Re: [v4l-dvb-maintainer] bttv driver errors

["Daniel Glöckner" <daniel-gl@gmx.net>]

On Fri, Aug 22, 2008 at 08:26:34AM -0400, Andy Walls wrote:
> What are the benefits of using the moving average filters before
> decimation?

The benefit is that the upper spectrum is attenuated before it is
mirrored into the lower spectrum by the decimation. After the decimation
you have no way of removing those artifacts in a mathematically correct
way.

There are better FIR filters than 0.5 * x(0) + 0.5 * x(1), which will
avoid aliasing even more and preserve more of the lower spectrum.
This is just the simplest.

Btw., enabling the chroma comb filter in bttv will result in alternating
0.5*x(-2)+0.5*x(0) and 0.5*x(-1)+0.5*x(1) for frame captures and
0.5*x(-1)+0.5*x(0) for field captures. I have once tried to modify
bttv_risc_planar to be closer to MPEG chroma subsampling when the comb
filter is enabled, because I was annoyed by the gray first line.

On Thu, Aug 21, 2008 at 01:50:05AM -0700, Trent Piepho wrote:
> A better question would be how does the bt878 do horizontal and vertical
> scaling?  If you look at the description of ultralock and the number of
> taps avaiable for the vertical scaling filters, the chip must have some
> kind of multi-line buffer before the scaler.  But this buffer, and the
> delay it must introduce, is never mentioned in the datasheet.

The buffer is "mentioned" in figure 2-4 of the Fusion 878A datasheet
available on the Conexant website. Its size can be derived from the
restrictions placed on the luma filters. It should be 768 luma samples.
The decimation then has another full line buffer for luma and chroma
to perform the linear luma interpolation and the chroma comb filter.

  Daniel

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