[mythtv-users] HDTV on Myth -- is it a "myth?"

Doug Larrick doug at ties.org
Mon Oct 11 12:55:33 UTC 2004

Cory Papenfuss wrote:
>>> What is the same, Doug?  The horizontal scanning
>>> frequency of 33.75kHz?  I agree.  How about the image?
>>>  With 1080i you get two different frames of
>>> information, 30 times each second.  With 540p you get
>>> two identical frames 30 times each second.  Shouldn't
>>> they look different?  I think they should.
>> No.
>> 1080i = 2 fields of 540 lines, at 30 Hz
>> 540p = 1 frame of 540 lines, at 60 Hz.
>> Six of one, half a dozen of the other.  Almost literally.
>> -Doug
>     Same frequencies for the TV, agreed.  It's not exactly the same, 
> perceptually, however.  An 1080i signal will resolve higher vertical 
> resolution (Kell factor notwithstanding) than a 540p signal, even though 
> the TV's H/V PLL's will by running at the same rate.  The difference 
> would be most apparent on a still frame with horizontal lines on it.... 
> annoying 30Hz flicker, granted, but higher resolution yes.

What I'm saying is that for 1080i, the TV electrically has *no way* of 
telling the difference.  The signals transmitted are identical, whether 
the hardware sends each field, or if you run bob deinterlace in software 
and send the exact same fields sequentially.

Note that this is untrue for NTSC and its 525 lines, where one frame has 
one more line than the other.

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