Re: Are purely-analog audio devices immune to aliasing?

On Thu, 15 May 2008 15:42:44 GMT, "Earl Kiosterud"
<someone@[EMAIL PROTECTED]
> wrote:

>Hi Don,
>
>Suppose we set up an amplitude-modulation system using a multiplier,
except that we reverse 
>the carrier (44.1 KHz) and the baseband audio.  We offset the 44.1 KHz
signal (so it is 
>always positive) but not the baseband -- it's allowed to operate in two
quadrants of our 
>multiplier.  You could say that the carrier is modulating the baseband,
instead of the usual 
>other way around.  The resulting signal would include the baseband
components, and a set of 
>AM sidebands around 44.1KHz, images of the audio components, just as with
CD audio (except 
>there wouldn't be sidebands around 88.2 KHz, 132.3 KHz., etc).  If we
allow our audio to go 
>past 22.05 Khz, we'll have spurious stuff in our baseband frequency
range.  For example, an 
>audio component at 30 KHz would produce a component at 14.1 Khz.
>
>Isn't that the same spurious component (same frequency) we'd get with
aliasing in the case 
>of CD audio?  This system is continuous (the audio isn't sampled -- it's
allowed to change 
>continuously), yet we have a sort of a Nyquist frequency under which our
baseband must stay 
>in order to not get signal components in our output that are ambiguous.
>-- 
>Earl

Let me think about this! I'll do the maths later. I think you are
right that you will get stuff all over the place that you don't want,
but I think I would tend to call them images rather than aliases.

I also suspect that the use of a synchronous demodulator might let you
recover the signals (as you can recover modulation over 100% this
way), meaning that the signals aren't truly jumbled - they just appear
that way. Aliasing is truly there for ever once it happens - there is
no way back.

OK, let me change my mind. I've just used Mathcad to look at this. I
made two FFT spectra - one with 44.1k as the carrier and 3k as the
modulation and the other with 3k as the carrier and 44.1k as the
modulation.

For the first I see a carrier at 44.1kHz, and sidebands at 41.1 and
47.1kHz. Exactly as you expect.

For the second I see a carrier at 3kHz and sidebands at 44.1 and
47.1kHz. Which is exactly as you expect once you know what to expect
;-)

Which will do for me. It would even be a nice way to generate
suppressed carrier AM - just filter away the 3kHz when done
modulating.

d


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