by George Jarrett
The EBU has again produced a stunning ‘must see’ format comparison presentation from its long tradition of observing and depicting the evolution of HDTV scanning technologies.
The three screens showing exactly the same clips presented as 720p, 1080i and 1080p at various bitrates applicable to the consumer market, are the latest stage of a history that started in the 1930s with the development of interlace scanning.
Jumping from that bandwidth compression of 2:1 to 1977, David Wood, head of New Media in the EBU’s technical department, set what we see today into context.
“In 1977, an interlaced scanning HDTV format was developed by NHK as an extension of SD TV. This is now called ‘O Generation HDTV’ because it was analogue,” he said. “By the 1990s, both progressive and interlace digital HDTV formats (576p, 720p, 1080i) were adopted in the United States and Australia. These were used in conjunction with MPEG-2 compression for broadcasting, and are now called ‘First Generation (digital) HDTV’.
“In 2006, broadcasting began in Europe with 720p/50 and 1080i/25 using MPEG-4 AVC compression. This is ‘Second Generation’. Then, between 2002 and 2006, all subjective evaluations done in Europe with WideXGA displays independently suggested that 720p/50 delivers marginally higher picture quality than 1080i/25 at high bitrates, and that there will be progressively more advantage for 720p/50 as the broadcast bitrate is reduced,” he added.
The EBU in its wisdom then recommended 720p/50 for broadcasting in the near term but its longer term perspective, as seen here at IBC, is to evaluate the use of 1080p 50 for both production and broadcasting.
“Possibly, this is ‘Third Generation HDTV’,” said Wood. “The purpose of our demonstration is to illustrate some aspects of the potential evolution of HDTV. There are two main dimensions to consider. Under ‘basic quality’, how does the format perform for making programmes? Under the ‘failure characteristic’, how does the format perform for delivering content at reduced bitrates?”
System performance in both cases is affected by the scene content, and basic quality advantages will be affected by the presence or absence of individual detail and movement in the scene. What about the failure characteristic advantages?
“They will be affected by the collective amount of detail and movement in the scene (or entropy)”, said Wood. “We allow the industry to draw its own conclusions about the advantages and disadvantages.
“The demos have the maximum transparency to allow fair and just conclusions to be drawn, but initial results suggest that even with next generation displays (1080p), 720p delivery will always give better picture quality than 1080i,” he added. “The fundamental reason is that there is no interlace to progressive scan conversion to be done in the display.”
Early test results suggest that bitrates needed for 1080p delivery (to a 1080p display) for average entropy and a given quality in the range of grade 4.5 to 3.5, are no higher than bitrates needed for 1080i/25.
“We know that 1080p/50 is virtually as efficient a broadcast format as 1080i/25. MPEG-4 AVC can be as efficient as interlacing,” said Wood.
“Initial results suggest that 1080p/50 production and 720p/50 delivery produces excellent results, even on 1080p displays. Is this the ‘dream ticket’ for HDTV delivery?”