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Multiple sub-nyquist sampling Encoding (MUSE) is the first HDTV compression and transmission system. It was designed by NHK's Transmission Research Section from the early 1970s till its unveiling in the late 1980s. It was marketed as "Hi-Vision" by NHK.
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.
Technical specification
MUSE is a 1125 line system (1080 visible), and is not pulse and sync compatible with the digital 1080 line system used by modern HDTV.
For terrestrial MUSE transmission a bandwidth limited FM modulation system was devised. Shadows and multipath still plague this MUSE transmission mode.
A satellite transmission system used uncompressed FM modulation.
Some decoders and recoders still exist for this system.
Other analog HDTV systems have been abandoned in the past for other technical reasons. France and Belgium abandoned the 819 line system for a more standard 625 line SECAM.
History
NHK after 20 years of research introduced MUSE HDTV into Japan the early 1980s, and in the US in the late 1980s.
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 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 for a composite FM signal in the 22 GHz satellite band. This was incompatible with satellite broadcast techniques and bandwidth!
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 was 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).
Real world performance issues
MUSE lowers the horizontal and vertical resolution of material that varies greatly from frame to frame.
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.
Exotic digital audio broadcasting subsystem
MUSE had a very exotic digital audio transmission subsystem. 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: //ieeexplore.ieee.org/iel1/30/2796/00085585.pdf?arnumber=85585
See also
"Multiple sub-nyquist sampling Encoding"