[MPlayer-users] BPP for Frame Size

[Mark Himsley]

On 19 January 2007 19:48:26 -0500 Jason Tackaberry wrote:

> Though I guess if Mark is right (I am skeptical and would like further
> references) and the correct pre-cropped display resolution is supposed
> to be 1050x576, the aspect would need to be adjusted, which you could do
> by prefixing the filter chain with dsize=1050/576.

I understand you may be skeptical, because so many bits of software get it 
wrong. I've worked in video at the BBC for 20 years, I'm an on-line and 
off-line video editor in BBC News, so I have some background in the field.

Another reference is this one 
<http://lipas.uwasa.fi/~f76998/video/conversion/> which is an excellent 
technical explanation.

Regarding the 13.5Mhz and 52us stuff I spoke of earlier, unfortunately you 
need to go back to first principles of analog TV, and I'm only going to 
talk about 625 line 25 frame per second PAL here because it's what I work 
with every day.

And now the maths.

If we have 25 frames per second then each frame is 40ms.
( 1 second / 25 frames: 1/25 = 0.04 == 40ms )

If 1 frame is 40ms then one line is 64us.
( 40ms / 625 lines: 0.04/625 = 0.000064 == 64us )

A line consists of the line sync pulse, back porch with colour sync, active 
picture, front porch and back to the sync pulse. The non-active part of the 
picture is 12us and the active picture is 52us [1].

Now we move into the digital domain. Just about every standard definition 
digital picture is sampled to CCIR 601 [2] (now called ITU-R BT.601-4) and 
that defines that we sample the picture at 13.5Mhz.

So, if we sample that at 13.5Mhz then we sample 702 times in 52us.
( 13500000 * 0.000052 = 702 )

This means the 4:3 or 16:9 picture is always 702x576. So the Pixel Aspect 
Ratios (PAR) for 4:3 and 16:9 PAL are 1.0940 and 1.4587 respectively (to 4 
decimal places)

But CCIR 601 says we record 720 pixels so we put extra at the start and end 
of each line. We actually encode 53.3333us (720/13500000) of the line and 
usually expect to find black in those extra 1.3333us (although it often has 
odds'n'sods of picture and graphic depending on the source of the material).

This means that the actual aspect ratio of the encoded content is NOT 4:3 
or 16:9 because of these extra pixels. The actual aspect ratio of the 
encoded content is 1.3675 for 4:3 (not the expected 1.3333) and 1.8234 for 
16:9 (not the expected 1.7778).

Hope that clears things up.

[1] <http://www.intersil.com/data/tb/tb368.pdf>
[2] <http://en.wikipedia.org/wiki/CCIR_601>

You may also find references in Quantel's Digital Factbook 
<http://www.quantel.com/resource.nsf/Files/Quantel_Digital_Factbook/$FILE/Quantel_Digital_Factbook.pdf>


-- 
Mark Himsley