes AN539 & AN564 may be downloaded for data on setting-up & using PWM.
Application Note AN654 implements PWM by software.
Application Note AN543 has data on Tone Generation.

The PIC16C781/782 also has a PWM mode, plus an 8-bit DAC, offering the interesting possibilities of generating custom waveforms (but don't get your hopes up too high).

___________________

Another possibility is to use the PIC's consistent instruction-timing to implement a software frequency generator. If you have a 16MHz. clock, and implement a 955-instruction delay (which ends by toggling an output pin) then you will have a top-C (8372Hz) within an absolute pitch of 1-cent (± crystal error).

The equal-temperament scale to within 1-cent relative is obtained by instruction count loops of:

C 955
B 1012 (Ich glaube das ist H in Deutsch, si en Francais/Espanol/Italiano, h im Svenska/Norsk/Dansk)
Bb 1072 (Deutsch =B, Francais =si bémol, Svenska =b, Nederlands =bes)
A 1136
Ab 1203
G 1275
F# 1351
F 1431
E 1516
Eb 1606
D 1702
C# 1803

For a "programmable note-generator", this data can be put into a table and the note selected by the state of input pins. Admittedly, this is only one note (I thought about how to generate several notes, but it gave me a severe headache).

What about 12 identical PICs, and a cascading method that showed the PIC its place in the chain, so that the input code transposed the entire instrument?

___________________

Yet another possibility is to use the PICs for what they were intended: controllers!
(What a revolutionary idea! Wonder if anyone else has though of that?)
The 8253 chip has 3×16-bit counters (plus control register) that can be programmed as square-wave generators. Four of these plus a PIC would enable a dynamically-progammable top-octave (where you could change not just the key, but the entire tuning system!)

You might just assign two or three 8253 chips to 6 or 9 active voices, if that's enough polyphony for you. In the UK, Bardwell is selling-off these chips "for a song"!

[blue hell]

[Tony Deff]

[blue hell]

[Gordon2]

from topic :
http://electro-music.com/forum/post-427676.html#427676

Not using a pic but this might be interesting:
It's quite easy to use a very cheap chinese altera CPLD board (eg EPM1270t144) to make a top octave generator. In fact you can get most of the lower octaves as well from one board, even all free phase if you want. The biggest difficulty is getting your head around the (free) 2GB Quartus II software for programming. I use schematic entry as opposed to Verilog or VHDL entry.
You don't even need to use optimised divider ratios as the clock frequency is so high - 48MHz - that practically any ratios are accurate enough (ie accuracy better than 1 cent). The main limitation is the number of I/P and O/P pins. Would be great if a midi decoder could be included in the design - a bit beyond me!
Gordon Nudd