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

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