[opendtv] Re: BBC Demonstrates HDTV Broadcasts over SD Channels

At 10:36 AM -0700 6/23/05, Don Munsil wrote:
>
>Your math is problematic. The macroblocks are 16x16 because the sub-blocks
>are 8x8. A macroblock contains 4 luma blocks in a 2x2 pattern, 1 Cb block,
>and 1 Cr block. So if the image size is a multiple of 16, the chroma size
>will be a multiple of 8. Thus a 720x480 image contains only whole
>macroblocks, with nothing left over. As does a 704x480 image.

The math is correct, but you are correct about the contents of a 
macroblock, and the relationship of the color DCT blocks to the Luma 
DCT blocks.

Thanks for clarifying this.

>
>Essentially all DVD MPEG2 is in fact encoded at 720x480, not 704x480. And
>most DVD masters push the image all the way out to the edges rather than
>limiting it to the central 704 pixels as intended by the video specs, but
>that's a whole other can o' worms.
>

I was aware that this is the common practice. Thic can be done 
because the film is typically scanned to fill the full 720 x 480 
raster, or oversampled, then resampled to 720 x 480.

The typical problem with "601" acquisition devices is that they do 
not have active image samples in the full 720 aperture of the format. 
There are many reasons for this, including the clean apertures that 
are defined for various standards. Some new SD cameras are using the 
full 720 samples for active image information.

Regards
Craig
 
 
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