[FFmpeg-user] Problem with vf pad ... help please

[Mark Himsley]

On 17/09/2012 17:53, Tim Nicholson wrote:
> On 17/09/12 16:36, Nicolas George wrote:
>> Le jour de la Vertu, an CCXX, Mark Himsley a écrit :
>>> You are falling into the trap that the all of the pixels on a 720x576
>>> line are part of the active 4:3 picture. Where as only 702 pixels are.
>>>
>>> If you wish to ignore the calculated scale and pad values I gave you
>>> above, and insist on sticking with your calculated numbers, then you
>>> (like many before you and many to follow) will get the will get the
>>> images 2.5% too narrow. I'm sure that will flatter most people, being
>>> 2.5% too thin, but it is wrong. Please believe the experience of 26
>>> years at the BBC.
>>
>> Do you have any normative reference about that? It seems to contradict what
>> I remember (I'll have to dig into the standards again) reading.
>>
> 
> You have to work it out from ITU-R BT.601-7 TABLE 4 and ITU-R BT.470-6 !
> 
> This gives (for 625/50):-
> 
> Number of samples per total line for each signal =864.
> 
> Analogue total line timing 64us
> Analogue active line timing 52us
> 
> Therefore digital samples per active analogue line = 864*52/64 =>702.

Or to put it another way.

The sample frequency of an analogue 'PAL' or 'SECAM' line is:

1 / 0.000064 * 864 = 13.5 MHz exactly.

Which makes the active 52 micro-seconds have exactly 702 pixels

0.000052 × 13500000 = 702

I've heard people say that the size of a digital line is different to
the size of an analogue line. But that is not how it was (is, for those
places which have not gone all digital yet) transmitted, so is a wrong
assumption.

> The 4:3 aspect ratio dates back to analogue days and so aspect ratio
> calculations must be based on the analogue picture width and height, i.e
> 702x576.

Exactly.

> Hope this helps.

Yes - it saved me a lot of typing. Thanks Tim.


I think I'd like to submit
http://lipas.uwasa.fi/~f76998/video/conversion/ as evidence too, as
Jukka has a good way with words and a good selection of references.

-- 
Mark