[sdiy] Interesting article on top octave generators.

[Richie Burnett]

I was thinking about this some more, and the "slightly out" values in the 
Juno 106 voice board Look-Up-Table can't be an attempt to match the 
Railsback curve.  This is because the same table of "divider values" is used 
for all three of the oscillator octave settings (16', 8' and 4')  As you 
said the actual resulting pitch is shifted up or down an octave by patching 
in additional D-type flip-flops to divide the master clock by 2 or by for 
for the 8' and 16' settings respectively, (and the resistors feeding the 
integrators are adjusted accordingly to get the corresponding 1/2 or 1/4 
charging rate.)  This would just transpose the same tuning curve up or down 
by an octave wouldn't it?

-Richie,



-----Original Message----- 
From: Richie Burnett
Sent: Monday, January 6, 2025 7:16 PM
To: Gordonjcp ; synth-diy at synth-diy.org
Subject: Re: [sdiy] Interesting article on top octave generators.

That is a curious shape indeed.  I had imagined all of the pitches would be
centred around the 12-tone equal temperament scale with an increasing amount
of scatter due to quantisation of the timer periods towards the top end of
the pitch range!

That does *look* like they were deliberately trying to tune the instrument
to some standard other than 12-tone equal temperament.  As you said though
the detuning is very subtle... The opposite ends of the scale (x1.005 and
x0.999) are only 10.4 cents apart, so it would take good ears to notice this
in isolation.

-Richie,





-----Original Message----- 
From: Gordonjcp
Sent: Monday, January 6, 2025 10:53 AM
To: synth-diy at synth-diy.org
Subject: Re: [sdiy] Interesting article on top octave generators.

On Sat, Jan 04, 2025 at 09:05:11PM +0000, Mike Bryant wrote:
> I've always shipped my custom 'all digital since 1984' synths with 
> Railsback as one of the tuning options.  I know that it has definitely 
> been used on several film scores.
>

So, now I know what the term "Railsback" is.

Let me show you an interesting thing:

https://gjcp.net/images/junotuning.png

This is a comparison of the divider coefficients in the Juno 106 voice board
ROM with a "perfectly" tuned twelfth-root tuning.

It's kind of the same shape, isn't it? I notice that the lowest notes on a
Railsback tuning are way flatter and way sharper being anything up to 30
cents different, and I'm not convinced that I can hear a difference between
"ROM" tuning and "perfect" tuning.

I suppose that's another experiment to add to the pile, eh?

-- 
Gordonjcp

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