Breakdown of Analog Broadcast Standards

Jump to: navigation, search

When working with analog assets, it's beneficial for archivists to have a deep understanding of the standards and technology which influenced their design. The following is a description of the various broadcast standards for analog television.

The two major analog standards are the 525-line system at 60hz frame-rate (NTSC) and the 625-line system at 50hz frame-rate (PAL). Before the switch from analog to digital television, North America, Japan, South Korea, and most of South and Central America employed the NTSC system. The majority of Europe, Africa, and the Middle East utilized the PAL system. A third standard, known as SECAM (625-lines at 50hz frame-rate), was used by many African, Asian, and Eastern European countries. There are a few notable outliers, however, such as Brazil, which used PAL-M: a combination of the 525-line system with PAL color. A more thorough list of standards by country may be found here. Although NTSC and PAL have become synonymous with both color and line specifications, it is important to note that PAL, NTSC, and SECAM refer to color transmission only (CVBS, CCVS) and do not directly relate to line count and frequency. This is because chroma and video signals are processed differently. Video signals are amplitude modulated, while in NTSC and PAL systems, chroma signals are IQ modulated. In SECAM, they are frequency modulated, as are audio signals. Despite these differences, it is typical for NTSC to utilize the 525-line system and PAL to utilize the 625-line system. Understanding the standards utilized by different geographic regions may offer clues as to the contents of an asset, and the equipment necessary to preserve it.

Visualization of analog bandwidth

For American 525-line systems, VHF and UHF stations are assigned channels 6 MHz wide, meaning all information included in a signal must fit within this bandwidth. This includes information on color, sound, luminosity, and sync signals. As a bandwidth-saving technique, analog television used two interlaced video signals to assemble an image. Although interlacing builds a single frame, it is using information taken at two different times (with differences of 1/60th a second NTSC and 1/50th a second PAL). Not all lines in the aforementioned systems are visible, however, as 50 lines are blanked out in a 625-line system and between 38 and 42 lines are blanked out for a 525-system. These lines fall within the horizontal and vertical blanking intervals, which contain information on captioning, sync pulses, and color burst. The resulting images are 480 interlaced fields of visible resolution for NTSC systems and 576 interlaced fields for PAL systems (known as 480i and 576i, respectively). Horizontal scan frequencies are measured in kilohertz and can be found by multiplying vertical resolution by refresh rate, with results being halved when using an interlaced system. For example to find the khz required to produce a 525 line system at 60 hz progressive scan we would multiply 525 by 60, resulting in 31,500 or about +/-31.5 khz of horizontal scanning. If this was an interlaced signal, the result would be 15,750, or about 15.75 khz. Side-by-side, the numbers demonstrate the amount of bandwidth saved by using interlaced fields over progressive scan. When deinterlacing an image, two fields with different information are forced together. Video consisting of fast movement and imagery will be horribly out of sync, resulting in screen tearing and other artifacts. For this reason, it is advised that interlaced video remain so after digitization.

NTSC was originally 30 frames per second when transmissions were in black and white. To account for additional color information, framerate had to drop to 29.97 fps so that all information could fit within the 6MHz of bandwidth utilized by the 525-line system (which was really 4.5MHz of visible image, as additional bandwidth was reserved for horizontal and vertical blanking pulses). A large reason for the smaller frame rate was due to the positioning of the chroma signal within the available bandwidth, which fell between the luminance and audio signals. Additional bandwidth could be saved by placing the chrominance signal within the luminance signal, which accounts for the change in chromaninance when luminance is adjusted. At the initial 30fps, sound and chroma carriers were falling in phase with one another, causing image distortion. A change in framerate solved this problem so that audio signals wouldn’t have to be shifted. PAL was created specifically with the issues of NTSC in mind. Their 625 line system solved the problem of a non-integer framerate and the phase-alternating line (PAL) technique solved problems caused by the phase distortion of chroma signals.

Personal tools
MediaWiki Appliance - Powered by TurnKey Linux