[mythtv-users] Image quality of HD3000 for OTA sources

Michael T. Dean mtdean at thirdcontact.com
Fri Jun 10 08:46:43 UTC 2005

Scott wrote:

> 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  
> stream

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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