No. 304 (October 2005)
Numerology: 525 + 625 = 601?
This issue of EBU Technical Review contains three articles marking the 25th anniversary of studies in the SMPTE and EBU that resulted in CCIR Recommendation 601 – which forms the basis of all modern digital TV standards. It is instructive to consider why Recommendation 601 has become so successful.
During the 1970s, digital TV equipment was generally confined to "digital islands" in an "analogue sea". In such circumstances, it was logical for the digital equipment to use standards that were closely aligned to the analogue standards. For example, in the PAL environment with the colour subcarrier at fsc near 4.43 MHz, the luminance signals might be sampled at 3 fsc = 13.3 MHz or at 4 fsc = 17.73 MHz. Similarly, in the NTSC environment with the colour subcarrier at fsc near 3.58 MHz, the luminance signals might be sampled at 4 fsc = 14.32 MHz. In the case of SECAM, the digital world had to use separate components because of the use of an FM colour subcarrier in the analogue signal.
In the PAL and NTSC environments, using a sampling frequency that was a multiple of fsc simplified the integration of digital islands in the analogue sea – but the need for different sampling frequencies for PAL and NTSC seemed to rule out the idea of a global standard. For a long time, it looked as if the digital era would perpetuate the existing divisions between PAL and NTSC.
At this stage, most engineers instinctively believed that the sampling frequency must be a multiple of fsc whilst a few suggested that selecting the sampling frequency to be an integer multiple of line frequency, fh, would yield many benefits. In terms of achieving a global standard, there seemed little merit in line-locked sampling because fh = 15.625 kHz for PAL had no obvious commonality with fh = 15.734 kHz for NTSC. Fortunately, Stan Baron observed that frequencies that were multiples of 2.25 MHz were also multiples of the line frequencies for both PAL and NTSC:
The above results could easily have been dismissed as mathematical curiosities. However, some years earlier, Chris Clarke and others in Europe had built digital standards converters which successfully used line-locked sampling frequencies to digitise both PAL and NTSC signals. This practical experience persuaded people to move away from sampling frequencies based on fsc towards a global standard using a sampling frequency that was a multiple of fh for both PAL and NTSC.
Joint activities between the SMPTE and EBU eventually led to CCIR Recommendation 601, based on a luminance sampling frequency of 13.5 MHz. It was the first digital TV standard to break free of the constraints imposed by the legacy systems for analogue colour TV, namely NTSC, PAL and SECAM. It also allowed manufacturers to produce digital TV equipment that would work on either 525-line or 625-line signals.
Despite endorsement by the SMPTE, EBU and CCIR, the success of Recommendation 601 was not rapid. One of the early and most impressive physical manifestations of Recommendation 601 was the D1 video tape recorder which easily demonstrated its superiority over analogue recorders: who can forget the “52 card trick” which involved 51 re-recordings? For many years, the D1 machines were heavily used to create special effects – but these machines were so expensive that they were generally only used for making TV advertisements – rather than TV programmes!
In the early days, much of the market for digital TV equipment was won by “inferior” digital products based on composite, rather than component, digital video. Some of those involved in the development of Recommendation 601 were naturally disappointed that their excellent work was not being implemented rapidly: I recall a fairly desperate suggestion that broadcasters should be asked to sign a declaration that they were not interested in digital composite VTRs! As digital composite recorders were much cheaper than digital component recorders, many broadcasters subsequently decided that digital composite recorders made economic sense as direct replacements for analogue VTRs (i.e. as digital islands in an analogue sea). Although digital component recorders were not an instant success, manufacturers of specialised equipment, such as graphics devices and frame synchronizers, rapidly embraced Recommendation 601 – many had great success because one single model could satisfy the needs of the PAL/SECAM and NTSC markets.
With the benefit of hindsight, we can now see that most of the digital TV pioneers under-estimated the importance of video compression for video recorders, as well as for distribution to consumers. Even so, it is obvious that digital components won the long battle. The real benefits of using digital components occur when the entire production chain has been converted to digital components. Furthermore, digital TV transmissions allow broadcasters who use Recommendation 601 throughout their studios to deliver high quality pictures without using PAL or NTSC at any point between the camera and the consumer.
Obviously, the pioneers of Recommendation 601 deserve our hearty congratulations.
* * * * * * * * * * * * * * *
Regrettably, I am forced to finish on a slightly sour note. Many
broadcasters spend much effort and money (e.g. by using Recommendation 601)
in ensuring good quality pictures in the studios – only to destroy much
of this quality by using very low bitrates
on digital TV. Too many broadcasters in Europe seem to think that MPEG-2 compression
at 2 or 3 Mbit/s can provide appropriate quality for consumers. Although it
is often said that “you can never under-estimate the public”,
ordinary members of the public have recently become more demanding in terms
of picture quality due to the adoption of DVDs and large flat screens. Broadcasters
must take care that they do not alienate the public!
Philip Laven
Director
EBU Technical Department
27 June 2005
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