EBU Technical Review

No. 307 (July 2006)

HDTV fever

As I write this, the 2006 World Cup football championships are under way in Germany. As well as generating huge sales of national flags and football memorabilia, this event is also powering the introduction of HDTV services in Europe. Even if your national team is knocked out in the early rounds, it is clear that the European HDTV services have grabbed the attention of consumers.

The World Cup gives us the opportunity to compare the technical quality of the HDTV and SDTV broadcasts. Obviously, HDTV is much better than SDTV – especially because, as previously noted in Editorial 304, many European broadcasters are squeezing too many SDTV programmes into a digital TV multiplex (some are using less than 2 Mbit/s for SDTV services). As football is fast-moving game with lots of detail in the pictures, it presents particular challenges for video compression systems. The inevitable result is that “ordinary” non-technical viewers notice serious problems on digital SDTV coverage of football: apart from the obvious “blocking” artefacts, you should look at the texture of the grass as the camera pans and zooms – and, especially, look for the improvement in quality when the camera stops moving.

It can be argued that the market will correct such problems. If consumers do not like the picture quality, they will stop using that particular service and, eventually, broadcasters will realise that their falling audience figures are related to poor picture quality. But, during the World Cup, many viewers typically have only one source of TV coverage.

For those of us who have the opportunity to see several SDTV versions of the World Cup broadcasts, the differences in picture quality on the same material are very large. Interestingly, this observation also applies to HDTV. Some services give excellent technical quality, whilst others give mediocre quality. The difference seems to be due to the data rate being used for the various services: some broadcasters have chosen to operate their HDTV services at 20 Mbit/s in order to guarantee good results – whereas others seem to be operating at half that data rate. As all of the HDTV services are using MPEG-4 AVC video compression, 20 Mbit/s is certainly a generous allocation of data rate.

It is often claimed that MPEG-4 AVC can provide similar performance to MPEG-2, whilst requiring only about 50% of the data rate required by MPEG-2. Nevertheless, it seems that the early implementations of MPEG-4 AVC do not achieve that level of performance. Based on our experience of MPEG-2, we can confidently expect improvements in the performance of MPEG-4 AVC encoders in the next 12-18 months. Although broadcasters introducing HDTV may be attracted by operating their services at relatively low data rates, thus saving money and bandwidth, consumers rightly expect HDTV services to have excellent quality. If these expectations are not met, they will not enthuse about HDTV to their friends and the adoption of HDTV will be delayed.

A key ingredient in the recipe for the success of HDTV has been EICTA’s HDTV logos, first mentioned in this column 18 months ago (Editorial 301) are now widely seen in shops.

Fig. 1 — EICTA’s HDTV logos

The logo on the left is attached to set-top boxes and integrated TV sets, whilst the logo on the right is attached to displays to indicate that they are “ready” for HDTV. These logos reduce confusion amongst consumers and, crucially, give them the confidence to buy HDTV products.

One of the key elements in EICTA’s HDTV specifications concerns the use of digital interfaces, such as DVI or HDMI. Most equipment is also equipped with analogue interfaces, such as SCART sockets (also known as Peritel). These interfaces provide legacy support for interconnections with SDTV equipment: for example, HDTV set-top boxes typically provide down-converted analogue SDTV output signals for use with SDTV displays. But how many people have connected their HDTV set-top box to their HD-ready display using a SCART lead? Did they notice that they were watching SDTV rather than HDTV? It is well known that engineers rarely read instruction booklets for new equipment. Let us hope that consumers will read, at least, the first few pages!

Another issue related to HDTV interfaces is the use of HDCP (High Definition Content Protection) on the digital connection (DVI or HDMI) between the HDTV set-top box and the HDTV display. The owners of high value content, such as Hollywood movies, are understandably concerned about the issue of piracy. In particular, they are reluctant to provide content to broadcasters if consumers can easily make perfect digital copies of HDTV versions of their movies. To prevent illicit recording, the SDTV set-top boxes used by pay-TV operators do not include digital outputs that can be connected to recording devices. In the HDTV environment, the situation is more complex: as shown in Fig. 2 where the broadcast signals are protected by conditional access systems (CA). The left side of Fig. 2 shows that the use of unprotected digital HDTV connections between the set-top box and the display is clearly a security hazard.

As shown on the right side of Fig. 2, this unprotected digital connection can be scrambled using the HDCP system. The appropriate descrambling mechanisms are included in HDCP-compliant display devices.

Fig. 2 — Use of HDCP on a pay-TV service

HDCP-enabled set-top boxes will refuse to deliver digital HDTV signals to devices that are not HDCP-compliant, thus preventing pirates connecting a digital recorder to an HDTV set-top box.

Some content owners have indicated that they will not provide content to broadcasters who do not actively support HDCP. This makes sense for pay-TV broadcasters – because pirates will naturally attack the weakest link in the chain, which on the left side of Fig. 2 is obviously the digital connection between the set-top box and the display. However, the situation is very different for free-to-air broadcasters – as indicated in Fig. 3 below.

Fig. 3 — Use of HDCP on a free-to-air TV service

The use of HDCP would protect the link between the set-top box and the HDTV display. But what is the value of protecting this particular link for free-to-air TV services? Any sensible pirate would realise that free-to-air broadcasts are, in fact, “free-to-air”! If you want to make a digital recording of a free-to-air broadcast, it is relatively simple to record the MPEG transport stream which is transmitted at a convenient data rate (e.g. 20 Mbit/s or less) for recording devices. It is much more difficult to record the uncompressed HDMI or DVI streams because they have data rates of more than 1 Gbit/s. In these circumstances, it is obvious that the HDMI or DVI connection is not the weakest link in the chain – and, consequently, piracy would be neither encouraged nor discouraged by the lack of HDCP on set-top boxes used for free-to-air broadcasts.

Piracy is certainly a legitimate cause for concern. The latest movies (e.g. less than 1 year old) included in pay-TV services need some form of copy protection. Free-to-air broadcasters rarely broadcast movies within 3 years of their first cinematic release. As this is also long after DVDs have been marketed by the content owners, it is inconceivable that commercial pirates would be interested in making illegal copies of free-to-air broadcasts. Introducing HDCP on free-to-air broadcasts would have no impact on commercial pirates who have the money and resources to produce illegal copies of DVDs within days of movies first being shown in cinemas.

Consequently, there is no value in requiring HDCP for free-to-air TV services.

Philip Laven
Director
EBU Technical Department

4 July 2006

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