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

Joseph Fry joe at thefrys.com
Fri Aug 30 14:11:52 UTC 2013


>> In which case the cable is faulty.  By definition, an HDMI cable must
>> put out a signal that meets the HDMI specification.  If it puts out a
>> corrupt signal, it is not an HDMI cable.  That is the fundamental
>> difference between an analogue signal and a digital one.  There is no
>> such thing as a degraded or corrupt digital signal - that is a faulty
>> digital signal.
>
> This is still an overly simplified view..
> While digital, it's still carried over an electric, fundamentally analog signal.
>
> so yes, there can be losses.
>
> certainly more so with spdif or dvi, where there's nothing in regards
> to packet loss detection

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".

The reason someone would favor toslink is, as stated previously, for
electrical isolation.  High end audiophile systems get much of their
credibility due to having extremely clean power supplies.  Connecting
a grounded conductor cable between devices WILL cause some level of
ground noise at the output stage.  Using a pure optical transport
ensures that the analog output stage remains isolated and all of that
effort to build clean power supplies isn't ruined by simply connecting
your source component.

The other advantage of toslink.... is that the signal will not be
degraded by emf.  Any electronic signal (not optical), be it on a
cable or within your CPU, is susceptible to noise.  If that noise is
strong enough to make it difficult for the receiver to correctly
determine what's a 0 and what's a 1... then it may guess wrong and
cause an artifact.  Optical signals are not susceptible to electrical
noise, so you can run them right next to any unshielded cable without
concern.  This can be important in recording studios (for example)
where the number of electronic devices in a small space makes it
impossible to route cables without them running along side a noisy
power strip, unshielded speaker and power cables.

Ironically, when you consider that fiber optics have much greater
range than copper transmission cables, toslink has a shorter cable
length rating than coxial spdif 20ft vs 30ft due to optical
attenuation.  Long run fiber signals use a completely different
technique of putting a signal on the fiber than toslink.  Toslink
operates similarly to using a flashlight to send morse code down a
hallway, it doesn't really take advantage of the internal reflective
properties of the cable.  This makes toslink extremely cheap to
implement, but greatly shortens its range.


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