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Definitive guide to reencoding HD for SD
http://forum.linhes.org/viewtopic.php?f=11&t=16875		 Page 1 of 1
Author:  jjoshua [ Fri Oct 05, 2007 8:17 pm ]
Post subject:  Definitive guide to reencoding HD for SD
Is there a definitive way to reencode HD for display on a 4:3 SD set?

I'm not ready to upgrade my frontends but I would like to watch content from HD (16:9) channels.

I'm pretty sure that I need to set up a user job. What command should I run?
Author:  mihanson [ Fri Oct 05, 2007 8:26 pm ]
Post subject: 
Not a definitive answer you requested, but I think if you use "Seprate Video Modes for GUI and Playback" (Somewhere in the Utilites/Setup > Setup menus) you can define what resolutions to display at a lower (or higher) resolution.

i.e. 1920 x 1080 @ 60 Hz > 640 x 480 (or 800 x 600?)
1280 x 720 @ 60 Hz > 640 x 480, etc...
Author:  jjoshua [ Fri Oct 05, 2007 8:29 pm ]
Post subject: 
I'm pretty sure that I want to convert the file to a lower resolution to save disk space.

I'm also pretty sure that my frontend can't convert the resolution on the fly like that.
Author:  mogator88 [ Fri Oct 05, 2007 8:45 pm ]
Post subject: 
You cannot convert and store to low resolution on fly without

1) real compromises in quality
2) not getting best compression
3) a TOL cpu

The best quality and compression come from analyzing the whole recording... meaning you can't do it on the fly.

Actually, I think some analog tuners and S-Video input cards are capable of this, meaning they can generate a lower quality video stream. But if you're taking an OTA or firewire HD stream, then you're not using that kind of equipment anyways.

I record OTA digital, so take all this with a grain of salt and bias. Myth's built-in transcode will do the job for you. But IME SD channels don't compress enough (10-15%), and an HD recording compressed about 90% but lost the sound once. I don't understand the settings well enough to know what would make it better.

I set up a job to run nuvexport and generate an Xvid file. However, I have to manually replace the original recording using myth.rebuilddatabase.pl, a PITA. One day I'll mod the script to do the replacement automatically, but with very rusty Perl skills, kids and a long list of home projects that might not be for a while. Also, now that my recordings are on a PATA disk, my Xvid conversions don't play smoothly, they pause every so often. Every 10-120 seconds I get it.

Now just for viewing, your frontend will convert the appearance of the video ("aspect ration") by pressing the W key on your keyboard. If you use a remote, the aspect ration setting is part of the OSD menu options.

Not sure if all this answers your questions, but I HTH. Post back if it don't.
Author:  cecil [ Fri Oct 05, 2007 9:12 pm ]
Post subject: 
As longs as your frontend is powerful enough, you don't have to reencode. The video will simply have borders on the top and bottom. Now, if you frontend isn't powerful enough, you can try transcoding to xvid. There are also a few script floating about here and on MythTV's archive.
Author:  jjoshua [ Fri Oct 05, 2007 10:37 pm ]
Post subject: 
I'm using an xbox frontend so it's not powerful enough to display HD.

I'll check out the xvid scripts but I was really hoping to keep everything as mpeg.
Author:  grante [ Sat Oct 06, 2007 9:02 pm ]
Post subject: 
mogator88 wrote:
You cannot convert and store to low resolution on fly without

Nobody said anything about doing it "on the fly", did they?
A normal job-queue transcode should work fine for him.
Quote:
1) real compromises in quality

I transcode from HD to SD, and I see no change in quality.
Quote:
2) not getting best compression

Even if I'm not getting "best compression", converting 8MB
files into 2MB files saves a lot of disk space.
Quote:
3) a TOL cpu

No clue what a "TOL" cpu is.
Quote:
The best quality and compression come from analyzing the whole
recording... meaning you can't do it on the fly.

The quality acheived from normal tanscoding is fine. I can't
tell the difference.
Quote:
Actually, I think some analog tuners and S-Video input
cards are capable of this, meaning they can generate a lower
quality video stream. But if you're taking an OTA or firewire
HD stream, then you're not using that kind of equipment
anyways.

Eh? I don't know what you're talking about about. We're
talking about HD signals, so analog tuners and S-Video input
cards don't really seem relevent.
Quote:
I record OTA digital, so take all this with a grain of salt and
bias. Myth's built-in transcode will do the job for you. But
IME SD channels don't compress enough (10-15%), and an HD
recording compressed about 90% but lost the sound once. I
don't understand the settings well enough to know what would
make it better.

I've always used a user-job that utilizes mencoder. I get
about 75% reduction in space when I transcode from HD MPEG2 to
SD MPEG2. I do have problems with sound loosing sync when the
OTA signal drops.

Since I'm no longer using my PVR-350 card, I don't need MPEG2
files, so I'll probably give the built-in tanscoder a try.

Quote:
I set up a job to run nuvexport and generate an Xvid file.
However, I have to manually replace the original recording
using myth.rebuilddatabase.pl, a PITA. One day I'll mod the
script to do the replacement automatically, but with very rusty
Perl skills, kids and a long list of home projects that might
not be for a while. Also, now that my recordings are on a PATA
disk, my Xvid conversions don't play smoothly, they pause every
so often. Every 10-120 seconds I get it.

Replacing the file isn't too difficult. Just rename it so that
it has the correct name. Then you need to at least delete the
seek table for the show. It would be nice if you could run
commflagging on the transcoded file to regenerate a seek table,
but that doesn't work. I had to write a custom seek-table
generator program in C -- which doesn't seem to work quite
right but it's close enough to be usable.
Quote:
Now just for viewing, your frontend will convert the appearance
of the video ("aspect ration") by pressing the W key on your
keyboard. If you use a remote, the aspect ration setting is
part of the OSD menu options.

His frontend isn't powerful enough to play the HD files (I have
the same issue), so his frontend won't simply convert the video
when he plays it -- he (and I) have to transcde to a lower
resolution.
Author:  jjoshua [ Tue Oct 09, 2007 1:57 pm ]
Post subject: 
So it's not looking like there is a definitive way to do this?
Author:  grante [ Tue Oct 09, 2007 3:06 pm ]
Post subject: 
jjoshua wrote:
So it's not looking like there is a definitive way to do
this?

Sure: the built-in transcode feature. You just need to set up
profiles for the output resolution you want.

Here's a good explanation of how to do it:

http://www.myhdbox.com/mythtips/2006/05 ... r-hdtv.php

Some of the external scripts that use mencoder are a bit
fancier and do things like crop 16:9 shows horizontally so that
they're less "letterboxed" when viewed on a 4:3 screen.
Author:  mogator88 [ Tue Oct 09, 2007 4:50 pm ]
Post subject: 
grante wrote:
mogator88 wrote:
You cannot convert and store to low resolution on fly without

Nobody said anything about doing it "on the fly", did they?
A normal job-queue transcode should work fine for him.


Yes jjoshua did. 2nd reply to original post.

TOL = Top of the Line
Author:  grante [ Tue Oct 09, 2007 9:28 pm ]
Post subject: 
mogator88 wrote:
grante wrote:
mogator88 wrote:
You cannot convert and store to low resolution on fly without

Nobody said anything about doing it "on the fly", did they?
A normal job-queue transcode should work fine for him.


Yes jjoshua did. 2nd reply to original post.

He's saying his frontend can't play HD content. That's
why he wants to transcode HD recordings.
Quote:
TOL = Top of the Line

Ah.
Author:  yamez [ Thu Dec 20, 2007 6:01 am ]
Post subject: 
You can automatically transcode HD TV and SD TV to DVD standard MPEG-2 after the recording. The original recording is replaced by the transcoded MPEG-2 file and you play through the MythFrontend normally.

Please look here...

http://mysettopbox.tv/phpBB2/viewtopic. ... highlight=

I have upgraded and simplified the script since then and will post my upgrade this weekend.

I have had this working for well over a year now. Another bonus, besides space saving, is all recordings are in a DVD ready format for archiving.
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