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

[Stefan Seyfried]

Stephen Davies <steve at daviesfam.org> writes:
 
> 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".

this is - in better english :-) - exactly what i have written :-))
 
> 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.

another plus for PAL, at least in germany sound is at 5.5Mhz -> more
bandwith for video ;-)
 
> "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." (!)

where 50Hz PAL is just lucky sincd line rate is 15625 which lets you
use sound carriers in 125kHz steps, so every country can protect their
own market / its own TV industry and still be mostly compatible (this
is probably the reason, so many different PAL Standards exist which are
only differentiated by their sound carrier). 

> 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

i didn't know this, very interesting.
 
> Its interesting to see how much pratical engineering savvy is behind
> these strange frequencies.

Yes. And it is also interesting to investigate the technology in old
TV sets, how this functions were implemented with a minimum of active
parts :-)

regards,

    Stefan

-- 
 Stefan Seyfried, seife at gmane0305.slipkontur.de
+----------------------------------------------+

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