Multiple sub-nyquist sampling Encoding system
From Wikipedia, the free encyclopedia
Japan had the earliest working HDTV system, with design efforts going back to 1979. The country began broadcasting analog HDTV signals in the early 1990s using an interlaced resolution of 1035 or 1080 active lines (1035i) or 1125 total lines.
The Japanese system, developed by NHK Science and Technical Research Laboratories (STRL) in the 1980s, employed filtering tricks to reduce the original source signal to decrease bandwidth utilization. MUSE was marketed as "Hi-Vision" by NHK.
- Japanese broadcast engineers immediately rejected conventional vestigial sideband broadcasting for well-founded technical reasons.
- It was decided early on that MUSE would be a satellite broadcast format as Japan economically supports satellite broadcasting.
Modulation research
- The idea of FM modulation of a conventionally constructed composite (Y+C, like NTSC and PAL) signal was first tested.
- MUSE initially had a signal structure similar to NTSC. Early MUSE had with the Y at the lower frequencies and the C at higher frequencies just like NTSC. This worked moderately well and was adopted for initial satellite trials in the 1980s.
- Separate transmission of Y and C components was then explored. The MUSE format that is transmitted today uses separated component signalling. The improvement in picture quality was so great that the original test systems were recalled.
- One more power saving tweak was made: 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.
Contents |
[edit] Technical specifications
MUSE is a 1125 line system (1035 visible), and is not pulse and sync compatible with the digital 1080 line system used by modern HDTV. Originally, it was a 1125 line, interlaced, 60 Hz, system with a 5/3((1.66:1) aspect ratio and an optimal viewing distance of roughly 3.3H.
For terrestrial MUSE transmission a bandwidth limited FM modulation system was devised. A satellite transmission system uses uncompressed FM modulation.
The pre-compression bandwidth for Y is 20 MHz, and the pre-compression bandwidth for chrominance is a 7 MHz carrier.
The Japanese initially explored the idea of FM modulation of a conventionally constructed composite signal. This would create 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 ratio for a composite FM signal in the 22 GHz band. This was incompatible with satellite broadcast techniques and bandwidth!
To overcome this limitation, it was decided to use a separate transmission of Y and C. This reduces the effective frequency range and lowers the required power. Approximately 570 W (360 for Y and 210 for C) would be needed in order to get a 40 dB of signal to noise ratio for a separate Y/C FM signal in the 22 GHz satellite band. This was feasible!
There is one more power saving that appears from the character of the human eye. The 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 then 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).
[edit] Sampling systems and ratios
The subsampling in a video system is usually expressed as a three part ratio. The three terms of the ratio are: the number of brightness ("luminance" "luma" or Y) samples, followed by the number of samples of the two color ("chroma") components: U/Cb then V/Cr, for each complete sample area. For quality comparison, only the ratio between those values is important, so 4:4:4 could easily be called 1:1:1; however, traditionally the value for brightness is always 4, with the rest of the values scaled accordingly.
Sometimes, four part relations are written, like 4:2:2:4. In these cases, the fourth number means the sampling frequency ratio of a key channel. In virtually all cases, that number will be 4, since high quality is very desirable in keying applications.
The sampling principals above apply to both digital and analog television.
MUSE implements a varible sampling system of ~4:2:1 ... ~4:0.5:0.25 depending on the amount of motion on the screen.
[edit] Exotic digital audio broadcasting subsystem
MUSE had a very exotic digital audio transmission subsystem. It was based in a bit-reduced stereo audio transmission technique that was notable in its design as it was not psychoacoustical like Musicam. Its details have practically been lost -- but the patents and innovations relating to it are still in effect. The audio subsystem had some similarities to NICAM, but a much more exotic compression scheme.
Because MUSE's audio compression scheme has been made technologically moot by Musicam, MP3 and AAC audio compression methods -- there is no freely available residual technical description of this compression technique on the Internet.
The methods of this codec are described in the IEEE paper: [1]
[edit] Real world performance issues
In the typical setup, three picture elements on a line were actually derived from three separate scans. Stationary images were transmitted at full resolution.However, as MUSE lowers the horizontal and vertical resolution of material that varies greatly from frame to frame, moving images were blurred in a manner similar to using 16 mm movie film for HDTV projection. In fact, whole-camera pans would result in a loss of 50% of horizontal resolution.
MUSE's "1125 lines" are an analog measurement, which includes non-video "scan lines" during which a CRT's electron beam returns to the top of the screen to begin scanning the next field. Only 1035 lines have picture information. Digital signals count only the lines (rows of pixels) that have actual detail, so NTSC's 525 lines become 480i, and muse would be 1035i.
Shadows and multipath still plague this analog frequency modulated transmission mode.
Considering the technological limitations of the time, MUSE was a very cleverly-designed analog system. Though Japan has since switched to a digital HDTV system based on ISDB, the original MUSE-based BS Satellite channel 9 (NHK BS Hi-vision) is still being broadcast. It broadcasts the same programs as BS-digital channel 103, but will end sometime in 2007.
[edit] See also
The analog TV systems these systems were meant to replace
Related standards
- NICAM-like audio coding is used in the HD-MAC system.
- Chroma subsampling in TV indicated as 4:2:2, 4:1:1 etc...
[edit] External links
- HDTV coverage of the Barcelona Olympic Games by M. Romero and E. Gavilan (EBU)
- The HDTV demonstrations at the Expo 92 by J.L. Tejerina and F. Visintin (EBU)
- European Broadcasting Union
- COUNCIL DIRECTIVE 92/38/EEC of 11 May 1992.
