[opendtv] Re: Math of oversampling
- From: Jeroen Stessen <jeroen.stessen@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Mon, 25 Apr 2005 08:40:58 +0200
Hi,
Don Munsil wrote:
> It's not something that's simply quantifiable. A very clean
> 480p image gathered with an excellent 480p camera or telecine
> could be every bit as good as a 1080p image scaled to 480p.
Maybe. You're comparing optical low-pass filtering with
electronic (digital) low-pass filtering. The former is done
in the linear-light domain by a gaussian filter, the latter
is (probably) done in the gamma domain by a sinc-like filter.
Both have opportunities for making sampling artefacts, so it
is impossible to tell a priori which would be best.
> Practically speaking, the 1080p->480p signal will often look
> better, largely because of preservation of edge sharpness.
The electronic sinc-like low-pass filter can have a sharper
transition band than the optical gaussian filter, and this
implies that it will create ringing. This ringing contributes
to the sharpness impression, but it is unnatural too...
> An image from a 1080p camera downconverted to 480p by a
> good-quality algorithm like bicubic
That is not a good filter for down-sampling ! You would need
at least a 12-taps polyphase FIR filter for decent performance.
You could use a transposed polyphase filter with fewer taps
(between 4 and 8), see our patent US5892695. The bi-cubic is
not good enough for transposing, unless you apply some further
tricks. Better to use a polyphase filter with lookup tables.
> will probably have better inherent edge sharpness than a
> raw unsharpened feed of a signal from a 480p camera.
Probably, but it is always a trade-off between sharpness and
(aliasing) artefacts. Sometimes less sharpness gives better
pictures. I find that one of the potential attractions of
HDTV: to give a bit more sharpness for a lot less artefacts.
> On the other hand, judicious application of quality sharpening
> algorithms to a native 480p image can quite possibly get it to
> very nearly the 1080p->480p image. It really depends on the
> cameras, the algorithms, the various formats the picture goes
> through, etc.
We call that PixelPlus, and every major manufacturer has his
own version of such algorithms. Sony started it with DRC.
Greetings,
-- Jeroen
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