What is HDMI?

The High Definition Multimedia Interface (HDMI) is a fully digital audio and video interface that is built to transmit uncompressed data streams. The advantage of HDMI over existing digital standards is that both video and audio can be transmitted over a single cable, which helps reduce the clutter of cables often found in home cinema environments. In addition the HDMI standard supports High-bandwidth Digital Content Protection (HDCP) Digital Rights Management technology.

HDMI provides a high definition video and multi-channel audio connection between compatible digital audio/video sources, such as the SkyHD High Definition receiver, DVD players, PC's, video game consoles (PlayStation 3 - PS3), AV receivers and a compatible digital monitor, such as high quality LCD or Plasma digital televisions.

The HDMI standard was designed to be a replacement for older analogue standards such as SCART, Composite Video, RF/Coax, Component Video, VGA, DVI-A, and RCA/Phono connectors, and to be an alternative to the existing separate digital video (DVI-D & DVI-I) and digital audio standards. Even in the computer world, HDMI is found on many audio/video devices and newer video cards, with adoption rapidly increasing.

What is the difference between 720p, 1080i and 1080p?

HDMI is able to support several different screen resolutions, depending on the source. For example the PlayStation 3 is capable of 720p, 1080i and 1080p.

The number tells you the amount of vertical lines that are drawn on the screen (obviously the more lines the more detail you'll get in the image) and the letter - "p" or "i" - stands for progressive or interlaced.

An interlaced picture draws half the amount of lines for one frame and then draws the of half of the lines on the next frame. This means that a 1080 image is possible with roughly half the data, although picture quality can suffer as a result (especially during scenes with a lot of movement). 1080i is actually comparable to 720p in terms of quality.

A progressive picture such as 1080p is the true 1080 resolution with no trickery used to draw the image on screen. However, not all televisions and displays can display a 1080p signal, so make sure you check your TV's manual before trying.

Still, regardless of resolution, 720p, 1080i and 1080p all look superb when compared with standard definition pictures.

Maximum HDMI cable length

The HDMI standard recommends that HDMI leads with a maximum length of approx 15 metres should be used to carry a signal without loss of quality, but longer lengths have been successfully tested.

The quality of the cable and connectors becomes critical When planning to transmit HDMI signals over greater lengths. Low quality HDMI cables will drop some of the data bits and you will see sparkles on the display, or even complete loss of signal.

Much longer cable runs can be achieved by using an HDMI repeater. This device plugs in between two HDMI cables and acts as a signal booster. HDMI repeaters can even be chained together to achieve extremely long cable lengths - upto 200 metres has been reported.