[opendtv] Re: Getting From 59.94 to 60
- From: Mark Schubin <tvmark@xxxxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Mon, 04 Apr 2011 10:08:58 -0400
On 4/4/2011 9:48 AM, Stessen, Jeroen wrote:
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.
It's convenient, but it's not strictly necessary.
Barbados used to transmit NTSC-N. That's NTSC color (at roughly 3.6
MHz) on 625-line (576 active lines), 25.00-frame/50.00-field pictures.
Tektronix made them the necessary sync generators. They switched to
NTSC-M (U.S. standard), and the only concern was the power supplies is
some older sets, which needed filtering (the power on the island is 50
Hz). The TVs had no trouble switching between 25.00 and 29.97 frames,
let alone between 29.97 and 30.00, and ALL of them used RF inputs.
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,
They are approximately 900 kHz apart, which makes for a very visible
beat pattern of lines in the image -- at least when the FM carrier isn't
modulated.
TTFN,
Mark
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