"Randy Yates" <yates@[EMAIL PROTECTED]
> wrote in message
news:m3hcd2p3cc.fsf@[EMAIL PROTECTED]
> "Earl Kiosterud" <someone@[EMAIL PROTECTED]
> writes:
>
>> "Randy Yates" <yates@[EMAIL PROTECTED]
> wrote in message
news:m31w46svgx.fsf@[EMAIL PROTECTED]
>>> "Chronic Philharmonic" <karl.uppiano@[EMAIL PROTECTED]
> writes:
>>>
>>>> "Randy Yates" <yates@[EMAIL PROTECTED]
> wrote in message
>>>> news:m3k5hzj58s.fsf@[EMAIL PROTECTED]
>>>>> "Green Xenon [Radium]" <glucegen1@[EMAIL PROTECTED]
> writes:
>>>>>
>>>>>> 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?
>>>>>
>>>>> Yes.
>>>>
>>>> Well... AM radio is not immune to aliasing, although we rarely
encounter it
>>>> in practice
>>>
>>> How would AM ever alias?
>>> --
>>> % Randy Yates % "Midnight, on the water...
>>> %% Fuquay-Varina, NC % I saw... the ocean's daughter."
>>> %%% 919-577-9882 % 'Can't Get It Out Of My Head'
>>> %%%% <yates@[EMAIL PROTECTED]
> % *El Dorado*, Electric Light
Orchestra
>>> http://www.digitalsignallabs.com
>>
>> An AM radio station operating at 1 MHz would have to have the baseband
limited to 500
>> KHz,
>> or aliases would appear. For example a 600 KHz baseband component
would appear at 400
>> KHz.
>> This obviously ain't gonna happen in any real world situation, but
imagine an AM station
>> operating at 10 KHz. It would have to keep the audio limited to below
it's Nyquist
>> frequency of 5 Khz.
>>
>> I'm using the term Nyquist frequency somewhat incorrectly, as it
technically applies to
>> sampled systems. But it still works, and we know what we mean! :)
>
> Hi Earl,
>
> I see what you mean now. I'd say that comes as close to an "analog
> alias" as you'll get. It is mathematically almost the same thing. (I say
> almost since the analog system involves only two Dirac delta functions
> while a digital one involves an infinite number.)
>
> But I really don't think this is what was in Radium's mind when he
> asked.
> --
> % Randy Yates % "...the answer lies within your soul
> %% Fuquay-Varina, NC % 'cause no one knows which side
> %%% 919-577-9882 % the coin will fall."
> %%%% <yates@[EMAIL PROTECTED]
> % 'Big Wheels', *Out of the Blue*, ELO
> http://www.digitalsignallabs.com
Hi Randy,
Oops. I spoke too soon. It occurred to me while I was working in the
yard. A 10KHz AM
station could modulate to 10 KHz. As Don pointed out in another thread,
there's no baseband
in the modulator output, so there's no concern about the difference
component (sideband)
overlapping with it, as there is with sampling. But if you try to
modulate above 10K, the
difference product ends up negative, and folds into the band. 12KHz
modulation would
produce a component at 2 KHz. I don't know of a demodulator that could
properly sort that
out. I've always considered that an alias, but it's not the same as in a
sampling system,
such as when the difference component creeps downward past Nyquist as the
baseband creeps
upwards past it.
I realize I've probably been misusing the term "alias." When in Rome. :)
--
Earl
|
