[sdiy] Is everything digital?

[Magnus Danielson]

From: The Old Crow <oldcrow at oldcrows.net>
Subject: Re: [sdiy] Is everything digital?
Date: Sat, 14 May 2005 22:24:15 -0400 (EDT)
Message-ID: <Pine.LNX.4.44.0505142209580.3063-100000 at crowsnest.oldcrows.net>

> 
>   My only other comment on this topic is how I perceive digital vs. 
> analog:
> 
>   discrete time domain = digital
> 
>   continuous time domain = analog

Nope, that's not it!

You can have discrete time domain in pure analogue too. There's bunches of
switch capacitor filters out there in real life to prove me right.

There is actually two independent (ortogonal) things happening:

* Continous vs. Discrete time scale
* Continous vs. Quantified signal level scale

In traditional "analogue" you have continous time and signal level.
In traditional "digital" you have discrete time and signal level.

Put a signal through a comparator, a schmitt trigger or quantizer and you have
continous time scale but quantized signal level. Not quite "digital".

Put a signal through a sample and hold stage, and you have continous signal
level scale, but discrete time scale. Not quite "digital".

Infact, you can have _both_ discrete time and signal level scale, without
really be "digital". In "digital" we transport signals in binary (or on odd
occasions other modulations forms like trinary), but many times we define the
signal as being one or more of the transporting bits, and for continous signals
(sampled) we often use Pulse Code Modulation (PCM).

I strongly recommend the reading of Shannons article on digital telephony from
1948. Briliant motivation for the concept. In the Appendix the now called
"Nyquist theormem" is being prooved, but with a error in the proof I might add.

Cheers,
Magnus