[opendtv] Re: 720P = 540i ?

  • From: Jeroen Stessen <jeroen.stessen@xxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Tue, 3 May 2005 10:24:09 +0200


Bob Miller wrote: 

> This thread says that received 1080i on a native 720P will see the 
> conversion of a single 540i frame into 720P by 1980. 

I should hope not ! 
Most if not all de-interlacers for HDTV are "motion adaptive". 
On still images, and on 24p or 30p film mode, they will weave two 
540i fields into one 1080p frame. No problem at all. If you consider 
that much material is originated from 24p film, or from 24p video 
productions, then that is really 1080@24p transported as 1080@30i. 
Perfect reconstruction of the 1080p is then trivially easy, at the 
cost of one or two field memories in the receiver. 

It is only with moving images in (50i/60i) video mode that there is 
a fundamental problem for de-interlacers. There will no longer be 
two fields from the same motion phase, so weaving does not work 
anymore. Then intra-field interpolation, indeed from 540 lines, is 
the easiest (but incorrect) solution. Motion adaptive de-interlacers 
will necessarily switch to intra-field mode. The smarter MA designs 
make such decision (between still/film and video mode) for each 
individual pixel, preserving weaving e.g. for still backgrounds. 

A yet better solution, which we don't typically see for HDTV, is 
based on application of the "Generalized Sampling Theorem". This 
says that any two sets of samples, they don't have to be spaced 
evenly as long as they don't coincide exactly, can be used for 
reconstructing the original signal at the double Nyquist bandwidth. 
With the use of motion vectors a "motion compensated" de-interlacer 
can get the missing lines from previous frames, at least if the 
vertical component of the motion is not near a "critical speed". 

(A vertical motion of 2N+1 lines per field period reduces a 1080i 
grid to a real 540p grid, it makes the two sets of samples coincide 
and thus it makes de-interlacing and reconstruction of the full 
vertical resolution according to the GST fundamentally impossible.) 

> How does this impact what we you were discussing about 
> 1080p>480P>720P? 

Not. We weren't discussing 1080i->1080p conversion, and it will 
only go wrong for moving images in video mode, which are can be 
largely avoided by choosing 720p for difficult (sports) programs. 
We should have some faith in today's decent de-interlacers. 

> How would that 720P compare to 720P converted in the former way 
> from 1080i? How would 640 x 480P compare to 1920 x 540i?

Well... using only half the lines from a sharp 1080 line image 
implies a serious aliasing problem. Re-sampling is not going to 
make that aliasing go away. On the contrary, it can make it 
forever impossible to reduce the aliasing by proper de-interlacing. 
So it would be a bad idea to apply scaling to an improperly de-
interlaced image, which is of course why all decent scaler ICs 
contain also a decent de-interlacer (at least MA) that can do a 
proper weaving of matching fields in film- or still image mode. 
The "DCDi" type of de-interlacer pioneered by Faroudja (who later 
seems to have denied its effectiveness, very strange) does already 
a good enough job for most applications. So don't worry, the 
statement on slashdot is a distortion of the truth. 

-- Jeroen 

| From:     Jeroen H. Stessen   | E-mail:  Jeroen.Stessen@xxxxxxxxxxx |
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