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

[Tom Wiltshire]

I think what you're describing might actually be fourier analysis looked at a different way.

You multiply by a specific frequency (your "precision frequency and precision multipliers") and then you can get the amplitude for that frequency.


On 26 Jan 2014, at 16:14, "cheater00 ." <cheater00 at gmail.com> wrote:

> 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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