Fibre 102; CWDM & encircled flux - The Engineer's Bench Podcast

Hugh and Phil talk about optical multiplexing as well as new methods for accurately testing fibre cables. A few tips on basic fibre cleaning as well.


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Digital path & SDi does not colour-accurate pictures make...

Recent years have seen many prosumer camcorders with HDMI outputs so that you can get access to the uncompressed RGB sensor output rather than having to make do with the H.264 (typically) encoded Y, Cb, Cr data (from the flash or disk-based recorder). This makes lots of sense and has given rise to HDMI -> SDi converters like the ones Mr. Blackmagic sells;

These take any HDMI 1.4 resolution (all the way to 4k UHD - 3840 x 2160 at a maximum of 30P in 4:2:2 colour space) and convert to single-link (1.5G, 3G or BM's home-brewed 6Gbit/sec) SDi. Excellent, you'd think; and they are so long as you only use them for video-type sources - camcorders etc. Don't assume they are of any use in turning the output of your computer into SDi!

So, here's the test rig; my Macbook Pro 15" running Mavericks with a Thunderbolt -> DVI breakout connecting to the HDMI input of the BM converter.

 

The SDi output is fed to my trusty Tektronix WFM7100-series and I'm running a known-good recording of 10-bit, 1080 50i 100% EBU colour bars on the 2nd display.

Now let's take a look at the state of the bars; not pretty - the luminance response is all over the place with a very funky gamma that has really gone awry in the bright parts of the picture. The blue colour-difference channel is not so bad, but the Cr (red colour difference) is really crushed in the cyan end of it's response; look at the vector display (top-left).

That's not to say that the pictures don't look good on the monitor; but they aren't colour accurate in the way they need to be if you're using this as a method of profiling an SDi display - and I have seen people use this method with Light Illusion to derive the colour space of a display and then generate a LUT to make the display look how they want.

So, my first thought was, head over to the display profile and see if it's just using the wrong RGB numbers; OS-X and Windows both support standard profiles like Adobe RGB or sRGB which are more suited to print and web graphics prep but not necessarily our beloved broadcast Rec.709 colour space. Imagine my horror when I realised that the colour display profile that OS-X had used was the one that shipped with the Blackmagic! How did they not even get that right?!

To be fair even the 709 profile that comes with the OS is wrong; the take-away is don't use these kinds of gadgets if you need accurate colourimentry. For XBoxes or just getting a high-quality desktop feed as SDi they are fine, but not if accurate broadcast pictures are needed.

Ongoing Barnfind notes

As I get more familiar with Barnfind's products I need to make a note of some of their gotchas!

1. The Tx and Rx lights on all SFP ports are not data lights, rather they are clock lights and as such only light when you're dealing with synchronous broadcast signals - HD/SDi, MADI, AES etc.
2. The CWDM multiplexer/de-multiplexer (they are exactly the same unit!) works both ways and if you have signals going bi-directionally on a fibre each port is an input or an output; that takes a bit of getting your head around!
3. In a similar vein the wavelength quoted on SFPs (1350nM in this image) only really applies to the transmitted signal; SFPs are "colourblind" - they don't mind what wavelength they receive. So, once a signal leaves the CDWM de/multiplexer you can take it into any of your SFPs for input to the crosspoint router - again, it's not particularly intuitive as we're used to "tuning" or "demultiplexing" other signals to the frequency they'll be used at.
4. The BarnStudio software; the manual says that it comes set up hard-set to a 192.168.0.1 address; our didn't, it was setup for DHCP and since it doesn't respond to multicast PINGs it took me a while to figure this out.
5. If you are routing ethernet out to the multiplexer it is always directional (i.e. it takes up the in and out of an SFP and needs two ports on the multiplexer). 
6. See note 1 above (no activity lights) for ethernet.
7. If you're using the same machine to drive BarnStudio and test an ethernet connection (by sending it via an ethernet SFP -> fibre -> multiplexer -> BarnMini -> SFP -> ethernet) you run the risk of an ethernet loop and subsequent broadcast-storm! Wesley & I suffered this and couldn't figure out why the entire workshop network was down. Much better to use your rucksack router & a RaspberryPi as a separate test network.
8. The BarnStudio software - although good, is a tad hard to read initially as if you give the ins and outs proper names they re-order alphabetically rather than in order of the SFPs and BNCs. Just remember - inputs are down the left axis and outputs are along the top (most things are present in both).

HD playback - how we did it a quarter of a century ago

In 1988 I went to a demo of HDVS in Studio 2 at Television Centre; the left-hand image shows the 1" uncompressed RGB recorders that ran around eight times the speed of regular C-Format videotape. On the right is current-model Blackmagic Hyperdeck Shuttle - a small HD/SD record/playback box that uses SSDs and can handle uncompressed, ProRes and Avid DNxHD. It's in pieces 'cause I'd just finished fixing it and was testing it. BUT, given that they are two-hundred quid you might wonder if they're worth fixing?! The Sony was definitely worth fixing as each machine came in at >£100K.

Makes me wonder what I'll be doing by the end of my career?!

Barnfind's high-speed data router and optical CWDM for TV infrastructure

I've been a very slack blogger over the last five weeks due to work (installations, running training, getting trained!) and holiday (splendid). I spent a few days last week in Norway as the guest of Barnfind in Sandefjord.

Norway seems to be a lovely country if not a tad expensive (£24 for a round of three pints).  Barnfind are a small company whose engineers used to be with Nevion - you've probably come across their VikinX range of HD/SDi and other facilities routers. 

I had a long Skype chat with Barnfind a month ago and kind of 'got' their range. It's not a one-for-one replacement for any other specific products rather a platform that nicely ties together all digital signals within a facility; synchronous (SDi, MADI, AES, etc) and asynchronous (ethernet - copper & fibre, fibre-channel). They also make CWDM very do'able in a broadcast environment. As we move towards an entire IP infrastructure these are the kind of platforms that allow an easy transition. 

The basic product (the BarnOne BTF1-01) is a 32x32 generic data router and 16 bi-directional SFP ports. The SFPs can be any MSA-compliant units but Barnfind manufacture their own at very reasonable costs (much less than Cisco!). You could insert Ethernet, SDi i/o, fibre or any of around 150 variations they offer. This allows you to route SDi in and out over fibre, insert AES into an SDi stream, convert ethernet to/from fibre etc etc. 

3G HD/SDi input/output SFP

Clearly some signal types don't sensibly convert; routing an SDi stream to a fibre channel-equipped port won't replace an HP workstation running Avid! But where is does make sense everything is taken care of for you. In the case of all video signals (composite, SDi and HDMI are all supported) the signal is converted in the SFP to 3G SDi before it is passed to the 32x32 router.

The BarnOne range extends to several variations - the lower board which carries the router and the first sixteen ports can be joined by two upper boards carrying BNCs, more SFP holes or (more interestingly) CWDM fibre modules. Essentially having BNCs on an upper board allows you to avoid SDi SFPs (it's marginally cheaper to do it on an 8-way board than an extra 8-holes with video-SFPs).

 

The other end of the link could be easily served by their BarnMini units - essentially replacing Blackmagic or AJA converters but integrating very nicely with the BarnOne. For less than £400 you can get either two BNCs with an SFP hole or two SFPs. 

The whole thing makes sense when you realise that all the signal intelligence is in the SFPs - the dual-port BarnMini can do anything that makes sense; maybe you need to route some ethernet coming in on single-mode fibre and send it out over existing multi-mode cable. Again, AES, SDi, MADI as well as all fibre and copper networking are supported. 

Before I start banging on about Course Wave Division Multiplexing it is worth including a photo of the insides of a BarnOne so you can see the control card they use.

That's right! It's a RaspberryPi! When re-invent the wheel; they claim they tested a few Linux SOC boards and found the humble Pi to be the most reliable and they make use of the watchdog timer to ensure it's always listening for config updates. Their BarnStudio software not only allows you to configure the system (including all the monitoring via SNMP) but you can also control the router. They also support several manufacturers generic control panels if that's what's needed.

CWDM

Single mode cable (more often than not) is used to carry a network feed, a 3G video feed or some other data. We quote wavelengths for fibre (typically 1310nM) rather than frequency and so often forget that the sidebands of the signal we put down a fibre are tiny relative to the centre frequency. 1400nM of wavelength is around 200Thz (yes, 200 x 10^12 Hz!) which makes the 4.5Ghz bandwidth of the best-quality HD video coax look very modest. So, we could divide up the single-mode range into many wavelengths and use each for a different purpose; a bit like the radio stations on the VHF band.

These are the standard wavelengths for CWDM working; sixteen channels and by convention the two colours run in different directions (SDi input/output or ethernet Tx and Rx for example). This allows you to have SFPs that put the signal they send/receive onto specific wavelengths. Then, with a simple passive optical splitter/combiner (which is all a CWDM multiplexer is). With all this in place you can use your Banrfind kit to multiplex sixteen functions in and out of a single 9/125u fibre. If that's all you have between premises then this is a lifesaver. The multiplexers are in the £400 range. 

We opened one up last week and it was a thing of beauty; all passive optical engineering with tiny dichroic filters. So, by careful planning you could send multiple SDi signals, ethernet, fibre channel and other things to and from a remote site over a single strand of mono-mode fibre. It could be up to 80kms away. 

There is a further development of the multiplexing technology called Dense Wave Division Multiplexing - DWDM which allows up to 192 channels and very far distances (by the use of Eridium-doped optical amplifier - but in the case of Barnfind (and other manufacturers) the cost of DWDM vs CWDM is fivefold!

The BTF1-07 is the box I've ordered as my demo unit; it has sixteen SFP holes, eight bi-directional BNCs as well as a CWDM multiplexer.

MPEG transport stream corruption; effect on pictures

I'm running training at the MET's video forensics lab soon and part of it is explaining how DCT-based compression works and particularly the effects of corruption on long-GOP MPEG transport streams as delivered in DVB-MUXes. One of the illustration videos is below. 

Three clips; each with a momentary corruption to the data stream and in each case you can see how the decompressor can't reconstruct a proper picture until the next i-Frame. The second half of the clip shows me framing through with the I, B, or P frame indicated top-left - you'll need to make it full screen to see that as the marker is quite small.

The challenges of modern picture quality analysis.

This is a recent article written for a trade magazine;

Engineers have sought to quantify the quality of the video signal since the birth of television. Since all aspects of the TV picture are represented first by voltages (analogue) and then by numbers-in-a-bit-stream (digital) you have to make measurements to really know anything about the quality of your TV pictures.

“When you can measure what you are speaking about, and express it in numbers, you can know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind: it may be the beginning of knowledge but you have scarcely, in your thoughts, advanced to the stage of science.” - William Thompson, Lord Kelvin.

In the dim and distant days of monochrome tele the only things the TV engineer had to worry about was the blacks and the whites. Common consent had us placing the blacks (dark areas of the picture) as a low signal (at zero volts) and the whites (bright parts of the picture) up at 0.7 of a volt. In addition we allocated the 0.3v below black to the synchronising pulses – the electronic equivalent of the sprocket holes in film; a mechanism that allows the receiving equipment to know when new lines and frames of video were starting so that the picture is “locked” and not free-running (“try adjusting the vertical-hold!”). Once all those things are well-defined then Mr. Sony’s cameras work nicely with Mr. Grassvalley’s vision mixer and the engineer at the broadcast centre can adjust the incoming signal from the OB truck such that it looks right on the wavefrom monitor and hence the pictures will match what left the truck. Dark shadows and bright clouds will look like what the camera operator saw.

Fig.1 – monochrome TV signal, two lines.

So far so good; but people wanted colour TV and so all of a sudden the way colour is encoded needs to be considered. With colour comes grading and the “look” of pictures and colourists need to see different representations of the colour parts of the signal for artistic reasons. Engineers need to ensure that the colour content of the picture is constrained to the legal gamut of colours that the transmission system can handle; nobody wants things to change colour as they get to air! Tektronix have always been the gold-standard for TV test and measurement and to this day if you ask an engineer or colourist what kind of test equipment they’d like it’s going to be a Tek.

Fig.2 – colour TV signal, several types of display

As we moved from analogue to digital working in the 1990s and then from standard definition to higher resolutions in the noughties the principle of looking at the lines and fields of the TV signal remained; we assumed that if one frame got through the system with minimal/acceptable levels of distortion then all subsequent frames would; and as we know - the illusion of television is that many frames make a moving sequence.

However – with the introduction of “long GOP” (lit. Group Of Pictures) video compression in the 90s it became apparent that we don’t treat every frame of video the same. On a compressed video-link there are I-Frames (the ones where the entire picture can be re-built) and other, more complex beasts, called B-Frames and P-Frames; these serve to convey the differences (by not sending all complete video frames, but merely the differences from previous and subsequent ones we achieve video data rate reduction AKA compression). You’ve no doubt seen the on-air fault where some parts of a picture seem to have become “stuck” in the previous scene where other parts of the picture are behaving normally. Then, suddenly the picture rights itself. What you have witnessed is a corrupt I-Frame; all your set-top-box can now do is show the changes as they arrive and you don’t get a re-built complete frame until the next I-Frame arrives (typ. half a second later). This is just one kind of “temporal” fault.

Fig.3 – MPEG multiplex fault

So, now we have to consider things happening in time as well as the pixels, lines & fields of video.  The colour-bars you’re looking at coming across the satellite link may look splendid, but perhaps the link is only able to convey pictures that change minimally between frames and as soon as moving video arrives it looks awful. Perhaps the fields of video have been reversed (it’s a more common technical fault than you’d expect) and you’ll only see that on moving pictures; and it’s not nice!

Test signals have always served to “stress” the system they are being run through; the traditional colour bars have their colours set at the extreme ends of what the system can handle – you never see that amount of saturated yellow in pictures coming out of a TV camera! We want our test signal to show faults; if the pictures look good it should be because the OB link or editing workstation is capable of carrying the worse-case pictures.

So, now we need a test signal that is not just the same frame of video repeated endlessly; we need a test that changes and serves as a challenge to compression encoders and is constructed in such a way as to have predictable picture effects that highlight when your production chain is sub-par. Ideally we could see these drop-offs on a picture monitor and not on a £10k Tektronix test set. Once we have an animated test signal we can test not just for the degradation of compression but also those field-cadence problems. We can also test for lip-sync errors, exaggerating the effect of audio being late or early with respect to the video and more importantly all of these tests can be done by an operator rather than the expensive engineer. We’d also like the sequence to be constructed such that faults are visible on a 24” video monitor from the across the other side of a busy machine room or studio gallery.

Fig.4 – SRI Visualizer - a still frame; it moves normally!

• Lip-sync: Measure and quantify the synchronization offset between audio and video – a single frame of sync is easily missed on camera pictures.
• Bit depth: Detect 10-bit to 8-bit truncation – in a modern facility a mix of eight and ten bit video is a fact of life, but no client wants unnecessary loss of dynamic range.
• Compression fidelity: Measure and quantify compression levels in real time; again, real camera pictures often make this effect hard to spot.
• Colour matrix mismatch: Determine high-definition (709) and standard-definition (601) colour space conversion errors. These colour shifts are subtle until the director is shouting about that shade of red he wants!
• Chroma subsampling: Determine the Chroma subsampling being used (4:2:2, 4:2:0, 4:1:1…)
• Chroma upsampling: Reveal how missing chroma samples are interpolated
• Field dominance: Definitively determine field order reversal.
• Chroma motion error: Demonstrate incorrect processing of chroma in interlaced 4:2:0 systems
• Subjective image fidelity: Perform a rapid check of system integrity
• Colour conversion accuracy: Verify colour-space conversions
• Display gamma: Measure monitor gamma quickly
• Black clipping: Reveal black clipping and accurately set monitor black level
• White clipping: Determine if highlights are being blown out
• Noise: See how an encoder handles noise
• Skipped frames: Detect repeated and dropped frames

The SRI Visualiser is available as a hardware product (the TG-100 which also includes equally innovative audio tests) which can be installed into a machine room to augment/replace existing SPG-type test signal generators. You can also purchase and download it as several kinds of video files which can prove very useful in file-based workflows; injecting the sequence at the start of the post/production workflow and confirming all is well at the very last stage. An hour of time paying attention at the start of a new production will pay dividends by highlighting exactly where any picture faults creep in. Did that colour-shift occur during editing, grading, VFX or when the show was transcoded for distribution? Without a test system like The Visualizer these problems are hard to track down.

Temporal-based video tests; The SRI Visualizer

We've recently take on SRI as a supplier and I'm very excited about their test system.


You can read the article I've written for a couple of industry magazines here.
SRI International

Someone who has some actual experience of file-based deliverables!

There are an awful lot of people touting themselves as "DPP consultants" and "file-based technologists" in the UK TV industry at the moment. For the most part they are riding the gravy train of the DPP roadshow and the fact that come October all the big broadcasters will expect file-based delivery AND material QC'ed to the DPP specification. The majority of these folks have not delivered a single minute of material for terrestrial broadcast but the one chap who really does know his stuff is my good pal Simon Brett (recently moved from National Geographic/Fox UK to NBC-Universal).

Here he is at a recent evening event we laid on; if you want to know some actual details (what bit of software to use to handle your metadata etc) then Simon is your man.  He's quite an engaging speaker (and he bigs-me-up for colourimetry!)

Near synchronous video over UDP/IP with Sony's NXL-IP55

Just about the only things that caught my eye at the recent Beyond HD Masters day at Bafta was Sony's IP Live system. This is a single product called the NXL-IP55 which puts four 1080i 4:2:2 signals over a gigabit connection - so modest 6:1 compression with a well-defined single field of latency. The camera channels can go either way (so three source cameras and a return preview monitor for example) and embedded audio plus tally & camera head (colour) and lens control are included. It's quite expensive ($10k per end) but is the only video-over-IP device which I've seen so far which is suitable for live production.  

http://www.sony.co.uk/pro/press/pr-sony-nab-av-over-ip-interface

Measuring fibre cabling and the problem of encircled flux loss

Last week I went on a very interesting training day courtesy of Nexans - data cable & parts supplier. I went looking forward to learning all about the new standards surrounding catagory-8 cabling for 40 and 56 Gigabit ethernet (a massive 1600Mhz of bandwidth down a twisted pair cable!) and the new GG45 connector; but those things will have to wait for another blog post! The thing that really tickled my fancy is the new standard for measuring the response of multi-mode fibre.

Multi-mode fibre works in a fundamentally different fashion to single mode (they are as different as twisted-pair and coaxial copper cable; but they look very similair). If you want a bit of a primer on fibre then Hugh & I did an episode of The Engineer's Bench a couple of years ago on the subject.

As we've gone from one-gig to greater than 10Gigbits/sec in OM3 and OM4 cable and engineers have often noted the lack of consistency between different manufacturers light-source testers. You might get as much as 0.5dB of difference between say an Owl and a JDSU calibrated light source and detector. We typically use a 20dB(m) laser at 850nM to test OM3 and we always just deliver the loss figres to the client, but it would be good to know if your absolute reading is of any use at all?

Well, the answer is that LED or VCSEL (Vertical-cavity surface-emitting laser) will tend to "overfill" the fibre and high-order modes of light travel (to a degree) down the cladding of the cable.

Launch conditions correspond to how optical power is launched into the fiber core when measuring fiber attenuation. Ideal launch conditions should occur if the light is distributed through the whole fiber core.

Transmission of Light in Multimode Fiber in Underfilled Conditions 

Transmission of Light in Multimode Fiber in Overfilled Conditions

An overfilled launch condition occurs when the launch spot size and angular distribution are larger than the fiber core (for example, when the source is a light-emitting diode [LED]). Incident light that falls outside the fiber core is lost as well as light that is at angles greater than the angle of acceptance for the fiber core. Light sources affect attenuation measurements such that one that underfills the fiber exhibits a lower attenuation value than the actual, whereas one that overfills the fiber exhibits a higher attenuation value than the actual. The new parameter covered in the IEC 61280-4-1 Ed2 standard from June 2009 is known as Encircled Flux (EF), which is related to distribution of power in the fiber core and also the launch spot size (radius) and angular distribution.

All the manufacturers are producing EF-compliant testers so you don't need to worry about inaccurate reading due to these high-order modes, but for now there are some suggestions.

Multimode launch cables allow for the signal to achieve modal equilibrium, but it does not ensure test equipment will be EF-compliant based on the IEC 61280-4-1 standard.
Multimode launch cables are used to reveal the insertion loss and reflectance of the near-end connection to the link under OTDR test. They also reduce the impact of possible fiber anomalies near the light source on the test.

If the fiber is overfilled, high-order mode power loss can significantly affect measurement results. Fiber mandrels that act as “low-pass mode filters” can eliminate power in high-order modes. It effectively eliminates all loosely coupled modes that are generated by an overfilled light source while it passes tightly coupled modes on with little or no attenuation. This solution does not make test equipment EF-compliant.

Mode conditioning patch cords reduce the impact of differential mode delay on transmission reliability in Gigabit Ethernet applications, such as 1000Base-LX. They also properly propagate the laser VCSEL light along a multimode fiber. This solution does not make test equipment EF-compliant

Oscilloscope Watch project update

I'm still monkeying around with the oscilloscope watch and hopefully I'll shoot a bit more video later this week, but it's still very much in Beta and I got this update of what the final case will look like from the Gabriel Anzziani, the developer.

HD/SDi physical layer measurements - gotta be Tektronix

Here's a brilliant document from Tek - tell you all you need to know about eye pattern, jitter etc.

http://www.tek.com/document/how-guide/sdi-eye-and-jitter-measurements...

Quick & dirty RS422 VTR control from your laptop

If you need to control a deck (or test routes through an RS422 matrix) then a very quick and easy way of doing it is via your laptop. Modern machines don't have RS232 ports and so you can't just hook up an RS232-422 balancer, but USB serial devices are cheap - the Addenda RS-USB4 is the one I keep in my rucksack.

It's worth noting some of the settings you need for the serial port in Windows.

You then need a bit of software that will talk the P2 protocol. WSony II has been around forever but still works under Windows7. You have to run it as admin for it to access the serial port.

Once it runs you have to tell it what port number gave it - It shows up as Com1 on my machine by default, but bear in mind that Windows will often assign it to Com5 or greater. The WSonyII will only talk up to Com4 so you might need to change that (see the first image above in the advanced tab).

Now you've got everything you need to drive a VT or test that an RS422 circuit is working.

Oh, if you don't have the CD that came with the Addenda adaptor cable you'll need to download the software here.

Remember; P2 over RS422 runs at 38,400 baud.

£3k for an Ethernet cable..!?

Just in case they take it down I had to do a screen grab! Yes, there are people selling "audiophile" grade network cables for three grand.

Buy yours here!

All sorts of dishonesty is written about high end audio cabling - directional conductors, "burned in" mains cables, even quantum effects are cited as important ways to "improve sound clarity" etc. You can believe that decent quality copper cable makes some difference at analogue baseband but by the time you get to AES and even network cables so long as the packets are getting through there is nothing the design of the cable can do to change the sound being re-produced. Even in the case of analogue audio (either line level or the speaker cable) good quality copper is all that's needed.

Still, a fool (make that an audiophile) and his money are soon parted and once you've spent thousands of pounds on cable you probably can "hear" a difference!

So, the lies peddled about this particular brand;

Perfect-Surface Technology applied to extreme-purity silver provides unprecedented clarity and dynamic contrast. Solid conductors prevent strand interaction, a major source of cable distortion. Extremely high-purity Perfect-Surface Silver minimizes distortion caused by the grain boundaries which exist within any metal conductor, nearly eliminating harshness and greatly increasing clarity compared to OFHC, OCC, 8N and other coppers.

Just nonsense!  Electrical signals travel down cables by the movement of electrons. Copper (Oxygen free or otherwise) is about the best conductor commonly available and the "grain" or "strand interaction" has nothing to do with the movement of electrons. BUT, let's not forget that this is a network cable and the transit of TCP/IP packets has NOTHING to do with the "harshness" or "clarity" of the sound.

Any solid material adjacent to a conductor is actually part of an imperfect circuit. Wire insulation and circuit board materials all absorb energy (loss). Some of this energy is stored and then later released as distortion. Solid High-Density Polyethylene Insulation ensures critical signal-pair geometry while minimizing insulation-induced phase distortion.

Are they hinting at capacitance here? "Energy stored and then later released as distortion" - where do you start. This is so misleading. Network cables are twisted pair construction (relying on common-mode rejection on each pair) - they aren't coaxial cables (if they are referring to capacitive effects?). "Insulation induced phase distortion" - have any electronic engineers ever measured that?!

All audio cables are directional. The correct direction is determined by listening to every batch of metal conductors used in every AudioQuest audio cable. Arrows are clearly marked on the connectors to ensure superior sound quality. For best results have the arrow pointing in the direction of the flow of music. For example, NAS to Router, Router to Network Player.

Please - audio cables are not directional! As many electrons flow in one direction as the other. This old chestnut was commonly trotted out for analogue cable, it was a lie then, and it's a lie now. With a network cable packets travel in both directions as the TCP/IP handshake brings up a connection, data is transferred and then the connection is closed. There is no directionality for the packets, the data they carry or the electrical signals that represent the bits. 

What this all comes down to is that these snake-oil salesmen NEVER subject this tat to a double-blind test. If they did you find out that there aren't a pair of ears in the world that can actually "hear" the difference between a £3k and £2 network cable.

Sony PVM-A250 OLED - some colour measurements

I have a pair of new PVM-A250s in ahead of an install and wanted to test a couple of things out. My first one was to see if the monitor does the right things between it's studio swing-range HD/SDi 4:2:2 input and it's full-range HDMI 4:4:4 inputs. Aside from a slight shift in luma (big Y) the difference is minimal (and definitely not the usual 601/709 matrix-mismatch you see on other monitors).

Y, Cr, Cb 4:2:2 HD/SDi input, peak white field before calibration - out of the box these monitors are a tad blue and a bit too bright.








R, G, B 4:4:4 HMDI input, same source.


We're running these over many hours to see if they drift at all. Will add some more to this post next week with those results.

What's the best setting for Gamma with Rec.709 video?

I've been calibrating TV displays for around twenty-five years and have always leaned on the BBC practice of setting peak white at 80Cd/m2 and the white point at 6504k. For all the decades of standard definition work a gamma of 2.2 has been used. This is required because television cameras don't have a linear transfer characteristic and to make a black-white ramp match (i.e. it looks the same on the monitor as well as to the naked eye) you have to invert the gamma response of the camera at the display end. 

If you get this wrong you wind up with blacks and whites matching but all the shades in between don't - pictures either appear washed-out or all the detail is crushed out of the dark areas of the picture depending on which way you get it wrong.

When Rec.709 came along in the early nineties (for HD television) they changed a few things; new colour primaries, new luminance transfer function, wider gamut etc. They also made a subtle change to the definition of gamma. Like sRGB (the Adobe colour space for graphics working) and Rec.601 the "scene-referred" gamma of 2.2 is specified (in 1992 TV cameras were still tube'd devices) but the display gamma is governed by a complicated transfer law which many people have taken to be closer to a gamma of 2.4. However, a straight power curve of 2.4 is correct only if the display has a zero black level and an infinite contrast ratio, which no real-world display has. The new BT.1886 specification (from 2011) is complex and its precise recommendations vary depending upon the white level, and especially the black level, of the display.

However, if you don't want to bother with a precise BT.1886 calculation a 'best approximation' of this would be a gamma of 2.2 in the low end of the curve rising to a gamma of 2.4 at the high end. In fact it's worse than that - it's linear for the first 10% of the range.

So, what's a colour-calibration guy to do? Once again Charles Poynton comes to the rescue - he's forgotten more things about colour than most of us ever knew and I take his advise whenever possible.

Colors change appearance depending upon absolute luminance, and upon their surroundings. A very dark surround at mastering will “suck” color out of a presentation previously viewed in a light surround. A colorist will dial-in an increase in colorfulness (for example, by increasing chroma gain). The intended appearance for an HD master is obtained through a 2.4-power function, to a display having reference white at 100 Cd/m2 – but that appearance will not be faithfully presented in different conditions! The key point concerning the monitor's gamma is this: What we seek to maintain at presentation is the appearance of the colors at program approval, not necessarily the physical stimuli. If the display and viewing conditions differ from those at mastering, we may need to alter the image data to preserve appearance. In a grading environment, you might set the consumer's display to 100 Cd/m2, matching the approval luminance. However, ambient conditions in an editing environment are somewhat lighter than typically used for mastering today. The lighter conditions cause a modest increase in contrast and colorfulness, beyond that witnessed at content creation.

So, it seems the choice is this - 2.4 gamma in well-controlled grading/mastering conditions (particularly if your monitor has good dynamic range; OLED or Dolby reference monitor) and 2.2 in brighter editing rooms with lower-end LCDs.

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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

Colour results from a Sony PVM-2541 TriMaster OLED monitor

Having spent a bit more time with the Klein K10-A probe I profiled the Sony monitor we hired to practice on.

Here you can see the colour gamut for the displayed compared to the Rec709 colour space for television.

It is an excellent match with the green extremity being a tiny bit mis-matched. I would not expect to see this from either a CRT or LCD monitor.

The white curved line is the "black body locus" and is where physicists define the colour of white.

1. Gamut measurements


White Field
Video  x    y    Y      
255  .313 .336   136.45 

Red Field
Video  x    y    Y      
255  .639 .332   28.35 

Green Field
Video  x    y    Y      
255  .296 .603   99.75 

Blue Field
Video  x    y    Y      
255  .149 .060   9.47 

EBU Overlap Gamut Value:  98.4%

2. Gamma measurements

White Field
Video  x    y    Y      
235  .313 .334   110.99 
207  .313 .334    83.10 
180  .314 .336    58.14 
153  .315 .337    37.74 
125  .317 .338    22.35 
 97  .319 .341    11.39 
 70  .324 .347     3.50 
 43  ---- ----     0.39 
 16  ---- ----     0.01 

Red Field
Video  x    y     Y  
235  .640 .332    24.30 
207  .641 .332    17.79 
180  .642 .332    12.87 
153  .645 .332     7.72 
125  .651 .333     4.20 
 97  .664 .332     1.88 
 70  .671 .325     0.65 
 43  ---- ----     0.09 
 16  ---- ----     0.01 

Green Field
Video  x    y     Y      
235  .296 .603    81.12 
207  .297 .603    57.73 
180  .297 .604    39.60 
153  .297 .604    25.74 
125  .297 .608    14.55 
 97  .297 .614     6.97 
 70  .296 .638     2.27 
 43  ---- ----     0.26 
 16  ---- ----     0.01 

Blue Field
Video  x    y      Y      
235  .149 .060     8.19 
207  .149 .059     5.73 
180  .149 .059     3.89 
153  .148 .058     2.39 
125  .147 .056     1.29 
 97  .143 .053     0.54 
 70  ---- ----     0.18 
 43  ---- ----     0.02 
 16  ---- ----     0.01 

You can see that the colour of white through grey tracks very well until you get to within 15% of black; you can generally accurately read the luminance level down to sub 1Cd/m2 but colour measurements become too noisy down there. The K10-A seems to perform better than our DK PM5639 in this respect.