Articles and whitepapers

HDTV Distribution Around the Home (3/10/2006)

By Jon Jane, Smart-e

High-definition television (HDTV) has the potential to provide around four times as much detail as standard definition, yet despite being technically possible for the last twenty years, it is only now starting to become adopted by the residential mid-market.

There are many proposed standards for HDTV and displays, with varying quality depending on the ability to display the correct resolution, and there is no consensus yet on how to 'future proof' a home for HDTV in terms of cabling. I will therefore outline the issues concerning digital versus analogue HD, and how to overcome some of the pitfalls.

Progressive versus interlaced

There are two common HDTV formats in use today, 1080i and 720p. The numbers refer to the number of horizontal lines (vertical resolution) in each frame of video. In a1080i signal, there are 1,080 lines per frame of video. In a 720p signal, there are 720 lines per frame. The 'i' and 'p' indicate whether the signal is interlaced or progressive.

With an interlaced signal, all the even-numbered lines of a frame are transmitted first, followed by all the odd-numbered lines. In other words, each frame of video is made of two fields - one field of even lines followed by one field of odd lines. The traditional format of PAL television is 625i.

This method of interlacing reduces the bandwidth required to transmit a signal. With the interlaced 1080i signal, only a field of 540 lines are recorded by the camera and transmitted at a time; which means that the bandwidth required to transmit this only needs to accommodate 540 lines. The even and odd fields are then reassembled at the time of display, producing the perceived higher resolution of 1080 lines.

In a progressive signal, all lines of the frame are transmitted sequentially in one go, therefore for a 720p signal, the bandwidth required must accommodate 720 lines.

There is some debate as to which picture quality is better. In theory, the more lines the sharper the image, but interlacing has the drawback that if the action moves between the capture of the even and odd fields, artefacts may appear in the picture.

720p works well in the current climate, as a lot of flat panel displays are progressive devices and support 768 lines of resolution. Generally 720p is better for fast moving images such as sports events, whilst 1080i is more suited to stationary or slow-moving shots. True HD (1080p) would be the best of all, but programmes and equipment supporting this are not yet common.

Truly HD ready

The majority of TV displays being sold in the high street sport the 'HD Ready' badge which the Consumer Electronics Association has specified should have active horizontal lines (i.e. vertical resolution) of 720p, 1080i or higher. However, to be truly compatible and receive HD programmes, a system requires specialist input connectors: an HD analogue component, and for digital distribution, a DVI (Digital Visual Interface) or HDMI (High Definition Multimedia Interface), both of which must support HDCP (High-bandwidth Digital Content Protection).

Connectivity for delivering HD content via digital distribution is either by DVI or HDMI. For video transmission, both have the ability to carry HDTV up to 1080p (with HDCP) which is beyond the currently agreed formats for broadcast, although possibly not for the next generation home cinema. With the current HDTV broadcasts and new HD-DVDs or Blu-ray movies, either connectors will be fine.

DVI

DVI cable

Connecting via DVI requires more than one cable if audio is required, and cable lengths are only restricted by cable quality. A DVI connection maximises the quality of digital displays. With a direct digital connection from a PC to the screen, the signal is not compressed and matches the screen format for maximum quality. By incorporating DDC (Display Data Channel), the picture can be enhanced, and the system supports HDCP. DVI connections are available for both digital and analogue systems.

HDMI

HDMI cable

HDMI is the easiest connection, requiring only one cable between the source and display as it also carries the audio channels and control features. HDMI is an industry-supported, uncompressed, all-digital audio/video interface, that 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/video monitor, such as a digital television. Cable lengths up to 15 metres are now available at reasonable prices and as technology improves, even longer stretches will be reached with fibre optic cables and active cable equipment such as amplifiers or repeaters.

HDMI has the advantage of being able to carry 8 channels of uncompressed audio, and is backward compatible with the single-link DVI 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 adaptor or cable, although the audio and remote control features of HDMI will not be available.

While DVI is likely to continue to be more popular within the computer market, within the current HDTV market, it looks likely that DVI connectors will slowly be phased out in favour of HDMI, due to the latter's 'multimedia' capability.

Digital content protection - HDCP

HDCP is a form of digital rights management (DRM) which was developed by the Intel Corporation to control HD video and audio as it passes along HDMI or DVI connectors. This is implemented to curtail the illegal reproduction of HD digital content. The HDCP specification is proprietary and a licence fee is payable to Digital Content Protection - a subsidiary of Intel, by all who implement the system. Without the ability to support HDCP, video quality and resolution may be downgraded by the player unit.

Analogue HD

The transmission of digital signals over cable works very differently to analogue signals. With analogue, the signal gradually degrades as the cable length increases, resulting in a softer picture. Digital signals on the other hand, are prone to the 'cliff effect' whereby the image is suddenly lost when the errors become too great.

Connecting HDTV using analogue HD signals eliminates many of the problems which arise with digital HDTV connections. The analogue outputs of most HD devices will replicate the resolutions of the digital outputs i.e. 720p and 1080i, providing the same clarity. Connectivity is normally via the standard VGA HD15 connector or the high-resolution component output using three RCAs. Using the analogue signal removes the HDCP problem as this is not present on the analogue outputs, therefore removing the problems of backward compatibility on older displays not being HDCP compliant.

The analogue HD signal can be distributed over standard Cat5 structured cable for several hundred metres. With the right distribution equipment, all HDTV formats can be handled as well as any analogue video. This makes it possible to mix and match the HD with standard component, Y/C, RGBS and even VGA signals up to 1600x1200+ resolutions. Additionally, such a system will be fully controllable via infrared and serial connections.

Future proofing with Cat5

One of the best methods of future-proofing a multiroom installation is to consider installing a Cat5 cabling infrastructure. It is relatively cheap and easy to install if procedures are followed correctly. It is suitable for both digital and analogue distribution, and a can be used for simple point-to-point systems through to full multiformat matrix switches. There are also several DVI and HDMI Cat5 extenders on the market able to transmit the signals up to 50 metres, however it is necessary to ensure that they support HDCP.

The Smart-e SNX8x8 compact multiformat switch extends up to 8 individual inputs and outputs up to 300m over cat5/5e/6 cabling with RS232/422 or infrared control of both switch and devices

Ultimately, many factors such as budget and the client's requirements will dictate whether to opt for digital or analogue connections. The one area of no contention, and one which should provide future-proofing is the cabling, i.e. Cat5. It is robust, cost-effective and there is an ever-expanding range of equipment all designed to meet this cabling standard.

Jon Lane is the managing director of Smart-e Limited, designer and manufacturer of high-quality audio and video distribution and switching equipment.

www.smart-e.co.uk