Field rate

525·60 = 15750

2

The flyback transformer scheme was the precursor to modern switchedmode power supplies (SMPS).

A second reason to lock television scanning to power line frequency concerns image capture. Many light sources – for example, fluorescent lamps – flash at twice the power line frequency. If a camera operates at

a picture rate unrelated to the flash rate, then the captured image is liable to contain artifacts owing to the flashing illumination. Various countermeasures can overcome these artifacts; however, the simplest approach to prevent them is to capture at a multiple or submultiple of the power line frequency.

Line rate

The total number of raster lines chosen for the 525-line television is the product of a few small integers: 525 is 7× 52× 3. The choice of small integer factors arose from the use of vacuum tube divider circuits to derive the field rate from the line rate: Such dividers were stable only for small divisors. The total number of scan lines per frame is odd. Equivalently, the field rate is an odd multiple of half the line rate. The 2:1 relationship generates the 2:1 interlace that I introduced in Interlaced format, on page 88. These factors combined to give monochrome 525/60 television a line rate of

30× (7× 52× 3), or exactly 15.750 kHz.

For 525-line receivers, a scheme was invented to develop high voltage for the picture tube using a transformer operating at the horizontal scanning frequency, 15.750 kHz, rather than the AC line frequency. This approach permitted a lightweight transformer, which became known as the flyback transformer. (The scheme is still used today; it can be considered as a precursor to the switch-mode power supply.) The flyback transformer was a complex component, and it was tuned to the horizontal frequency.

When European engineers started designing receivers, it was a practical necessity to fix the field rate at 50 Hz to match the local power line frequency. Rather than develop flyback transformers from scratch, European engineers imported them from North America! Horizontal frequency was thereby constrained to a narrow range around 15.750 kHz. The total line count was chosen as 625, that is, 54. Monochrome 625-line television had a line rate of 56, or exactly 15.625 kHz; that rate was unchanged with the addition of colour.

Hazeltine Corporation (1956),

Principles of Color Television, by the Hazeltine Laboratories staff, compiled and edited by Knox McIlwain and Charles E. Dean

(New York: Wiley).

455 fSC,NTSC = 2 fH,480i

Sound subcarrier

In about 1941, the first NTSC recognized that visibility of sound-related patterns in the picture could be minimized by making the picture line rate and the sound subcarrier rest frequency coherent. In monochrome

525/60 television the sound subcarrier was placed at exactly 2000⁄7 (i.e., 2855⁄7) times the line rate – that is,

at 4.5 MHz. Sound in conventional television is frequency modulated, and with an analog sound modulator even perfect silence cannot be guaranteed to generate an FM carrier of exactly 4.5 MHz. Nonetheless, making the FM sound carrier average out to

4.5 MHz was considered to have some value.

Addition of composite colour

NTSC and PAL colour coding both employ the frequency-interleaving technique to achieve compatibility with monochrome systems. With frequency interleaving, the colour subcarrier frequency is chosen to alternate phase line by line, so as to minimize the visibility of encoded colour on a monochrome receiver. The line-to-line phase relationship makes it possible to accurately separate chroma from luma in an NTSC decoder that incorporates a comb filter (although

a cheaper notch filter can be used instead).

NTSC colour subcarrier

In 1953, the second NTSC decided to choose a colour subcarrier frequency of approximately 3.6 MHz. They recognized that any nonlinearity in the processing of the composite colour signal with sound – such as limiting in the intermediate frequency (IF) stages of

a receiver – would result in intermodulation distortion between the sound subcarrier and the colour subcarrier. The difference, or beat frequency, between the two subcarriers, about 920 kHz, falls in the luminance bandwidth and could potentially have been quite visible.

The NTSC recognized that the visibility of this pattern could be minimized if the beat frequency was lineinterlaced. Since the colour subcarrier is necessarily an odd multiple of half the line rate, the sound subcarrier had to be made an integer multiple of the line rate.

The NTSC decided that the colour subcarrier should be exactly 455⁄2 times the line rate. Line interlace of the

525 60 Hz 1000 4552 1001 2

=315 MHz

88

≈3.579545 MHz

=315 MHz

22

≈14.318181 MHz

Prior to the emergence of framestore synchronizers in the 1980s, every major broadcast network in the United States had an atomic clock to provide 5 MHz, followed by a rate multiplier of 63⁄22 to derive its master 14.318181 MHz clock.

beat could be achieved by increasing the sound-to-line rate ratio (previously 2855⁄7) by the fraction 1001⁄1000 to the next integer (286).

Setting broadcast standards in the U.S. was (and remains) the responsibility of the Federal Communications Commission. The FCC could have allowed the

sound subcarrier rest frequency to be increased by the fraction 1001⁄1000 – that is, increased by 4.5 kHz to about 4.5045 MHz. Had the FCC made this choice, the colour subcarrier in NTSC would have been exactly 3.583125 MHz; the original 525/60 line and field rates would have been unchanged; we would have retained exactly 60 frames per second – and NTSC would have no dropframes! Since sound is frequency modulated, the sound carrier was never crystal-stable at the subcarrier frequency anyway – not even during absolute silence – and the tolerance of the rest frequency was already reasonably large (±1 kHz). The deviation of the sound subcarrier was – and remains – 25 kHz, so

a change of 4.5 kHz could easily have been accommodated by the intercarrier sound systems of the day.

However, the FCC refused to alter the sound subcarrier. Instead, the colour/sound constraint was met by reducing both the line rate and field rate by the frac-

tion 1001⁄1000, to about 15.734 kHz and 59.94 Hz. The

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colour subcarrier was established as 3.579545 MHz. What was denoted 525/60 scanning became 525/59.94, though unfortunately the 525/60 notation is still used loosely to refer to 525/59.94.

The factors of 1001 are 7, 11, and 13. This numerical relationship was known in ancient times: The book 1001 Arabian Nights is based on it. The numbers 7, 11, and 13 are considered to be very unlucky. Unfortunately the field rate of 60⁄1.001, about 59.94 Hz, means that 60 fields consume slightly more than one second: Counting 30 fields per second does not agree with clock time. Dropframe timecode was invented to alleviate this difficulty; see Timecode, on page 399.

NTSC sync generators historically used a master oscil-

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lator of 14.318181 MHz. This clock was divided by 4 to obtain the colour subcarrier, and simultaneously divided by 7 to obtain a precursor of line rate.