"Don Pearce" <nospam@[EMAIL PROTECTED]
> wrote in message
news:482caf60.46834968@[EMAIL PROTECTED]
(snip)
>>
>>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
Don
The reason I posed my little AM modulator, using a multiplier with some 0
Hz added to the
44.1K carrier was to show that it's exactly the same in this continuous
(non-sampled) system
as the aliasing in CD audio. The result is exactly the same in either
system, and my point
is that it's for the same reasons. It's all about modulation products. I
don't understand
why you say it can happen only in a sampling system. Could you elaborate?
Do you agree that the output of the DAC will contain the baseband
components, and a set of
sidebands around 44.1K patterned like the baseband? (Also around 88.2K,
etc.)
--
Regards from soon-to-be-raining -- again -- Virginia Beach,
Earl
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