26 May 2017

A brief guide to the most important image standards

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Some of the most important aspects of producing video content in the modern age are governed by standards. Here is a brief rundown of some of the numbers we're most likely to encounter, and a quick summary of what they mean.

Years ago it was possible to plug a camera into a monitor and be reasonably sure that the resulting picture was correct, simply because there were only a few camera types and a few monitor types. Messing things up required extra, expensive equipment. Now, it's almost impossible to glance through an instruction manual without running into numbers, which reference obscure documents that can each describe a bewildering array of things.

Here then, is a rundown of some of the numbers we're most likely to encounter and a quick summary of what they mean. It's important to remember that these are standards documents trying to express things with great specificity. They are often only referred to casually in technical writing without really elaborating as to how the standard, or some part of it, may relate to what we're talking about. This can cause confusion. To avoid causing any more confusion, we should be clear about this being a brief overview that will gloss over many details in the interests of giving an easily-digested overview.

If this seems to overlook any standards you may have encountered, let us know and we'll do a second round.

Rec. BT.709 – High-definition basics

Title: “Parameter values for the HDTV standards for production and international programme exchange”
Organisation: International Telecommunications Union
First approved: 1990
Latest revision: June 2015
Documents: https://www.itu.int/rec/R-REC-BT.709/en

If you've ever wondered why high-definition TV usually has 1920 by 1080 pixels and goes at 25 or 29.97 frames per second, this is the standard saying so. It's essentially comparable to (and in some ways based on) BT.601, which specifies the same things for standard definition TV and dates back to the early 80s.

Frame rates include both interlaced and progressive scan plus the fractional frame rates required for compatibility with US (and derivative) colour broadcast systems. Interlacing is something later standards started to overlook, possibly in favour of simply running progressive scan images at higher frame rates. Even so, 709 does actually specify progressive scan 60fps modes, making 1080p60 legal from a standards perspective. 709 specifies both 8-bit and 10-bit encoding, as well as the valid range of values within those ranges.

Among the more complicated things 709 specifies are the colour primaries and the ways to convert between different colour formats. It's easy enough to pick a red, a green and a blue. The ones given in 709 are based on those possible with CRT monitors, leaving us using colours we probably wouldn't have picked otherwise. The more complex part of it is the conversion between R'G'B' and Y'CrCb (usually just called YUV or component) encoding. The differences between (basically) RGB and YUV are easy to google and a bit beyond this article, but suffice to say that they're crucially important to colour accuracy. Furthermore, they're used in every camera which has HD-SDI output.

There are some dead-ends in 709; things that were specified but fell out of favour. It talks about the long-dead 1035-line interlaced system which never really went anywhere and an 1152-line 25fps interlaced system which never went beyond the experimental stage.

Rec. BT.1886 – Brightness for (standard dynamic range) monitors.

Title: “Reference electro-optical transfer function for flat panel displays used in HDTV studio production”
Organisation: International Telecommunications Union
First approved: March 2011
Latest revision: As published
Documents: https://www.itu.int/rec/R-REC-BT.1886/en

1886 is a comparatively obscure piece of paper which we almost might say exists to correct an omission in 709. Perhaps notoriously so, 709 did not define (broadly speaking) how much light should come out of a monitor compared to how much signal level goes into it. Until the publication of 1886, the de facto standard had been the behaviour of CRT displays. With CRT reference displays becoming unavailable and manufacturers emulating them to various degrees of accuracy and with various different priorities, some sort of standard was clearly required.

There are only about five pages of actual content in 1886, although it does clear things up quite nicely. The only issue is whether we should really be limiting our current, vastly better technology to the level of emulating an approach that's been out of date for decades — which is sort of where 2100 comes in (see below.)



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Phil Rhodes

Phil Rhodes is a Cinematographer, Technologist, Writer and above all Communicator. Never afraid to speak his mind, and always worth listening to, he's a frequent contributor to RedShark.

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