CCIR System M

CCIR System M, sometimes called 525–line, monochrome NTSC, NTSC-M, or CCIR-M,  is the analog broadcast television system approved by the FCC (upon recommendation by the National Television Systems Committee - NTSC) for use in the United States since July 1, 1941, replacing the 441-line TV system introduced in 1938. System M comprises a total of 525 interlaced lines of video, of which 486 contain the image information, at 30 frames per second. Video is amplitude modulated and audio is frequency modulated, with a total bandwidth of 6 MHz for each channel, including a guard band.

It was also adopted in the Americas and Caribbean; Myanmar, Philippines, South Korea, Taiwan and Japan (here with minor differences, informally referred to as System J). System M doesn't specify a color system, but NTSC (NTSC-M) was normally used, with some exceptions: NTSC-J in Japan, PAL-M in Brazil and SECAM-M in Cambodia, Laos and Vietnam (see Color standards section below).

The letter M designation was attributed by the ITU at the 1961 Stockholm meeting (see ITU identification scheme).

In 1965, Thailand decided to replace System M with 625-line CCIR System B, which started in 1967, adopting PAL at the same time.

Since 2015, System M is being replaced by digital broadcasting, in countries such as the Americas, Japan, South Korea, Taiwan and the Philippines.

Specifications
Some of the important specifications for System M are listed below:
 * Lines (total): 525
 * Lines (visible): 486
 * Frame rate: 29.97 Hz (NTSC color), 30 Hz (monochrome and PAL-M color)
 * Channel bandwidth: 6 MHz
 * Visual bandwidth: 4.2 MHz
 * Sound offset: +4.5 MHz
 * Vestigial sideband: 0.75 MHz
 * Vision modulation: Negative
 * Preemphasis: 75 μs
 * Sound modulation: FM
 * Color standards: NTSC-M, NTSC-J, PAL-M, SECAM-M, Clear-Vision, B-MAC

NTSC-M and NTSC-J
Strictly speaking, System M does not designate how color is transmitted. However, in nearly every System M country NTSC is used for color television. This combination is called NTSC-M, but usually simply referred to as "NTSC", because of the relative lack of importance of black-and-white television. In NTSC-M and Japan's NTSC-J, the frame rate is offset slightly, becoming $30/1.001$ frames per second, usually labeled as the rounded number 29.97.

PAL-M
The main exception to System M's being paired with NTSC color is Brazil, where PAL color is used instead, resulting in the PAL-M combination unique to that country. It is monochrome-compatible with other System M countries, but not compatible with other PAL countries, which use 625-line based systems.

SECAM-M
Between 1970 and 1991 a variation of the SECAM color system, known as SECAM-M, was used in Cambodia, Laos, and Vietnam (Hanoi and other northern cities).