[sdiy] [synth-diy] lock-in amplifiers

cheater00 . cheater00 at gmail.com
Sun Jan 26 17:14:10 CET 2014


On Sun, Jan 26, 2014 at 4:50 PM, cheater00 . <cheater00 at gmail.com> wrote:
> Hi guys,
> I've recently stumbled upon the concept of a lock-in amplifier.
> Basically, it is a way to recover an AM signal from a carrier, even if
> the carrier is buried deep in noise.
>
> what you get is something like a filter that's got a Q of about 10000.
> A normal band pass of Q=100 is considered absurdly steep.
>
> This helps in two ways. For one thing, you can uncover this carrier
> frequency from within a huge amount of noise.
>
> Also, it's much easier to build a precise oscillator than it is to
> build a precise high-Q filter. You can do it in digital without being
> penalized by terrible things like aliasing or decimation.
>
> Cheers,
> D.

A natural application for this would be a resonator. So you recover
the amplitude envelope of a specific, very narrow frequency, within a
complex sound. Because of what was said in the previous email (the
low-pass filter needs to be able to easily disjoin the near-0
frequencies from any frequencies derived from where f0!=f), the
amplitude envelope needs to be much slower than the frequency in
question.

If you need to be able to do quick amplitudes at low frequencies, you
might need to up-convert the audio first, so that what was e.g. 20Hz
is now 20020 Hz - and therefore even an amplitude envelope at 19 Hz is
going to be easy to recover.. at least in theory.

Anyways, what this approach gives you is that you don't need to use
steep Q filters any more. Instead, you use a precision frequency and
precision multipliers. Then, you can use the recovered amplitude
envelope to modulate a sine wave at that frequency, or at another
frequency (e.g. an octave up), interval (e.g. using the frequency + a
6th is pretty cool), or a chord, and the waveforms can be sine waves,
or other waves.

Since none of the oscillators need to change pitch or amplitude
abruptly, it's possible to create them in digital in such a way that
makes them sound as pure as analog waves. Amplitude modulation and
mixing should be done in analog. You need a lot of DACs, but they can
be low quality, since bit depth necessarily matter for repetitive
waves. This means you can use codec or multi-output DAC chips which
are inexpensive.

Cheers,
D.



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