Audio over cat6a cable?

My podcast-partner in crime, the mighty Hugh Waters asked me how often I run audio signals over twisted-pair data cable in media facility builds. Has has a customer who is eager to do it this way.

I’ve done it a few times and it’s fine with a few considerations.

1. Earthing is still important and since IT people have no idea about proper grounding it can be an issue. If it’s a cat6a install in a TV facility done by me I’d have no worries, but the average IT install might have issues. But as four twisted pairs with individual screens cat6a is ideal for audio. It’s the other parts of the facility I’d worry about,
2. RJ45s don’t have the same DC/LF performance as a good old B-gauge (or Bantam) connector; if the circuits are going to be patched often I’d be wary – mechanically they aren’t great next to traditional audio connectors, 
3. From a wiring perspective; how do you nicely terminate into XLRs from a piece of cat6a?
4. Track shuffling is hard,
5. AES – just fine. Cat6a has an impedance of ~100Ω per pair, ideal for twisted-pair AES. Same observations as 1, 2 & 3.

There you go. I did work at one facility where the engineer had got obsessed with structured cabling and did everything he could over cat6 – analogue video (via baluns), audio, RS422 etc. He liked the idea that you could patch an offline i/o with two RJ45s (stereo i/o on one and video i/o and remote on the other). It didn’t work well and I put in proper cabling and patching after a year.

Sometimes convenience blinds you to fitness for purpose.

Earth Leakage - in water!

I recently bought an Agilent U1191A clamp-meter. This is a piece of test equipment that can measure current flowing in a conductor without having to break the circuit (how you would if all you had was a digital multimeter). The jaws physically couple around the conductor in question and by induction you can measure the electrical current flowing in the conductor. 

Clamp meters have moved on somewhat since I last had to buy one (a Fluke; sometime in the late nineties). This one is a pretty competent DVM as well as being able to sample and hold min, max and average values across all setting. For most days it could definitely do double duty against my Amprobe 37XR multimeter - EXCEPT the Agilent doesn't have non-contact voltage detection (the Amprobe does!). Anyhow - how do you get four and a half digits of resolution across multiple ranges on a brand-name test set for less than a hundred quid? Engineers today, don't know they're born...!

Today I was called to a customer's site where they have three canal-barges, each with two or three edit rooms on board. In the bilges of each boat there is room for little half-height equipment cabinet where they have the shared-storage chassis, network switches etc. They've been suffering an unusual number of equipment failures (motherboards dieing etc) and since they also seem to have RCDs tripping out as a regular feature my first thought was earth leakage.

Here is a picture of the electrical termination point for each boat - two 32A feeders go into the hull, one for the pumps and one for the mains distribution board. The cables are permanently suspended in the water (and have been for many years!) and the ones I inspected had clearly been submerged for so long there has been lots of water ingress into the rubber jacket of the cables. One felt almost ready to crumble in my hands.

Insulation has both electrical resistance and capacitance – and it conducts current through both paths. Given the high resistance of insulation, very little current should actually leak. But -- if the insulation is old or damaged, the resistance is lower and substantial current may flow. Additionally, longer conductors have a higher capacitance, causing more leakage current. Attaching the clamp meter to the incoming earth bond (pre-the consumer unit) measured a massive 100mA of leakage current. This not only risks the equipment being fed off this supply - there is an imbalance between the live and neutral cores and Class-1 equipment is often upset by this, and power supplies can pass this residual current to the earth-plane on PCBs.

More worryingly you've also compromised the safety action of any RCD (Residual Current Devices) in the feed. 

So - my advice was; replace those 32A feeders with marine-grade power cable as soon as possible.

Three phase electrical supplies and technical earth

I had a customer ask a question yesterday about why single phase in a machine room is considered a good thing and why technical earths need to be unified. Here's what I wrote; it's not a complete chapter & verse but it's a good excuse to put in a link to the episode of the Engineer's Bench that Hugh & I did in 2012.

He's confusing the phase that his various supplies are on with the "best practise" requirement of a unified technical earth. Twenty years ago the 16th Edition of the IEE regs forbade powering bays next to each other with different phases. There is more than 400v of difference between the mains phases and so it was thought that having that much potential difference within reach of each other was more dangerous than should be allowed; hence the practise of powering the machine room off a single phase.

However, the other consideration is that the power company charges you 3 x your most heavily used phase. So, if you're averaging 100A on the red (your machine room maybe?) but only 10A on the blue & yellow phases you'll be paying for 300A of current whilst only using 120A - if you don't "balance the phases" your electricity can (worst case) cost you three time per KW/h than it should. So, what with the improvement in RCDs and the cost issue the 3rd revision of the 16th Edition (and it's carried into the 17th) allows for mixed phase supplies in machine rooms. Looking up section 514 all that is required is labelling;

"6.1 Labels to be Provided The following durable labels are to be securely fixed on or adjacent to equipment installed in final circuits. (i) Unexpected presence of nominal voltage (U or Uo) exceeding 230 V Where the nominal voltage (U or Uo) exceeds 230 V, e.g. 400 V phase-to-phase, and it would not normally be expected to be so high, a warning label stating the maximum voltage present shall be provided where it can be seen before gaining access to live parts. (ii) Nominal voltage exceeding 230 volts (U or Uo) between simultaneously accessible equipment For simultaneously accessible equipment with terminals or other fixed live parts having a nominal voltage (U or Uo) exceeding 230 volts between them, e.g. 400 V phase-to-phase, a warning label shall be provided where it can be seen before gaining access to live parts."

On the subject of a technical earth - it's an entirely different consideration to mains phases but best practice is that you run all your tech feeds - bays and edit desks etc, back to the same earth bus-bar in the MCR. This is then run to the incoming feed supply. It's not good to try and tie it to the domestic "cooking" earth somewhere upstream of the incoming supply. The attached PDF is what we spec to customers' electricians.  It’s worth pointing out that getting the earth’s wrong is not unsafe; you can have volts of difference between two earths and they still both work as effective safety earths, but a hundred mV of earth differential between and edit suite and MCR will be a problem – the HD/SDi signal is only a volt big after all and zero level analogue audio is 775mV.

I did a podcast on the subject; http://youtu.be/rL1ZiciXRBg

Chassis vs Signal earth on RS422 remotes

Grounding is essential to reliable operation of any RS422 connections. It is also the most overlooked and least understood. The easiest way to ground your RS422 equipment is to simply use "Earth" ground as your return path. Although easy this may not be the best method for grounding your application, because current leaking from equipment, electro-static discharge (ESD), and lightning all drive current through this path which results in high noise content. The reason for this increased noise level is due to the fact that "Earth" ground presents a relatively high resistance. RS422 is designed to operate normally with a ground potential difference of +/- 12 Volts. During normal operations this is typically not a problem, however during fault conditions or lightning strikes even within ½ mile the ground potential difference can reach hundreds and in some cases thousands of volts. This will most likely result in damage or failure of one or more devices on the RS422 router.

In TV facilities I most often come across three methods of earthing;

1. No earth - assume that the mains return is good (all equipment is class-1 and bolted into it's bay and that signal and chassis earths will be close)
2. Use pins 1 and/or 9 on the 9-pin D-type to couple the chassis earths together; please, don't get me started about intentionally connecting mains earths between different areas! Do you like dealing with induced hum between different areas or buildings?!
3. The best way; using pins 4 & 6 - the signal screens (it's what they're there for).

The job I'm finishing at the moment had a problem with Digital Rapids workstation which wouldn't run through the RS422 router although patching around the router worked - the PC could control a VTR. My first port of call was to test the cabling/router patch by sticking an old Sony RM450 edit controller at the back of the Rapids and pretend the RM450 was the workstation - all good; VTR control and timecode return worked fine (so Tx and Rx doing their things). 

So my first thought was to measure the impedance between the signal ground on the router and the mains earth - high Z so no return patch for the RS422 via the router if it was relying on the mains earth (scenarios 1 & 2 above) and since the router is optically isolated on it's data inputs I wasn't surprised. That is the way it should be done.

Now then; most PCs that are running video apps and have to control a piece of broadcast kit use an RS232 port with an external RS232-422 adaptor. These essentially just balance the Tx and Rx pins and there are several models. None of them (in my experience) actually use a pair of rep coils to properly balance rather they use a pair of op amps in a differential input configuration. This is fine but doesn't have the noise immunity that you get with coils (common mode rejection). What it does mean is that all of the noise immunity of the circuit comes from the electrostatic shielding of the earth and so you better get it right!

I cracked open the cheap'n'cheerful '232-422 adaptors supplied with the rapids and they had the screen connected to the shield of the 9-pin on the RS422 side (so relying on scenario 1 above). Moving that to pins 4 & 6 (scenario 3) fixed the problem. 

As an aside the Adenda "Rosetta Stone" adaptors that Avid supply do the right thing!

Some interesting domestic electronic fixes

Fridge/Freezer earthing conundrum

I've had my Miele fridge for maybe six years and it's one of those frost-free models; I assume an extractor fan maintains negative pressure inside so that moist air that has entered when you open it gets drawn out. About three years ago (after I'd re-arranged some of the mains in the kitchen) it started icing up every few months like an old-style freezer. Of course it was outside it's warrantee so we lived with it. Around eighteen months ago whilst cleaning behind it I noticed the mains cord had been tugged and the earth was disconnected. Of course I re-ended the cable but since then the freezer has stopped icing up.
I've searched online for a schematic for this model to see if I can figure out what is going on. I'm reluctant to disconnect the earth to prove it, but I'm left wondering (and no other engineer I've mentioned it to can give me an answer) why the anti-frost mechanism didn't work without the safety earth? I stuck the freezer on my trusty Martindale 2100 PAT tester and it was fine; no excessive residual earth current and all isolation good; even at 8A.

When is a micro-switch not a switch?

My Valiant combination boiler has the usual configuration of a butterfly valve that routes hot water to either the taps or the heating pump. There is a micro-switch on the valve such that when you open the hot water tap the boiler is forced on (it's unlikely it was on heating the radiators exactly when you needed to wash your hands). The boiler is fifteen years old and I've replaced that micro-switch two or three times before. Earlier this year it seemed to have gone again yet removing the connections and testing for continuity when the valve opened showed the switch (seemed!) to be working. In the end I admitted defeat and got a heating guy in. He concluded the same as me and he 'phone the Valiant tech support line.

"Replace the micro-switch"

"No, no - it's not the switch, it buzzes out correctly when the valve closes"

"Never mind that, replace the switch"

Because it's a common model of boiler and the micro-switch goes often he had the kit in his van. Sure enough, he replaced the switch and the boiler starting working correctly. I was now beginning to doubt a lifetime of electronics knowledge! I got him the call the helpline back and the chap at the other end explained that it's not just a DC voltage that gets switched but the i2c data link! Yes, I was amazed; I can only assume it's part of a wider control system (maybe common across much more complicated boilers); but if there is any electrical noise or impedance on the switch data doesn't get back to the control board and the boiler CPU doesn't fire up the gas.

XBox 360 and failed DVD drive.

What would imagine would be the part most likely to fail in any kind of consumer device? The mechanical part that has a 50p laser diode AKA the DVD drive. Given that it has two connection - SATA and power it should be a user-serviceable part and given you can buy them from eBay and spare parts suppliers home tinkerers like me should be able to fix their XBoxes. But no, the XBox OS is keyed to the firmware serial number of the drive that is installed at the factory and if it sees another drive it not only refuses to read the disk, but it informs the XBox Live! (why the exclamation?) mothership and your account is banned for being a game-stealing pirate.
In the end you have to get the same model of DVD drive (there have been four revisions over the lifetime of the '360) and swap the board between drives so that your new mechanics have an old firmware. It worked for me. 

RS422 / Sony P2 protocol and serial stuff!

When running P2 protocol over RS422 (i.e. Sony VTR remotes) there is no hardware handshaking so RTS and CTS (Request To Send and Clear To Send) aren’t used; a bit like the old 3-wire XModem/YModem/Kermit protocols used in RS232 (remember RS422 is just a balanced version of RS232).

We base our RS422 wiring on the Quartz remote standard (Quartz were one of the first firms to use RJ45s & cat5 for RS422 remotes):

However – I know for certain that Probel use a different standard and many places are wired to whatever the local standard is; remember – until ten years ago most places wired ‘422 on star-quad cable rather than cat5e/6. I don’t know if current model Evertz routers have maintained the Quartz standard – I bet they have given they bought Quartz for its router business.

Whatever wiring standard is used always make sure that pins 2 & 7 are a twisted pair and likewise 3 & 8 otherwise you lose all the advantage of common-mode noise rejection that balanced RS422 brings.

Finally you need to be certain if a place is wired for chassis earth (pin 1 on a 9-pin) or signal earth (pins 4 & 6 on 9-pin). Signal earth is best as there is always a chance of earth-hum between areas when you tie chassis earths together but hopefully properly designed kit with balanced lines have the signal earth floating WRT to power/chassis ground. BUT, you have to stick with the local standard; if the engineer has wired only chassis earths you need to continue using pin 1 or even shorting pins 1, 4 & 6 at the remote end.

Varying standards...!

Recently we installed five bays worth of equipment at a big facility - of course we did all the usual; Scope of Works, Method Statement, etc and when we'd finished all the usual test results - particularly electrical safety (since we all live in a 17th Edition world now). Now this machine room (one of several) was more than fifty cabinets and so you'd think they've sorted out all of their standards. However - when were handing over we were met with the following criticisms;

• "You've mounted all the storage chassis flush with the front of the bays; our standard is that they are proud of the rack-strip". I went around the all other cabinets and they were an almost 50/50 mix of mounted proud and mounted flush...?
• "You've attached the earthing straps to the front mounting-point of the bay, our standard is to the rear". I went around the all the other cabinets and discovered the only bays in the whole comms room with earthing straps were the ones we'd just installed.
• "Our standard for PDUs is for 10-amp IEC outlets - even when they're feeding C19 (16A input) equipment, you've installed 16A PowerConn outlet PDUs" - we have to certify what we do to appropriate standards (that pesky 17th Edition again!) - no death-leads when you use us...

So I left confused, they'd taken us on to do a job they clearly didn't want/weren't able to do but they had nothing but ridiculous criticism by the end of the job. There doesn't seem to be any camaraderie amongst engineers anymore.

Requirements for Electrical Supply for Systems Integration projects

If we are obliged to power equipment sited in a machine room and/or edit suites we ask that the customer’s project manager and electrician read and sign these notes to ensure a proper configuration for the mains supply. The difference between an optimal arrangement of mains power and one that merely satisfies the requirements of safety legislation will be the difference between a smooth-running facility and one that is bedeviled by hum on signals and corrupt data streams. Attention to detail initially will save money and result in a robust system.

1. Circuit breakers - Our requirement is that the customer’s electrician provides a separate spur connection for each bay and all feeds are provided via a D-rated 16A MCB. We recommend the area is protected by an Earth Leakage Breaker. For the edit rooms an MCB-protected 16A mains feed terminated in a Commando connector is required. Since most equipment used in modern television production represents inductive loads C-rates breakers found in domestic and office premises will results in unnecessary supply interruptions.
2. It is important that the customer’s electrician runs the earths for the edit rooms back to the same earth bus-bar as the mains feeds to the bays thus creating a technical supply for all production/editing equipment.
3. The practice of tying the domestic ‘cooking’ earth to the technical earth should be avoided as a quick and cheap way of unifying the earths between the edit suites and machine room. Although this satisfies the requirement of a safety-earth it means that the technical earth is now united with the dirty earth.
4. Be aware that new projects that start after June 2008 have to conform to 17th Edition of the IEE regs (BS7671:2008). These notes are meant as additions to legal requirements and should be included in Root6’s Scope Of Works submission.
5. Testing- we will regard demarcation of responsibility for the machine room cabinets and edit suite desks as being at the 16A Commando connector – we will provide a standard set of tests results (earth continuity, Insulation, run-current, leakage, and flash-test) from that point for every circuit. We ask that the electrical contractor provided us with a copy of his test results as detailed in IEE.17th.ed and Part-P.

The importance of a consistent earth with technical mains

This is a paragraph from my standard Scope of Works document that goes to customers of build where I'm not responsible for the technical power;

Electrical requirements for all rooms part of the installation
We recommend an MCB-protected 16A mains feed terminated in a Commando connector. It is vital that the customer’s electrician runs the earths for the rooms back to the same earth bus-bar as the mains feeds to the bays thus creating a technical supply for all production/editing equipment. Failure to observe this request will cause mains hum on all video signal distribution around the new facility.

So we're quite explicit about the need for the need for a proper technical earth and what will probably happen if it's neglected. One of our recent builds has been having niggling problems with corrupt video captures and despite me having twice tested the physical layer performance of all the cabling (using the eye pattern on a Tek WFM7120) the attitude from the customer has been "…it must be the cabling or the routers you provided". After lots of haggling I was there recently and I measured a full 400mV of hum between the 'technical'(!) earth in the suites and the power distribution in the machine room. Given that an HD/SDi signal is only a volt I'm amazed they weren't seeing more corruption.
Now it's the inevitable "..why didn't you test for this and spot it earlier"? Perhaps there's a lesson here in not splitting the job up into many parts and using the cheapest contractor for each. If they'd been my electricians I'd have briefed them and made sure they ran proper tech earths - and I'd have made sure they tested them before handing over to the customer. That's why we'd have been a tad more expensive. As it is we'll no doubt wind up fixing this for free.

For a class-1 device you need a mains earth!

We had a monitor that was giving people mains shocks off the SDi BNC connectors. I opened it up and there was no earth from the mains inlet to the metal chassis. No then - if you want to have no internal earth you have to have a plastic case and one other layer of insulation between the user and any current-carrying conductors (that can be a layer of air - but the device must be 'double-insulated' AKA 'class-2'). I hope this was a manufacturing fault and not a design oversight!
It's not unusual for equipment (particularly with a resonant or switch-mode power-supply) to have the internal earth float at half-mains (with a very high-impedance to the power-source, no real ability to deliver any current), but that's why you need a safety earth connection if you can touch the chassis-earth (on the BNCs, for example) and not get a little belt!

before - oh dear! - after - much better!

Double-insulated (class 2) mains equipment

A Class II or double insulated electrical appliance is one which has been designed in such a way that it does not require a safety connection to electrical earth.
The basic requirement is that no single failure can result in dangerous voltage becoming exposed so that it might cause an electric shock and that this is achieved without relying on an earthed metal casing. This is usually achieved at least in part by having two layers of insulating material surrounding live parts or by using reinforced insulation. The IEEE regs say that one of the layers can be an air gap and so as long as the chassis is predominantly made of plastic it's not hard to get class 2 certification for a piece of equipment (you'll see the sign on your DVD and probably even your TV). Now then, read the article linked (above) - it's possible to get residual voltage on the screens of video and audio cables, but once connected to other equipment (particularly broadcast VTRs which are class 1 - proper mains earth) you'll not even be able to measure them as they have no current carrying capacity.
Today I had to go to a facility where folks were getting shocks off the audio jackfields and at the edit desk monitor power supplies were blowing. I traced the fault to the DVD recorder AND the VHS machine - both were dumping around 90 volts onto the screen of the signal connectors.
I thought i was missing something fundamental - my first action was to PAT test all of the bay and desk mains and it all checked out. In the end it really was those two domestic machines. I got another DVD machine off the client which didn't have the problem and so I can only assume that some event in the past has compromised the internal isolation.

Fridge-freezer mains earth defrosting connundrum!

I have an upright fridge-freezer in my kitchen and about a year ago I moved it to clean underneath. Since then every month I've had to defrost it (despite it being a frost-free model) and was getting ready to maybe replace it. However, three months ago I noticed that the extension cable that powers it (from a socket in the lader) had been tugged (I assume when I moved it out to clean) and on further inspection the mains earth had come out in the trailing MK13 socket on the end of the extension. I re-wired it and since then the thing hasn't frosted up once!
I'm at a loss - how does the presence (or not) of a mains safety earth cause this behavior? What about those countries that don't have an earth?

If you've got a thought then I love to hear it - have a party in the comments!

The 17th Edition IEE wiring regs BS7671:2008

I went to a superb training day at my institute to get all tooled up for the 17th Edition of the IEE wiring regulations - BS7671:2008, standards fans! These updated regs replace the venerable 16th Editions (4th revision!) in July and although they will only apply to new designs and aren't retrospective for older installations I thought it was important to start getting up to speed. A lot of the changes are due to the IEE / IET's involvement with the European standards body CENELEC.
When I started at the Beeb the 15th Edition was still in force and some of the things that came in 1991 with the 16th were universally mistrusted by engineers at the time. I remember reading a couple of days before a senior engineer job interview (internal) that they had relaxed the regulation on how close bays powered by different phases could be. That question came up in the interview and with a but of quick thinking I stretched out my arms and replied about this far! - the engineer questioning me saw I understood the principle and I got the tick! (In case you're wondering - you don't want to be able to put one hand on one phase of live and our other hand on a different live phase!).
Anyway - as mentioned it was a very interesting day at Savoy House (how do they afford to keep such a prestigious building?!). I have a scan of the 1st Edition from 1882 here - it runs to four pages! I've also stuck the various presentations on my server here as they are well worth looking at if your interested in electrical safety and standards.

Anyway, some of the significant changes are;

• Terminology - There are several re-definitions. The phrases direct and indirect contact are replaced with basic and fault protection. These cover the protective measures you design-in for normal operation and for fault conditions. Section 4 of Darrell Locke's presentation covers this well.
  There are also changes to the 'zones' found in bathrooms (and elsewhere).


• Expanded use of RCDs - They pretty much insist that RCDs need to be incorporated into all domestic circuits unless there is a good reason not to. They have this concept of experienced and instructed persons and if you're not one of those your an ordinary person who who only be exposed to RCD-protected circuits. Apparently this aspect was fought by the banks who worry about computers being wrongly taken down by residual current detectors. Whilst chatting over lunch every engineer I spoke to thought this was a good addition and if it makes us a bit more innovative in how we lay out supplies all the better.
  

• Recognising the problem of harmonics - This is something that is a perennial problem when every piece of equipment has a switch-mode supply (like a typical datacomms or television machine room). I've often hooked an oscilloscope across the mains in the various facilities where I've worked and what you see is rarely a sine wave! These reverse-leaked harmonics cause RCDs to misbehave and mean that you don't get predictable heating effects in linear (i.e. non-inductive) loads. In inductive loads the acronym CIVIL tells us that the voltage leads the current;
  
  
  


• New specs for allowable voltage drops - previously voltage drop from supply point (consumer unit) to the furthest point was allowed to be 5% - this is now set at 3% for lighting circuits. Apparently this was a French specification.
  


• New special location definitions - in the case of a TV outside broadcast vehicle an NIC recognised inspector now has to check every time the supply is re-connected - that may be problematic! I'm sure there has to be a way around this. Either that or every SIS-Link/BBC/Sky engineer will become an inspector!
  


• Phase sequences - Incredibly up until now the regs don't have anything to say about maintaining the sequence of the three phases. You don't want suckers to become blowers!
  


• New earth loop impedance (0.3 ohms) and minimum insulation impedance (0.5M ohms)

The regs aren't legally binding but if you are in court under EAWR (electricity at work regulations) the man in the curly wig will look on you favourably if you took them seriously and showed you abided by them.
Best phrases heard today; A corpse changes everything and (in relation to a current-carrying conductor) Is is as hot as a cup of tea?!