[sdiy] Polyphonic issue revisited.

[Magnus Danielson]

From: "Byron G. Jacquot" <thescum at surfree.com>
Subject: Re: [sdiy] Polyphonic issue revisited.
Date: Sat, 13 Dec 2003 16:16:20 -0500
Message-ID: <1.5.4.32.20031213211620.00b0d97c at surfree.com>

> >> There is also the NCO which can have the best of both worlds (digital and 
> >> analog)....but, it also has its very own problems to solve as well.
> >
> >Can someone explain me what an NCO is?
> 
> Numerically Controller Oscillator.
> 
> Poke a tuning value into a register, and it sets a combination of integrator
> charge time, reset threshold & makeup-gain.  This generates the usual
> sawtooth, which can be further waveshaped for square, PWM, tri, etc.  The
> Roland Juno synths used this sort of setup.  Fairly easy to control from a
> microprocessor, with circuitry that's not too far from a garden variety VCO.

The NCO is made up of only an adder and a set of flip-flops acting as a
register. The adders feeds the inputs of the register, takes the outputs as one
of the input values and the free input is used for frequency control. The neat
thing about this setup is that it is both simple and has a linear frequency
control.
              fval
f    = f    * ----
 out    clk     N
               2

where N is the number of bits in register and adder, fval is the frequency
control value, fclk is the frequency of the clock of the register and naturally
the fout is the output frequency. Real handy thing this. Sits in the heart of
for example the PPG Wave 2.3 and the Commodore 64 SID-chip.

When you work in the digital domain, NCOs are really friendly kind of stuff.

Cheers,
Magnus - who accidentially enabled a switch-ASIC to play melodies... on a NCO!