[mythtv-users] One HDMI question from (Hijacked): HDMI sound issues

Joseph Fry joe at thefrys.com
Fri Aug 30 21:21:25 UTC 2013


>> First of all, most "electric" conductors don't carry "fundamentally
>> analog signals".  While there are certainly places where a digital
>> signal is encoded on an analog carrier frequency (modems)... most
>> digital interconnects are pure digital.  Essentially, if you hook a
>> o-scope to the lines in the cable you will see a nice square signal of
>> a fixed pulse rate and voltage differential.  This is no different
>> than the signal traveling between transistors in your CPU, other than
>> it having a much slower pulse clock and much higher voltage
>> differential, and I don't think you would argue that signals within a
>> CPU are "fundamentally analog".
>
> Actually, those "square" waves are anything but.  With a
> "real" scope, you will see a small ramp up, and usually an
> overshoot, and some (minor) ringing, etc.  Voltage does
> not change instantaneously (except in those "friction-less
> pulley" textbooks).  While one can model digital circuits
> as being on/off at lower speeds (even though there is a
> point at which those on/off transistors are in their linear
> region), at very high switching speeds what you have is
> an analog transmission line, with all the issues that
> implies.  Sure, the result is interpreted as a 0 or 1, but
> sometimes one is actually looking at the slope of the
> signal (ex. rising) to determine the next bit value.

Your splitting hairs, there is no such thing as a perfect square wave,
but anyone with any training in electronics still calls them square
waves.  In reality, even a transistor, the building block of the
digital age, cannot switch instantly and creates imperfect waveforms.

All digital circuits are susceptible to the same issues as an analog
one... it doesn't mean they are analog... they work off of a
completely different methodology.  In analog, the received data is
NEVER an exact duplicate of the transmitted data... in digital, if the
received data isn't an exact duplicate, than there is something wrong.

An analog signal has an infinate number of valid states, digital has
only two... just because both types of signals can be carried across
copper wire, and both can be effected by interference, does not mean
that both are "fundamentally analog"


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