OT: discussion on TV frequency history, was RE: [MPlayer-users] R e: which deinterlace filter

[Stephen Davies]

Hi,

I took my "Digital Video and HDTV - Algorithms and Interfaces" by
Charles Poynton off the shelf.  (Great book by the way).

He says that:  "Television systems originated with field rates based
on the local AC power line frequency ... In the 40s and 50s, coupling
of the ripple of a receiver's power-supply into circuitry - such as
video amplifiers and high voltage supplies - had an effect on the
instantaneous brightness of the display.  If the vertical scanning
frequency was different from the power line frequency, interference
caused artifacts called hum bars at the difference in frequency
between the two"

He goes on to say that using the same frequency meant that the hum bar
was stationary or moving very slowly, making it "not objectionable".

525 is 7x5^2x3 - allowing fairly simple vacuum tube divider circuitry
to get from line to field rate.

Where did the 1001/1000 adjustment come from?  He explains that:

When color was to be added, NTS committee (NTSC) realised that due to
nonlinearities in TV circuitry, a practical TV would have
intermodulation distortion between the sound subcarrier at 4.5MHz and
the color subcarrier that they wanted to put at about 3.6MHz.  The
difference is around 900kHz which would be clearly visible in the
luminance.

"They recognized that the visibility of this pattern could be
minimized if the beat frequency was line-interlaced.  Since the color
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." (!)

Now various frequencies could have been slighly changed in order to
get these magic ratios.  Simplest would have been to adjust the sound
subcarrier slightly.  EG the sound carrier could have been adjusted by
1001/1000 - 4.5kHz.  The sound was frequency modulated anyway so
existing TVs wouldn't have noticed the change.  But that was the FCCs
responsibility and they wouldn't let it be changed.  So instead the
line and field rates were adjusted slightly down - 15.750kHz became
15.734, 60Hz became 59.94Hz.  Color subcarrier was 3.579545...MHz

Its interesting to see how much pratical engineering savvy is behind
these strange frequencies.

Steve