News:

If you have difficulty registering for an account on the forum please email antespam@gmail.com. In the question regarding the composer use just the surname, not including forenames Charles-Marie.

Main Menu

Top Octave Synthesizers

Started by Nuddy, January 19, 2016, 08:40:03 AM

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.

Nuddy

TOS - Top Octave Synthesizer (or TOG - generator)
I don't know if this has been mentioned before somewhere - but now that we don't need top octave synthesizers any
more*, they have become extremely easy to make! (*as using computers we can form the output waveforms directly etc
etc). The only use I can think of is maybe to repair or improve some old organs.
Presumably a TOS may be made also using computers, however playing around with CPLDs** I programmed a few.
(**CPLD - a form of FPGA, which contain hundreds of logic cells, but also containing a non volatile memory that quickly loads the
programming of the chip on turn on). Boards with the chip already mounted can be bought from China at under £20. Load
program in, finished!
A cheap programming 'connector' from the computer to the board is necessary. Also you have to get your head around the
2GB software as well - unless of course you have the program from somebody else.
You don't even have to use optimum lists of dividers*** as the frequencies the CPLDs can run at are so high that
practically any series will be accurate enough. ***As I used for my previous boards - the last one being a 8x10cm board
with 30 SOT HCMOS chips with outputs of 5V cosmos and 1:1 mark to space ratio.
This board and all the CPLD ones as well have accuracies better than one sixth of a cent.
Using the larger CPLD it is also possible to produce the next lower 7 octaves as well from the same board, ie all 96
notes from one small chip. There is even enough 'room' on the chip to make around 5 of the octaves free phase -
although I can't imagine that the very slight differences in the frequencies (accuracies still as above) can make any
difference to the sound. (Of course one could also program much bigger differences in, if one didn't adhere to the
accuracy requirement).
CPLDs used: Altera EPM570t144 and 1270t144. Altera Quartus software (free). Outputs 3.3V cosmos, 1:1MS.

David Pinnegar

Hi!

This is incredibly useful and helpful.

Could unequal temperaments be programmed in?

The reason for "free phase" is to avoid synchronicity that causes all vibrations to be locked together which then causes full buildup of full organ merely to add harmonics rather than full sound, and many electronic organs suffered from being underwhelming on full organ.

There has been much talk among members of EOCS (Electronic Organ Constructors Society) about programming PICs for this sort of thing but the chip that you're suggesting presumably has advantages in this direction?

Best wishes

David P

Nuddy

Hi David,
Yes, any temperament could be programmed in. I use schematic entry (as apposed to verilog or VHDL code) with the software which goes quite quickly if you already have one as a template. Just calculate the dividers necessary - the chinese boards have a 48 or 50MHz clock oscillator on board, so for example 50MHz/7040Hz gives the top A divider needed, and change the schematic as necessary. The programming then just takes seconds. Mostly try to select an even divider number so that the last stage can be a divide by two to get a 1:1 mark-space. (There is another trick if this isn't possible).
I don't know if this is easier than using a PIC. I know that some computers cause drop outs when they suddenly decide to do something else - not a problem with CPLDs obviously.
Just had a look at the free phase trial that I did - I had the 3520Hz (A) octave the same phase as the top 7040 octave. The next 3 octaves 1760/880/440 all slightly out of phase. I think probably there wasn't enough room then to put the remaining octaves in in this fashion. One would have to use a second board for the rest. (The biggest CPLD - EPM2210x256 - in this series doesn't seem to be available on a cheap board).
(One other possibility would be to use the smallest CPLD board - EPM240t100 - one for each octave)!
Regards,
Gordon