Re: Complex to 2x real - how?

dbd <dbd@[EMAIL PROTECTED]
> writes:

> On Oct 10, 6:15 am, Randy Yates <ya...@[EMAIL PROTECTED]
> wrote:
>> Jerry Avins <j...@[EMAIL PROTECTED]
> writes:
> ...
>>   2. Mix to new sample rate (2*Fs) / 4: this ****fts the
>>   spectrum by Fs / 2, so that what was at -Fs/2 to 0 is
>>   now at 0 to Fs / 2, and what was at 0 to Fs / 2 is
>>   now at Fs / 2 to Fs. Since the new sample rate is
>>   2*Fs, Fs is the highest frequency available.
>>
>>   This preserves the information that was in the
>>   negative frequencies of the old complex signal
>>   in the positive frequencies of the new real
>>   signal.
>> ...
>
>>
>> The bad thing about this approach is that it inverts
>> the spectrum, as seen by the step 2 explanation.
>>
>> %  Randy Yates
>
> Randy
>
> This doesn't invert the spectrum, it merely ****fts it linearly up in
> frequency. 

That's what I meant. "Invert" may not have been the best term for this
process, but my point still remains: it moves stuff around. For example,
DC is no longer DC.

> When used to demodulate an upper sideband this process
> follows a translation of the center of the sideband to DC and a
> desampling lowpass filter stage to reduce the bandwidth to match the
> bandwidth of the sideband. This complex to real process puts the
> entire sideband back above DC and allows use as an audio channel of
> the sideband content.

So this approach may or may not be a problem depending on the goal. I
didn't see the OP stating this was being used to demodulate a SSB AM
signal.
-- 
%  Randy Yates                  % "Remember the good old 1980's, when 
%% Fuquay-Varina, NC            %  things were so uncomplicated?"
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%%%% <yates@[EMAIL PROTECTED]
>           % *Time*, Electric Light Orchestra
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