[mythtv-users] Image quality of HD3000 for OTA sources
Michael T. Dean
mtdean at thirdcontact.com
Fri Jun 10 08:46:43 UTC 2005
> On Jun 9, 2005, at 7:20 PM, James Stembridge wrote:
>> If you play interlaced content on a progressive display then it will
>> indeed look bad. Use deinterlacing.
> Sure. I know this :) But the question is more of is the tst.ts file
> provided on the pchdtv.com download page a 1080i or 720p native HDTV
Yes. 1080i60 direct from the broadcast source.
> or was it a 480i upconverted to either 720p or 1080i? I'm not sure
> how I could tell w/o knowing the details of the source it was
> recorded from.
That's the beauty of the pcHDTV 3000 (and *all* ATSC and *all* DVB tuner
cards)--they don't record images. Digital TV tuner cards simply
demodulate a signal and dump the data from within that signal to the
ATSC specifies that the data within the signal will be an MPEG-2 encoded
stream. The specification defines 18 formats, of which 2
high-definition formats are commonly used--720p60 and 1080i60. The
720p60 format has 1280x720 pixels per field, 60 fields per second, and
60 frames per second (i.e. each field is a complete frame). The 1080i60
format has 1920x540 pixels per field, 60 fields per second, and 30
frames per second (i.e. 2 fields are interlaced together to create a
single 1920x1080 pixel frame). For more information see
http://www.hdtvprimer.com/ISSUES/what_is_ATSC.html . Therefore, the
pcHDTV is not encoding video and the video it provides to the computer
is the *exact same* video that was transmitted by the broadcaster.
It's also a very good thing that the pcHDTV doesn't have to encode the
video because hardware capable of real-time encoding of a 1080i or even
a 720p format is currently /way/ too expensive (>$3000). Both 1080i60
and 720p60 have about six times the number of pixels as 720x480 SDTV
(480i30), so real-time high-def encoding would basically take 6
PVR-150's each encoding a portion of the video plus the circuitry
required to coordinate the encoders. Put another way, simply decoding
the stream takes pretty good quality hardware and decoding is
significantly easier than encoding.
While the pcHDTV does have an NTSC tuner, it's a frame grabber--so even
for NTSC, the pcHDTV doesn't encode the signal (the encoding is left to
your CPU). The card doesn't do any upconversion of SDTV.
> Either way, the tst.ts clip is of very poor quality it seems and I'm
> concerned that it represents the same quality levels I can expect
> from the HD3000 adapter. I hope that's not the case.
The clip is not meant to show off the quality of the image (because you
get exactly what's transmitted), but to allow you to determine if your
hardware has what it takes to decode a 1080i signal.
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