On Thu, 15 May 2008 21:38:09 GMT, "Earl Kiosterud"
<someone@[EMAIL PROTECTED]
> wrote:
>
>
>"Don Pearce" <nospam@[EMAIL PROTECTED]
> wrote in message
news:482e5bea.25475953@[EMAIL PROTECTED]
>(snip)
>>
>> 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
>>
>>
>> --
>> Pearce Consulting
>> http://www.pearce.uk.com
>
>
>Hey Don,
>
>Here's how I see this. In your first case, the 41.1K and 47.1 K
components are sidebands of
>the 44.1K carrier. In the second case those same components are the
(folded) sidebands (or
>negative-frequency, which is perfectly valid)) of a 3 KHz carrier. The
result is the
>same -- they're the same sum and difference frequencies -- they don't
really care who's
>modulating whom! :) A sum frequency is a sum, and a difference is a
difference. The only
>real difference is if you add 0 Hz (DC offset) to either the audio or to
the carrier. In
>the case of the former, you get the 44.1K carrier component as well as
the sidebands around
>it, and in the latter you get baseband audio as well as the same
sidebands around 44.1K.
>And that's the result of the sum and difference frequencies too. Ain't
no getting away from
>it!
>
Yes - that is how I see it.
>You mentioned generating suppressed-carrier generation, by filtering out
the 3K component.
>I think you can just multiply the signal with the baseband, adding 0 Hz
(DC) to neither, and
>get that. You have to use a four-quadrant multiplier (handles both
negative and positive
>signals). You should get no carrier and no baseband in the output --
just the sidebands.
>ATSC TV adds a little DC to get a little bit of carrier (if I understand
it correctly, and
>I'm not at all sure I do) for recovery. At least that's one way of
implementing it.
>
Right - I've done this with a double balanced mixer and a bit of
adjustable DC bias to null the carrier completely (more or less).
>Forgive my excess of clarification comments in all this -- I don't mean
to be pedantic, but
>I try to be as clear as possible, and I think they might be useful to
some of those who
>might be following this thread.
>
No, that's fine. This can all get very complicated unless you have
some simple way of visualising it.
>The terms "mirror" and "image," I think, at least as used in superhet
radio, refer to such
>things as a modulation product that is the sum frequency, where only the
difference was
>wanted. It's a mirror around the local oscillator frequency. But in a
more general sense,
>any modulation product that winds up in your frequency band of interest
(audio, in our case)
>is an ambiguous component, in that we can't distinguish it from a real
signal component.
>I've always thought of any as an alias.
>
An alias is a specific kind of image with a specific set of
properties. It can only occur as a result of discrete-time sampling.
All the stuff that comes of a normal modulator is an image - or an
intermod product if we have any non-linearity.
>Here's my main thrust: I think the aliases we get in sampled audio where
the audio goes
>above Nyquist are simply the difference modulation products, which creep
into our baseband
>if the audio goes above Nyquist. As the audio gets higher in frequency,
the sidebands get
>lower, from 44.1K. Nyquist is simply the midpoint, where they meet.
Sampling is a case of
>general modulation theory.
>
>Hoping to hear your comments.
Well, since sampling is a form of modulation, I have to agree. But as
I say it is a form of modulation product that can only exist in a
sampled system, so I think it is fair to treat it as a special case.
Also it is occurring in a situation we would not usually think of as
modulation (even though it is). Anywhere two signals are multiplied
(an ADC multiplies the audio by the sampling pulse) there will be
modulation products.
d
--
Pearce Consulting
http://www.pearce.uk.com
|
