television traffic

[aerowolf]

'traffic'. When we hear this word, we tend to think of "that stuff that we drive through", aka "that stuff that makes us late and irritable and and miserable". However, when it comes to television (and radio, I would presume, but I don't know for certain on this score) it's a word that means "keeping the signal from being black

Comments

[aerowolf]

The 29.97 thing (technically 1000 frames over the time that 1001 frames would normally play) is specifically related to getting stationary dot patterns off the screen as quickly as possible.

From http://en.wikipedia.org/wiki/NTSC#Color%20encoding:

When a transmitter broadcasts an NTSC signal, it amplitude-modulates a radio-frequency carrier with the NTSC signal just described, while it frequency-modulates a carrier 4.5 MHz higher with the audio signal. If non-linear distortion happens to the broadcast signal, the 3.58 MHz color carrier may beat with the sound carrier to produce a dot pattern on the screen. To make the resulting pattern less noticeable, designers adjusted the original 60 Hz field rate down by a factor of 1000/1001, to approximately 59.94 fields per second.

The 59.94 rate is derived from the following calculations. Designers chose to make the chrominance subcarrier frequency an n + 0.5 multiple of the line frequency to minimize interference between the luminance signal and the chrominance signal. They then chose to make the audio subcarrier frequency an integer multiple of the line frequency to minimize interference between the audio signal and the chrominance signal. The original black and white standard, with its 15750 Hz line frequency and 4.5 MHz audio subcarrier, does not meet these requirements, so designers had either to raise the audio subcarrier frequency or lower the line frequency. Raising the audio subcarrier frequency would prevent existing receivers from properly tuning in the audio signal. Lowering the line frequency is comparatively innocuous, because the horizontal and vertical synchronization information in the NTSC signal allows a receiver to tolerate a substantial amount of slop in the line frequency. So that is the route the color standard took. In the black and white standard, the ratio of audio subcarrier frequency to line frequency is 4.5 MHz / 15750 = 285.71. In the color standard, this becomes rounded to the integer 286, which means the color standard's line rate is 4.5 MHz / 286 ~ 15734 lines per second. Dividing by 262.5 lines per field, this gives approximately 59.94 fields per second.