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The Secret Life of HDMI - Part2

Fri 21 Jan 2011

The Secret Life of HDMI - Part2

In our last article we looked at a little of the pre-release history of HDMI. Since it arrived on the market HDMI has gone through a number of revisions, each adding more functionality. Not all features are deployed by all equipment though. For exact details of what a particular piece of kit does a detailed look through its specification sheet is needed.

These added features are possible because the basic HDMI system has a very high and flexible bandwidth. In this weeks article we’ll take a look at what we mean by bandwidth and how it relates to HDMI and other digital interfaces.

The “bandwidth” of a system is a value that represents the volume of data per second that a system can pass. Since all digital signals are essentially a stream of ones and zeros we can lump all signals (audio, video, other data etc.) together when looking at bandwidth calculations, what they represent is irrelevant. In electrical terms the ones and zeros are represented by the presence of a voltage (one) or absence of voltage (zero) at any given time.

HDMI is not the first high capacity interface to offer a digital video and audio connection. It’s possible to use computer interfaces like USB2, IEE1394 (otherwise known as Firewire or iLink) or Ethernet to move any digital data around, basic audio and compressed video included. However, their respective bandwidths are a bit limited, especially if we need to move high definition pictures.

Before we look at actual numbers lets clarify a couple of things:

  • Digital signals contain a number of “bits per second”. This is literally the number of ones and zeros that arrive during that time
  • Some systems (storage systems particularly e.g. hard drives, memory sticks etc.) use “Bytes”. A “Byte” is a collection of 8 bits, being moved and are stored together
  • For bandwidth considerations we use bits per second, which is abbreviated to “b/s”
  • For larger numbers we can also use “Kb/s” (thousands of bits per second), “Mb/s” (millions of bits per second) or “Gb/s” (billions of bits per second)

Looking at the headline figures for the computer formats mentioned above the maximum data rates are:

USB2 – 400 Mb/s
IEEE1394 – 400Mb/s
100BaseT Ethernet  - 100Mb/s
1000BaseT (Gigabit) Ethernet – 1000Mb/s (or 1 Gb/s)

At first these numbers look pretty big! But for video we need to move a very large amount of data around and we can quickly reach the limits of these interfaces, especially with high definition video.

As a quick and dirty illustration, this is how we work out the number of bits per second we need in order to move  a 1080p 50 frame video picture in real time:

Number of pixels per frame = 1080 (high) x 1920 (wide) = 2,073,600
Number of bits per pixel = 8 (bits per colour) x 3 (colours per pixel) = 24
Number of frames per second = 50


Total number of bits per second = 2,073,600 x 24 x 50 = 2,488,320,000
Or to put it more simply approx. 2.5 Gb/s

As you’ve probably already figured out, looking at the above figure, none of the existing data interfaces mentioned above have enough bandwidth to carry 1080p video! In actual fact the data requirement for such a picture moving along an HDMI cable would be even larger as it also carries audio and control data plus the data for HDCP copyright protection and other “house keeping” information…

Next week we will take a look at the how the different versions of HDMI have added specific features over time.