On Tue, 24 Aug 2004, Selva Nair wrote:
> Agreed, if its sampling at 14.75Mhz no scaling is needed, only a 1
> pixel padding.
Oops - slight misunderstanding there. For PAL if it was sampling at
13.5MHz there would be no need for scaling because the card would be
giving 704 samples/line. At 14.5MHz it is giving 768. In analog to
digital conversion fractional and odd sample counts get rounded up to an
easier to implement (in hardware) multiple of 8 - thus the 702.xxx
gets bumped up to 704.
Perhaps a little arithmetic will clear things up (and besides, I
feel like improving my typing skills ;)).
PAL
---
Square pixel sampling freq: 14.75 MHz
Rectangular (Rec.601) sampling freq: 13.5 MHz
NTSC
----
Square pixel sampling freq: 12 + 3/11 MHz
Rectangular (Rec.601) sampling freq: 13.5 MHz
Notice anything interesting? The rectangular rates are the same.
You can look those numbers up in any broadcast TV reference.
From those numbers the ratio of the rectangular/square pixels is
derived. I'll omit the reduction of common factors - I assume we've
all have basic math skills :-)
Thus, for PAL
square / rect = (14 + 3/4) / (13 + 1/2 ) = 59/54 EXACTLY
And for NTSC
square / rect = (12 + 3/11) / (13 + 1/2) = 10/11 EXACTLY
Q.E.D.
So, to convert from square to rectangular sampling you use y4mscaler
with "-O sar=PAL" or "-O sar=NTSC" which mean "-O sar=59:54" and
"-O sar=10:11" respectively.
All more clear now? ;)
> > Even though I switched to the DV capture method quite some time ago
> > I still use y4mscaler a lot when converting HDTV to widescreen DVD
>
> Oh, HDTV.. well, so its getting harder and harder to avoid scaling, eh?
Yep - 1920x1080i over the air. I use a Samsung T-165 receiver which has
a couple IEEE1394 ports on it. Attach it to my Powerbook and run a
"Virtual D-VHS" program available (free) from Apple and capture the
transport streams. Takes a minimum of a 2GHz G5 to play back a ~14Mb/s
stream so I usually grind the data thru a decode|scale|encode cycle.
Oh, in the US (not sure about Canada and other neighboring NTSC
countries) there's the 'broadcast flag' that the FCC is imposing
(greed/pressure from the MPAA and so on). After July 5 2005 you won't
be able to buy HDTV receivers without copy protection !&$*(@# stuff.
Existing equipment's being 'grandfathered in' so hardware you buy today
that doesn't honor the broadcast flag will still work. Thus if you're
ever going to be interested in HDTV reception on a computer running
other than windoze you may want to get the equipment now while the
getting is good.
http://www.pchdtv.com is a PCI card (that I'm thinking of getting before
it's too late). Linux Journal has mentioned that in 2005 they'll be
covering projects that require "pre-ban" cards...
Cheers,
Steven Schultz
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