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MUSE - Japanese HDTV

As of today, Japan is the only country actually broadcasting HDTV services. Approximately 30,000 receivers and 100,000 converters have been sold to customers of this service. It is widely believed that the establishment of this analog broadcast service essentially eliminates the possibility of starting a digital satellite HDTV service in Japan.

The history of this begins in 1968, when Japan's NHK began a massive project to develop a new TV standard. This 1125 line system, is an analog system which uses digital compression techniques. It is a satellite broadcast system which is not compatible with current Japanese NTSC terrestrial broadcast. (This actually makes a lot of sense for Japan, as they are a single group of islands easily accessed by one or two satellites).

The MUSE system as originally developed by NHK was a 1125 line, interlaced, 60 Hz, system with a 5/3 aspect ratio and an optimal viewing distance of roughly 3.3H. The pre-compression bandwidth for Y is 20 MHz, and the pre-compression bandwidth for chrominance was 7 MHz. As time has passed, this standard has been altered and upgraded.

The various standard MUSEs are summarized below:

            lines per   field rate  Y           C           C           aspect      
            frame                   bandwidth   bandwidth   bandwidth   ratio       
                                                - wide      - narrow                
NHK-1980    1125 lines  60 Hz       20 MHz      7 MHz       5.5 MHz     5/3         
MUSE 1986   1125 lines  60 Hz       20 MHz      6.5 MHz     5.5 MHz     5/3         
SMPTE       1125 lines  60 Hz       30 MHz      30 MHz      30 MHz      16/9        

In considering how to broadcast this signal, Japanese engineering immediately rejected conventional vestigial sideband broadcasting (i.e. broadcasting methods similar to NTSC). They immediately jumped to the idea of using satellite broadcast (no doubt helped by the geography of the Japanese Islands, which economically support satellite broadcast.)

The Japanese initially explored the idea of FM modulation of a conventionally constructed composite signal. (This would be a signal similar in structure to the Y/C NTSC signal - with the Y at the lower frequencies and the C above.) Approximately 3 kW of power would be required, in order to get 40 dB of signal to noise for a composite FM signal in the 22 GHz satellite band. This was virtually incompatible with satellite broadcast!

So, the next idea was to use separate transmission of Y and C. This drops the effective frequency range and dramatically reduces the required power. Approximately 570 W of power (360 for Y and 210 for C) would be required in order to get 40 dB of signal to noise for a separate Y/C FM signal in the 22 GHz satellite band. This is much more feasible!

There is one more power saving that appears from the character of the human eye. Lack of visual response to low frequency noise allows significant reduction in transponder power if the higher video frequencies are emphasized prior to modulation at the transmitter and de-emphasized at the receiver. This method was adopted, with crossover frequencies for the emphasis/de-emphasis at 5.2 MHz for Y and 1.6 MHz for C. With this in place, the power requirements drop to 260 W of power (190 for Y and 69 for C).

As mentioned earlier (see the section on compression) - the problem of fitting the combination Y/C signal into the 8.15 MHz satellite bandwidth was solved by digital compression. Summarizing the previous discussion, the NHK HDTV signal is initially digitally sampled at 48.6 Ms/s. This signal controls two filters, one responsive to stationary parts of the image - one responsive to moving parts. The outputs of the two filters are combined and then sampled at the sub-Nyquist frequency of 16.2 MHz. The resulting pulse train is then converted by to analog with a base frequency of 8.1 MHz.

next up previous
Next: All digital HDTV and Up: HDTV Previous: Compression
Dave Marshall