[sdiy] Really Low, LFO

[Tom Wiltshire]

On 3 Apr 2010, at 02:10, cheater cheater wrote:

> Tom,
>
>> I agree with what others have said, it isn't hard to do *low*  
>> frequencies with digital.
>
> Well... depends on the waveshape. A sinewave is not trivial to get
> going: either you need to have a *huge* table of values (not going to
> happen) or need a good interpolator, or need an algorithmic way of
> deriving the sinewave.

For a sine wave specifically you can use a quarterwave table and take  
advantage of the symmetry. That increases the apparent size of the  
table by a factor of four straight off the bat.

A 'good' interpolator would be nice, but even linear interpolation is  
fine for sub-audio. It's not like the 'vocal' part of the audio range  
where the ear is very sensitive. Instead we're talking about things  
you *can't* hear. Consequently, you could set up an arbitrary  
waveshape of 256 (1024, 2048, you choose) points and linearly  
interpolate between them and no-one will ever know the difference.

> But given the very low sampling rate that you
> can get away with (you only need 2x freq of LFO... so not a lot!), you
> can make those algorithms as good as you want.

Yes, that's it exactly. We've got *ages* between samples, so we can  
do whatever calculations we like! Maybe we *could* even do a full- 
blown FIR filter interpolation on a PIC given the time we've got  
available - nuts, but probably possible.

> The problem is that
> it's fairly boring math that, chances are, you won't use anywhere else
> in life - this is coming from a mathematician who normally finds
> boring stuff fascinating - so, if someone just wants an lfo with basic
> shape, I strongly suggest not reinventing the wheel and having a
> *good* look at specialized function generators

There are much better ways of reducing the frequency of my LFO, but  
that was why I suggested slashing a factor of 256 out of the sample  
rate. It involves changing about one line of code and it's done. If  
you filtered the resulting 76Hz PWM output at somewhere down below  
1Hz, you'd have a truly infrasonic LFO without any serious work or  
expensive capacitors/op-amps.

Regards,
Tom