[opendtv] Re: Europe now debates i vs p
- From: Doug McDonald <mcdonald@xxxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Mon, 13 Sep 2004 09:16:24 -0500
jeroen.stessen@xxxxxxxxxxx wrote:
>
>
>>It's not aliasing ... aliasing only starts at 37 MHZ, and at the
>>next "wrap around" the 18 MHZ analog cutoff would get rid of it.
>>There was stuff in the first alias range above 37 MHZ, by the way.
>>The stuff in the 30-33 MHz range was real. It may have been
>>garbage, but it was not alias and not noise. In one case there
>>was stuff at 30MHz that was clearly the 3rd harmonic of
>>stuff at 10 MHz ... the stripes were visible on the screen.
>
>
> I'd say that Doug is almost right, at the end. It IS aliasing.
> Even if the transmission signal is bandwidth limited to 18 MHz,
> there will still be higher harmonics that are generated by the
> gamma correction function. The latter is needed because the signal
> is transmitted "in the gamma domain", whereas the light is
> generated "in the linear-light domain". Here lies the cause for
> aliasing.
>
The signal I measure is at the output of the STB. I don't think
that STBs are anything but linear.
And as I said, the 18 MHz filter is an ANALOG filter at the
output of the STB. This is clearly visible ... there is a
staircase type signal output with a rise time of 9 nanoseconds,
single pole. (That's the 1-1/e lifetime.)
The signal above 25 MHZ is NOT aliasing. It is real signal.
Jeroen is of course right about the possibility of it being
a harmonic that would go away after the gamma correction
inside my (LCD) TV set. However, the clearest high frequency
signal I saw was a high frequency square wave, and a gamma
correction would have little effect on such a thing.
I think it fair to say that the real signal has content to
30 MHz. And why should it not? These are the sharpest and
clearest pictures I have ever seen on a TV ... the static
logos and score blocks are extremely sharp on the screen.
Both ABC and Fox apparently take pride in having the sharpest HDTV
pictures around, and they most certainly do, at least on my TV.
(Note to the time-impaired ... yes, Fox does HDTV (present tense)).
Doug McDonald
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