Atomic official guide to AV cables, cabling and tech

If all of this seems like a terribly complicated way to send red, green and blue colour signals, it’s because it is. RGB is a way of sending analogue video as three separate streams of red, green and blue data. It was also the way you saw pretty much everything on your computer up until you got that fancy new DVI-enabled video card and display. RGB comes in a number of variants relating to how synching is implemented – VGA uses RGBHV.

VGA started as a simple standard, and we still use the term to refer to the video 640 x 480 resolution that it was intended to display. RGB is typically sent out over that trusty old 15-pin (DE-15) connector – forever known as the ‘VGA’ connector (for reasons that really should be obvious) – with three pins for R, G and B, two pins for horizontal and vertical sync (hence the RGBHV) and the rest of the pins used for grounding or not at all. Over the years, the standard has been tweaked and modified to the point where it is now capable of pumping out 2048 x 1536 at 85Hz (although you’d have a hard time finding a CRT to monitor to display that – there are none currently in production). Such longevity for such an old standard is really quite commendable, and a tenfold increase in pixels on screen is nothing to sneeze at.

Indeed, if you care to push it further there is no technical limit to the resolutions that VGA can support. As far as the connection is concerned, the maximum resolution is limited only by the available bandwidth, which is in turn limited by how fast you can switch from one value to another. In an analogue connection, this means that you need to allow time for your signal go from peak to trough to peak in the time it takes to scan from one pixel to the next. And sorry, but there’s no choice but to resort to maths for this one.

Let’s take a common resolution: 1280 x 1024, also known as ‘SXGA’. We’re talking bandwidth, which is data over time, so framerates matter – let’s make it 60Hz, or 60 frames per second.

A basic representation of a video signal. Note the periodic dips to 0V for blanking -- when CRTs move the electron gun back to the top left of the frame. And yes, analogue video can go above 1 volt, but these hot signals are normalised to 1V before broadcast so they don't cause interference with other parts of the electro-magnetic spectrum when they're broadcast. Look up broadcast safe colour to learn more.

60 Hz here actually runs at ~59.895 Hz, and if we take the reciprocal of that we get ~16.7 ms to draw each frame, which consists of 1024 image lines plus the 39 blanking lines that allow the hypothetical electron gun to travel back from bottom-right to top-left without leaving a mark on the screen. Dividing the 16.7 by 1063, we find that each line has only ~15.7 microseconds to be drawn. That’s the time in which 1280 (image) + 432 (blanking) = 1712 pixels need to be individually processed and delivered. A quick trip back to the calculator, and we get 9ns for each pixel to get painted. Take the reciprocal of 0.000,000,009 and you get a figure of around 109 MHz (or 109 million changes per second) for a signal with sufficient bandwidth to handle 12804 x 1024 @ 60 Hz. And that’s per colour. As a very rough guide, with standard analogue timing (which will be either GTF (General Timing Formula) or CVB (Cumulative Virtual Blanking – not to be confused with Composite Video Blanking and Sync or CVBS)) you will need to add 50 per cent to the required bandwidth for a given resolution and framerate to take blanking into account, although this can vary up to around 70 per cent for some combinations.

n’Sync
Every analogue video signal needs to have a sync signal embedded in it somewhere. The sync signal tells the display to start a new line or a new frame, and ensures that the electron beam travels back to the start of the line or frame without leaving an image on the screen. To do this, the sync signal must occur during the ‘blanking interval’ – a period of zero RGB values. Different connections embed the sync signal in different places, e.g. in the luminance channel, in the green channel, or on one or two separate wires. If the display fails to detect or use the sync signal properly, the image will distort. In the case of the horizontal synch, the display will ‘skew’ diagonally as each successive line is drawn a few pixels too early or too late and a portion of the image may appear at the edge of the screen back-to-front and stretched as the electron beam travels rapidly back to the other side of the screen while image data is being sent. In the case of vertical sync, the display will ‘roll’ up or down as each frame cumulatively includes or removes extra lines.

Digital signals still need this sync data and accompanying blanking space, but since there’s no need to wait for an electron gun it is possible to achieve a substantial reduction in the blanking period, and the blanking period itself can further be used to carry non-video data, like audio.