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
"Don Pearce" <nospam@[EMAIL PROTECTED]
> wrote in message
news:9K2dnS2KKdBNgLTVnZ2dnUVZ8rCdnZ2d@[EMAIL PROTECTED]
> Earl Kiosterud wrote:
>> "Green Xenon [Radium]" <glucegen1@[EMAIL PROTECTED]
> wrote in message
>> news:4828deb3$0$5698$4c368faf@[EMAIL PROTECTED]
>>> Hi:
>>>
>>> Is it true that purely-analog audio devices [such as analog cassette,
AM radio, and the
>>> pre-digital telephone systems*] are immune to aliasing?
>>>
>>> *By pre-digital telephone systems, I am referring to how these systems
operated prior to
>>> using digital technology. Nowadays, many analog phone systems do use
DSP somewhere along
>>> the line. Similar applies to the analog AM radio, it used to be just
analog but now it
>>> utilizes some amount of DSP indirectly.
>>>
>>> Thanks,
>>>
>>> Radium
>>
>> The pre-digital telephone system used frequency-division multiplexing,
each voice channel
>> in a 4 KHz slot. Nyquist was 4 KHz. Without pre-filtering, they'd
have the very
>> aliasing that digital systems can have. Without post-filtering, you'd
hear sidebands
>> around 8 KHz. No difference, except the sampling frequency.
>
> No that isn't aliasing, it is adjacent channel interference - a totally
different thing.
> Aliasing is a product of sampling, an ambiguation of the signal caused
by the fact that it
> is sampled at discrete points and whatever is in between those points
must be filled in
> with assumptions about its nature. In audio, the assumption is generally
that the signal
> between those points contains the lowest possible frequency solution
below the Nyquist
> frequency. But the other solutions, involving higher frequencies are
equally valid from a
> mathematical point of view - they are the alias solutions.
>
> d
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