[sdiy] Reconstruction Filters

[James Hughes]

This excerpt from the Shruthi-1 site talks about their filtering strategy:
mutable-instruments.net/static/documentation/smr4_analysis.pdf

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All input signals, be it the raw oscillators signal or the control
voltages, are 0 / 5V PWM signals
with a carrier frequency of 20M Hz = 39062Hz/512.

This implies that the control signals have to be smoothed. You
shouldn’t feel bad about it: the
MCU generates the control signals at a “slow" rate of 976Hz anyway, so
a simple 1-pole low-pass
with a cutoff frequency of 1.25kHz kills the PWM carrier by 30dB while
still tracking fast enough
the fastest transitions the MCU can create.

The oscillators signal also has this 39kHz carrier. It is attenuated
by the main 4-pole LPF itself,
and by an extra 1-pole LPF with a fixed cutoff frequency nearing 20kHz
at the final output buffer.
In the worst case (cutoff set to its maximum value), the carrier is
thus attenuated by 30dB. In
most cases, however, the cutoff frequency is set to a lower value, and
the carrier is attenuated more
strongly. Anti-aliasing filters at the input of your soundcard,
speakers with a limited bandwidth, or
your ears will be doing the rest of the filtering.

-------

Does that help? Synth noob here, so disregard if it looks like I have
no idea what I'm talking about (I don't).

Best,
James

On Sun, Feb 13, 2011 at 1:52 PM, Matthew Smith <matt at smiffytech.com> wrote:
>
> Quoth Tristan at 13/02/11 22:40...
>>
>> If you don't want to dynamically change the filter frequency after the note has started, only require a small number of frequencies and/or a steep filter with a high number of poles, then then switching resistors may be acceptable. But for something like a 4 pole LPF the VCF and DAC might actually work out to be less complex and much more flexible.
>
> I'm actually treating reconstruction filtering and any 'creative' filtering separately - the reconstruction filter being part of the DCO, module. Just to tidy-up the waveform before it goes to the VCF module for fancy stuff.
>
> --
> Matthew Smith
>
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