[Ffmpeg-devel] Analog video grabbing chipsets

Michael Niedermayer michaelni
Sat Apr 14 22:31:06 CEST 2007


On Sat, Apr 14, 2007 at 09:22:21PM +0200, Alex Beregszaszi wrote:
> Hi,
> I need to digitalize old family videos, which are 10-15 years old and
> recorded on VHS-C.
> My question would be what chipset/cards would the folks here use for
> this work.
> Michael, I remember you had done some patches regarding better chroma
> for BT8x8 chipsets. What was the real issue? 

the first issue was that with 4:2:0 YUV the wrong chroma lines where

YUV 420
Luma:      Y y Y y Y y Y y Y y Y y Y y Y y
old:       C c     C c     C c     C c
correct(1): C   c   C   c   C   c   C   c
patch:     C     c C     c C     c C     c
(1) definition from the mpeg2/mpeg4 standard
upper case letters are even lines, lower case odd

so my patch improves it but its not perfect either, perfect would need
a filter which doesnt belong into the driver, simply capturing 4:2:2 and
then downsampling should (assuming the scaler isnt buggy) be ideal

the second problem was that the in hardware filters where set up totally wrong
that is the image is either noisy or over blured, the filters should be
set up based on the output height chroma subsampling and 1 vs 2 fields
being captured, they just set it to either disabled or to the strongest
possible filtering amount

simply capturing at full resolution and (optionally) scaling down in software
afterwards should be ideal and sidestep these issues

> Do they finally merged it?

no, i think not, but i think they did something, if they fixed things i
dont know, but they definitly didnt like my simple patch, it didnt fit
well in their code which had several pages of code instead of a simple
 ... +x)&y or something like that i dont remember exactly, i only
remember that the code was incredibly bloated, like it has been
written by some guy who doesnt know C, the real sad thing is that the
older V4L1 bttv code was much cleaner

> Actually I would prefer an USB2 capable chip, as for the most time I am
> using a notebook. But if there are no good USB2 grabbing cards, a PCI
> one would be ok aswell.
> Any comments and ideas welcome. Even grabbing setting tips :)

hmm how much time do you want to waste with this? if you seriously want
the best possible quality then you could try to remove various artifacts
introduced by teh recording to VHS -> playback -> PAL-> ...

that is record test images and then write a filter which performs
some linear least squares based reconstruction, that should improve
color, sharpness and completely remove any chroma vs. luma shift errors
also you could write a filter which attempts to fix slight hsync jitter
which is common with VHS IIRC, ...

either way id just capture it all in a lossless codec first if i had the
space ...


Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Observe your enemies, for they first find out your faults. -- Antisthenes
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