[opendtv] Re: 4:3 transmissions of syndicated programming
- From: "John Shutt" <shuttj@xxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Wed, 5 Mar 2008 09:37:43 -0500
----- Original Message -----
From: "John Willkie" <johnwillkie@xxxxxxxxxxxxx>
You said that -- not excerpted here -- AFD and pan/scan - were redundant
to
sar and dar.
No, John, I did not say that. Show me my quote. I said that DAR and AFD
were different critters. Here's a big hint for you: Read A/53 part 4,
table 6.11
You will note that this table is a list of valid values for the variable
<active_format> which is the AFD variable. You will also note that a single
value has two meanings, depending on if the frame is 4:3 coded or 16:9
coded. Therefore, AFD all by itself is not enough information for the
decoder to know how to properly display the encoded video. The decoder also
needs to know the the variable <aspect_ratio_information> which is the
source material's native format (either 4:3 or 16:9 which is encoded into
the MPEG bitstream as per ISO/IEC 13818-2 [3], and is referred to in the
ATSC document as "DAR") AND the decoder needs to know what sort of display
the decoder is connected to (either 4:3 or 16:9, and entered by the user
when setting up the decoder.) The decoder can then calculate "SAR," and can
also use AFD, if present, to know exactly how to best present the encoded
information to the viewer.
But, and this is a big point that you need to understand, John, AFD all by
itself is not enough information for a decoder to do it's job properly.
To qoute A/53 part 4, paragraph 6.2.4.2:
"The combination of source aspect ratio and active_format allows the decoder
to identify whether the "area of interest" is the whole of the frame (e.g.
source aspect ratio 16:9, active_format 16:9 center), a letterbox within the
frame (e.g. source aspect ratio 4:3, active_format 16:9 center), or a
"pillarbox" within the frame (e.g. source aspect ratio 16:9, active_format
4:3 center)."
Note the use of the word "combination" in the paragraph above. AFD all by
itself is not enough information for a decoder to properly display the
encoded material.
On the other hand, SAR and DAR, all by themselves, is enough information for
a decoder to properly present the encoded video, and this is routinely done
billions of times a day. No AFD required. The displayed video may not be
optimized, i.e. you may get the "postage stamp effect" on your display, but
the image presented to the viewer is not horizontally or vertically
distorted.
John
----------------------------------------------------------------------
You can UNSUBSCRIBE from the OpenDTV list in two ways:
- Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org
- By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word
unsubscribe in the subject line.
Other related posts:
