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

 dpierce.cartchunk.org@[EMAIL PROTECTED]
 wrote:
> On May 15, 6:04 pm, Don Pearce <nos...@[EMAIL PROTECTED]
> wrote:
>> geoff wrote:
>>> Don Pearce wrote:
>>>> 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.
>>> That was the analogy I was drawing with my "LSB/USB" post.  That the
sum and
>>> difference frequencies are in som way analogous to the frequency
products
>>> from aliasing.
>>> geoff
>> Yes they are analogous in that a sampling device will also produce
them.
>> The difference is that the sampler also produces a comb of them up to
>> infinity. You could say that the analogue image products are the limit
>> case of aliasing when the dead space approaches zero.
> 
> The reason is very simple. IN the case of AM modulation,
> the carrier is a sine wave, which has but a single component,
> the fundamental of the carrier.
> 
> When "sampling," in the context you're using it, the
> "carrier is actually a train of narrow pulses. Look at
> it's specturm: it's a series of components evenly
> space out to infinity (assuming 0 rise and fall time),
> so you have the images spread out to infinity as
> well.
> 
> Remember that what we are talking about "sampling"
> is often also referred to as "pulse code modulation,"
> or PCM.
> 
> It's exactly the same (save that the generated "sidebands
> are reallu "images," not "aliases"
> 
> The reason you have aliases with "sampling (or modulating
> a pulse train) is because of all those other components:
> something far out of band will also generate an infinite
> number of images, which can fall back or "alias" into
> your basebnand and thus be indistiguishable from a
> true base-band signal.
> 
> Can it happen in an all-anolg system? Sure, just come up
> with a carrier that's a sufficiently comples waveform.
> 
> Condiser, for example, your carrier having TWO components:
> a sine at 44.1 kHz and one at 88.2 kHz. Now, modulate that
> with a signal at. oh 88.2+-43 kHz, Where do the sidebands
> end up?!

That's another way of looking at it.

d