Articles and whitepapers

Preparing for HDTV (3/10/2005)

By Jon Lane, Smart-e (UK)

Although HDTV has been technically possible for more than 20 years, it is only now that it has become commercially viable. This has been due mainly to the availability and cost-effectiveness of flat panel displays coupled with the new compression techniques of MPEG 4 and VC-1, which allow for artefact-free HD material at bitrates as low as 8Mb/s.

Ever since the invention of television, we have been trying to increase the quality of the pictures. From 405 lines to 625 and colour! With the advent of large flat panel displays however, these resolutions are stretched to the native resolution of the display, and any flaws are magnified.

HDTV gives us TV pictures with the potential of about four times as much detail as the existing service. Subjectively, the picture quality moves from 'fair/good' to 'excellent', plus it is delivered digitally, so the pictures are clean and solid.

HDTV is upon us - it is already being transmitted, there are HD DVDs and camcorders, and with the forthcoming services from Sky next year, we need to prepare. But beware, not all things are the same - there are many proposed standards of HDTV and displays, so quality will vary depending on the ability to display the correct resolution.

What is HDTV?

There are two common HDTV formats in use today, usually referred to as 1080i and 720p. The numbers refer to the number of horizontal lines in each frame of video. So in a 1080i signal, there are 1,080 lines per frame of video, and in a 720p signal there are 720 lines per frame.

The 'i' and 'p' indicate whether the signal is interlaced or progressive. In an interlaced signal, all of the even numbered lines are transmitted in one batch, followed by all of the odd numbered lines. Interlacing is a method of reducing the bandwidth necessary to transmit a signal - and thereby increasing the number of channels - whilst keeping the perceived higher resolution. Current PAL television is 625i. So, with the interlaced 1080i signal, only 540 lines are recorded by the camera and transmitted at a time; they are then reassembled at the time of display.

In a progressive signal, all lines of the frame are transmitted at once in sequence. So, with 720p, all 720 lines are recorded and transmitted in sequence. Given an aspect ratio of 16:9, there will be 1080 x 16/9 = 1920 horizontal pixels for a 1080i signal, giving a resolution of 1920x1080. Similarly, for a 720p signal, there will be 720 x 16/9 = 1280 pixels, giving a resolution of 1280x720. This format works well in the current climate as a lot of flat panel displays are progressive devices and 768 lines in resolution.

Care must be taken when choosing a display, as those with a lower native resolution will need to compress the signal so that it can be displayed. However, help is at hand as the Consumer Electronics Association has specified that any 'HDTV ready' HDTV (which includes a decoder) or HDTV monitor must have active vertical scanning lines of 720 progressive (720p), 1080 interlaced (1080i) or higher. So look for the badge!

HDMI

The cable and connector of choice is the High-Definition Multi-media Interface (HDMI) - an industry-supported, uncompressed, all-digital audio/video interface. HDMI provides an interface between any compatible digital audio/video source, such as a set-top box, DVD player, and A/V receiver and a compatible digital audio and/or video monitor, such as a digital television (DTV).

HDMI connector

HDMI supports standard, enhanced, or high-definition video, plus multichannel digital audio, on a single cable. This video data is then encoded into TMDS (Transition Minimized Differential Signaling) for transmission digitally over HDMI. TMDS incorporates an advanced coding algorithm which has reduced electromagnetic interference over copper cables and enables robust clock recovery at the receiver to achieve high skew tolerance for driving longer cable lengths as well as shorter, low-cost cables. HDMI specifies the required performance of a cable, but does not specify a maximum cable length. Cable manufacturers are expected to sell reasonably-priced copper cables at lengths of up to 15 metres. As semiconductor technology improves, even longer stretches can be reached with fibre optic cables, and with active cable technologies such as amplifiers or repeaters.

HDMI also includes support for 8-channel uncompressed digital audio, the DDC (Display Data Channel) for communication with the display, and the CEC (Consumer Electronic Control) channel which is the standard AV Link protocol.

Molex 500254-1907 HDMI Type A Connector Pin Assignment

Type A HDMI is backward-compatible with the single-link DVI (Digital Visual Interface) used on modern computer monitors and graphics cards. This means that a DVI source can drive an HDMI monitor, or vice versa, by means of a suitable adapter or cable, but the audio and remote control features of HDMI will not be available. Additionally, without support for HDCP, the video quality and resolution may be downgraded by the player unit.

HDCP

HDCP (High-bandwidth Digital Content Protection) has been developed to prevent copying of the digital HDTV signal. The movie studios were so paranoid that anyone could make perfect copies on a mass scale that they invented a method of protecting the digital signal. Unfortunately they spent so much time developing it that manufacturers started producing equipment with digital outputs and inputs using DVI connectors without HDCP. This has created a real problem for backward compatibility!

HDCP works by the video source inserting 'keys' into the digital video signal. These keys are used by both the source device and display to calculate a value. This value is then compared and, if equal, the video image is displayed correctly. This method requires both the source and display to communicate together, which is achieved using the DDC connection.

The problem comes when the display in not HDCP compliant. In this situation, the quality of the image is downgraded to a lower resolution or to a snowy picture, as shown below.

Non-HDCP compliant monitor

System considerations

Connectivity for digital HD will therefore be via either DVI or HDMI, depending on the age of the display. HDMI is the easiest connection, requiring only one cable between the source and display as it also carries the audio channels and control features. Cable lengths up to 15m are now available at reasonable prices. Connecting via DVI will require more than one cable if you need audio, and cable lengths are restricted to 5m.

The transmission of digital signals over cable works very differently to analogue signals. With analogue, the signal gradually degrades as the cable length increases, so you still get a picture but it becomes softer. Digital signals are prone to the 'cliff effect', whereby the image is suddenly lost when the errors become too great. This is not particularly helpful for fault finding!

Although HDMI has an improved cable length over DVI, it still restricts the maximum distance between source and display. For a multiroom video distribution system, the cable length must be dramatically increased. It is early days, but equipment manufacturers are trying to catch up with the technology, and there are some interface products around. A popular method of distributing signals is via Cat5 structured cabling which is easy to install and cheap. While there are some DVI and HDMI Cat5 extenders on the market that are able to transmit the signals up to 50m, make sure that they support HDCP!

There are also distribution amplifiers and some clever interface boxes such as the Spatz DVIMAGIC. It claims to be not only a DVI distribution amplifier but it can adapt non-HDCP displays to HDCP sources. The input is HDCP compatible and can accept DVI input signals from 480i to 1080p.

The spatz DVIMAGIC interface box

Analogue HDTV

Up to now we have been discussing the issues of digital HDTV. Most of these problems however, are eliminated if we connect using the analogue HD signals. The analogue outputs of most HD devices will replicate the resolutions of the digital outputs i.e. 720p and 1080i, so you will still get the same clarity. Connectivity is normally via the standard VGA HD15 connector or the high-resolution Component output using 3 x RCAs.

Using the analogue HD signal removes the HDCP problem as this is not present on the analogue outputs, and so removes the problem of backward compatibility of older displays not being HDCP compliant. The analogue HD signal can then be distributed over standard Cat5 cable for several hundred metres. An example of a solution for distributing HDTV throughout a building is the Smart-e SmartNet X. This product can not only handle all of the HDTV formats, but also any analogue video. It allows you to mix and match the HD with standard component, Y/C, RGBS and even VGA signals up to 1600x1200 resolutions, and is also fully controllable via infrared and serial connections.

The Smart-e SmartNet X distribution system

Summary

We all need to prepare for HDTV since Sky will start transmitting HD content in 2006 with sports and movies. Once this happens, the take-up of HD will increase, so it is important to consider the impact now. Make sure that all displays you buy now are 'HD ready' as this will ensure that there is a HDMI connection and it will accept 720p or 1080i signals.

Although HDMI is the standard connection method for HDTV, you will also find DVI and analogue connection available, but be careful as older displays might not be HDCP compliant.

If you want to future-proof a multi-room installation, then consider installing a Cat5 infrastructure. There are many manufacturers of video and audio distribution systems using Cat5 and any future developments will certainly use Cat5 cable.

Jon Lane Bsc (Hons) MIEE is the Managing Director of Smart-e (uk) Ltd, manufacturer of Cat5 multimedia solutions.

www.smart-e.co.uk