Resolution Issues in Downconverting HDV
J. Beale Feb. 15 2006
The Sony FX1 camera can record a lot of
detail. Sometimes, when downconverting to produce
a standard-definition DVD, you have too much. Here is one
an interview that I did, examining a detail area by a shirt collar. The
images below are a 1:1 crop from an original HDV frame (using square
pixels at 1920x1080). The camera "sharpness" was set to 14; a high
setting which for some cases yields good results. In the
original frame shown below at top left, the fine pattern of a shirt is
reproduced with some
moire patterns, but the image looks fine at the original resolution on
a laptop screen. When I simply exported the project from Vegas 6
as a 720x480 widescreen AVI for conversion to DVD, I saw severe moire
the shirt (see below). This was further compounded by the MPEG2
compression on the
DVD, causing the moire bands to pop in and out of view depending on the
bit-budget in each separate half-second GOP. As a result, the DVD
playback looked truly awful on the same laptop that showed the HDV
looking just fine.
You can reduce the fine detail by checking "reduce interlace flicker"
properties but even that may not be adequate. I used a linear blur
filter set at 90 degrees (vertical) and a strength of 0.001 (the
minimum Vegas allows), as shown at the lower left. This resulted
in a reasonable looking downconverted video for DVD.
Crop from original HDV frame (1920x1080)
Original with Properties / Reduce Interlace Flicker" checked
Original plus linear filter at 90 degrees, strength 0.001
Both linear filter (90/.001) and Reduce Interlace Flicker
Below are three photos of the same Dell Inspiron 9300 laptop (1440x900
pixel, 17" LCD display) playing the two DVDs and the original HDV for
comparison. At left is the first DVD created from the original HDV
pre-filtering. On the right, another photo with everything the same but
playing a DVD using the linear filter as described above, prior to
downconversion to SD resolution. The original HDV file is shown
below left. Note that these images are only a small portion of the full
HDV frame, as indicated below right. In each case I am using Nero
Showtime 2 for display, which turns
out to be particularly bad for moire artifacts.
WinDVD does not show as severe a
problem playing this particular DVD.
photo of laptop screen showing moire pattern
photo of same laptop playing corrected DVD
photo of laptop showing original HDV image
detail area shown relative to full HDV frame
Ideally, interview subjects shouldn't wear clothing with fine patterns. In this particular situation I couldn't
ask my subject to go home and change.
I didn't realize the
problem until I got home, since the shirt looks fine on the
FX1 LCD screen and in HDV playback on the laptop.
Incidentally, I only blanked out the interviewee's face since I didn't
ask permission to put his photo online. He's not suspected of