Filthy Casual

Entry #002 - Unshielded CAT5/6 cabling and Digital Audio

By PodeCoet - Sunday, January 11th 2014 @ 00:01:01 ADST

UPDATE: Made it to Hack-A-Day's Fail of the week!



I wrote this post mainly for entertainment value (and I was deseperate to try my new amp with whatever cable I had lying around)



I could have said nothing, but I figured someone out there is bound to try this exact same thing; might as well save them the headache.

TL;DR / Page Summary for the impatient / Spoilers

Don't use Ethernet cable for Digital Audio. It'll appear to work at first, then piss you off by only intermittently working later.

I've acquired this fancy-schmancy KRF-V6200D Hi-Fi Amplifier from a friend at work (who totally doesn't specialize in street pharmaceuticals):

This is a HUGE upgrade for me, as for more than a decade, I've used a car amplifier attached to a Laboratory PSU for my home HiFi needs (limited to Stereo sound):

Now I faced a problem - I needed to get 5.1 channels of Audio from my PC, to the amplifier, which is seven metres away underneath my plasma television

I already have a HDMI feed directly to the TV, so you'd think I could just use HDMI audio, and then the TV's SPDIF optical audio output to feed the amplifier, right?

WRONG.



...Well, kind of right.

The TV is a 60" Panasonic Plasma from 2007 (ie: old enough to have been installed on Noah's Ark, but I like it) - It does support HDMI audio, but only stereo, so the optical audio output only contains two channels of audio.

It's a long weekend, late in the evening, everything is closed, I'm overly excited, and I want this to work today so that I can bask in glorious 5.1 channel audio

The most viable solution would be to run a shielded seven-metre coaxial lead from the digital/coaxial output on my PC, to the digital/coaxial input on the amplifier:

Slight problem, I don't have any coax left. What I do have is loads of unshielded CAT-6 cable, which is still kind of shielded, right guys?

Something-something twisted-pair, single-sideband, standing-wave black magic, and gigabits of datas will surely work for comparitively slow audio... Right?

FUCK IT. I can take failure. Let's run this shit.

So I get to soldering up a patch lead, careful of course to use the 'twistiest' pair for the greatest shield-action, and a bit of heatshrink so as not to crimp through the insulation:

I run the cable across the floor roughly, plug everything in, and the amplifier auto-detects 5.1 channels of audio from the DVD/6CH (coaxial audio) input:

Audio! I have audio! 5.1 Channels of audio, no-less. COAX, SCHMOAX!

I neaten up the cable by throwing it behind the couch, make some black tea, prepare a hookah, and sit down to binge-watch me some Arrow

A few minutes in, I noticed a relay in the amplifier clicked, and the audio disappeared for a few of seconds. Something transient, I figure, probably gamma rays from the freemasons.

A further few minutes in, again. And again.

... SON OF A BITCH SOLD ME A FAULTY AMP!

Or so I thought. I had the split-system aircon switched on, and it's an old clunker. There's probably some transient BS induced onto the line from the compressor switching in and out

Figured I'd pry myself off of the couch and have a wank run some tests, to see what else could induce the fault:

Light switch: YES

Stun-Baton: YES

Soldering station: YES

Rubbing two sticks together: YES

Using the word Arduino in a sentence: YES



Time to break out the trusty port-a-cro, and look at the signal:

...And what the hell is this shit?! Turns out my ground clip was loose, sorry -

The signal looks clean(ish), except for some ringing. Let's set it to one-shot mode, increase the trigger level and flick some light switches:

Sigh. Look at those spikes. The amp goes into standby mode when the digital audio signal drops, which explains the clicky relays.

Seems this was a bad idea all-along. I'll have to wait til tomorrow to run some tri or quad-shield coax instead.

EDIT: It's tomorrow, and I bummed some Tri-Shield coax off a mate who does Satellite installs. No surprise that it works perfectly now, despite flicking the shit out of every switch I could find!

Fun fact: Foxtel Approved Tri-Shield coax appears to have a copper cladded steel core, and an unsolderable (probably steel'ish) shield.





See you in the next post!



-PodeCoet

