[opendtv] Re: Getting From 59.94 to 60
- From: "Stessen, Jeroen" <jeroen.stessen@xxxxxxxxxxx>
- To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
- Date: Mon, 4 Apr 2011 15:48:31 +0200
Hi,
Craig wrote:
> This should have happened as part of the DTV transition. Actually the
> frame rates should have been 24/36/72 as we proposed in 1992.
>
> Unfortunately, the powers to be saw interlace and 59.94 as barriers
> to competition for the computer industry, That didn't work out for
> them either...
Your government wanted subsidized converter boxes for everyone
with an old NTSC TV. If it must have a CVBC (Cinch) or even RF
output, then it is very convenient if all the frequencies are
NTSC compatible. If you had first standardized on components
inputs, like Europe did with the Scart RGB connector, then it
might have been a non-issue. Well, for Europe it's a non-issue
anyway, as our field frequency was already a nice round number.
ATSC had to remain compatible with NTSC for the converter boxes !
The history of the 59.94 Hz frame rate is a bit more complicated
than I read here. System M started as 60.00 fields per second,
15750 lines per second, and a sound carrier at 4.500 MHz, i.e.
at 285.71 times the line frequency.
For NTSC, the chroma carrier was added at 227.5 times the line
frequency. This had to be an odd multiple in order to suppress
the cross-color and cross-luma artefacts. Cross-luma exists
also on B/W TVs, which could not be retrofitted with a chroma
trap. Hence the (smart !) choice for the odd multiple.
(BTW this also makes it possible to apply "comb filters".)
For some reason which I can not reconstruct, there was going to
be some interference between the chroma and (FM) audio carriers,
so the sound had to be moved to an even multiple of the line
frequency. However, due to the way that FM demodulators work,
it was unwise to move the audio carrier itself, and therefore
the line frequency was changed by -0.1% instead. Line sync
circuits are much more tolerant than FM audio demodulators.
Now the audio carrier is at 286.0 times the new line frequency.
And now you understand why Charles Poynton charges exactly
USD 59.94 for his book... For the same sad reason...
The video - audio relationships matter only on RF links, but
those are common on older TVs because they do not require a
chassis that is galvanically separated from the 110 V mains.
(A simple small capacitor in the antenna input is sufficient.)
The line frequency - chroma carrier relationship matters only
on composite video (CVBS) links, but those have not completely
been replaced by the Y/C (S-VHS) links. CVBS is still the norm.
Interestingly, my latest TV does not have a single Y/C input
anymore. It has 4 HDMI, 1 analog Y'PbPr, and 2 Scart RGB
(and CVBS) inputs. But nothing for my two S-VHS recorders...
Best regards,
-- Jeroen
Jeroen H. Stessen
Specialist Picture Quality
Philips Consumer Lifestyle
High Impact Innovation Center
High Tech Campus 37 - Office 8.042
NL 5656 AE Eindhoven - Nederland
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