[sdiy] [synth-diy] Frequency shifter which rejects negative frequencies

[cheater00 .]

On Sun, Jan 26, 2014 at 6:10 PM, Magnus Danielson
<magnus at rubidium.dyndns.org> wrote:
> On 26/01/14 16:55, cheater00 . wrote:
>>
>> Hi guys,
>>
>> In a parallel thread, I have explained what lock-in amplifiers are and
>> how they work.
>> I have asked myself: what if rejecting negative and positive
>> frequencies, leaving only frequencies near DC, you could reject only
>> negative frequencies?
>>
>> I'm not sure, but I believe in the digital world you can do the
>> four-quadrant multiplication and then take a Fourier transform of
>> that. But I'm very hazy on the details. Can anyone confirm or deny?
>
>
> It works, if you have the phase quadrant of both signals.

I don't know what this means. Could you explain a bit more please?

> This is why you have an elaborate filter to achieve this for the audio
> signal.

Again.. not sure where to begin with that. Please bear* with me :-)

>> Is there an analog process that allows you to do this?
>
> Yes.

What is it? I'm not talking about high-pass filtering before the
mixing, if that's what you mean. That would give no advantage to just
a simple high-pass filter.

Cheers,
D.

* here's a very bad ascii art of a couple teddy bears for you:

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