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

[Andy Walls <awalls@xxxxxxxxx>]

On Fri, 2008-08-22 at 22:32 +0200, Daniel Glöckner wrote:
> 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.

Ah, of course. Thank you.  

I should have known that. :P  I didn't mentally connect decimation with
pulling the alias spectra in closer to overlap the main spectrum.


Regards,
Andy


> 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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