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explanation of transcoding
http://forum.linhes.org/viewtopic.php?f=6&t=2361		 Page 1 of 1
Author:  adrianbc [ Wed Sep 08, 2004 2:12 pm ]
Post subject:  explanation of transcoding
Hi,

I'm afraid that I've got some n00Bish questions. Could someone please explain the issues assossiated with transcoding. I've searched the forum and the wiki but I can't seem the find a straightforward answer.

I understand that transcoding reduces filesize - which is a major benefit. However, what are the tradeoffs involved -- besides the time taken for the transcoding . Is there a significant falloff in quality when a) watching recordings on the MythBox or b) burning the transcoded file to a DVD, VCD, SVCD when compared the original file? Can one even make a DVD of the resulting transcoded file?

Also I understand that transcoding also let's you get rid of comercials with the help of a cut list.

I've got a PVR-250 (Hardware Encoding Card). Does that change anything?

Thanks,

Adrian
Author:  Xsecrets [ Wed Sep 08, 2004 2:34 pm ]
Post subject: 
well I don't think on NTSC tv you will notice much if any difference in a transcoded file.

Now it can remove the cutlists, but you would need to check them before you transcoded and not use the automatic, because in my experience the commercial flagging is less than spectacular particularly on some shows and some channels.

As for burning them to DVD/VCD/SVCD if you wanted to burn them they would have to be transcoded back to MPEG2/1 from the MPEG4 that they got transcoded to. MPEG4 will not work directly for DVD/VCD/SVCD.
Author:  adrianbc [ Wed Sep 08, 2004 2:50 pm ]
Post subject: 
Thanks XSecrets. Would the quality suffermuch if a file was transcoded twice; once from Mpeg2 to Mpeg4 and then back again?

Adrian
Author:  gr8nash [ Wed Sep 08, 2004 3:48 pm ]
Post subject: 
it would definitly suffer some. how much depends on the eye of the behonlder. Xsecrets doesnt notice imperfections much.. (he has vision problems see his other posts) :wink: lol anyway.. mpeg2 and mpeg4 are NOT lossless compression, meaning both times its compressed more data is thrown out.. leaving less and less actualy data to work with.
Author:  ceenvee703 [ Wed Sep 08, 2004 7:15 pm ]
Post subject: 
I would say it would definitely suffer a LOT, quite honestly.

It of course depends on the resolution and bitrate you transcoded your MPEG4 into, but transcoding will definitely introduce compression artifacts, and would destroy the interlaced video fields of the MPEG2 recording... plus it would take a fair amount of time overall to encode to MPEG4, then back again to MPEG2.

Bottom line, the recordings of the PVR-250 are MPEG2 to begin with and can go right onto a DVD and look great on playback on a DVD player, as long as you record at 320x240 or 352x480 or 720x480. Interlaced fields are preserved and motion is correct--no pseudo-film look for live shows.
Author:  tjc [ Wed Sep 08, 2004 7:55 pm ]
Post subject: 
Since this is a case of cummulative generational error which is essentially mutiplicative, minimizing the loss for your first step can hold the degradation to acceptable levels...

I've run into the same issue with digital photography. Since the raw mode on my camera isn't supported under Linux (at least not last time I checked) I'm stuck using JPEG. Editing JPEG files means you're inevitably going to lose quality, but starting with the best JPEG resolution available and not making intermediate saves in a lossy format can mitigate this very substantially.
Author:  spyderr911 [ Sat Oct 01, 2005 7:13 pm ]
Post subject: 
From reading these posts, I realize I don't know what 'transcoding' is exactly used for.

First off, I have a FX5200 as well, and I gather it 'records' a broadcast in mpeg-2 format automatically (default). Now, since a video file needs to be in mpeg-2 format to burn a DVD, VCD, etc., then why would you want to 'transcode' the files to mpeg-4???

Doesn't mpeq-4 format files take up more disk space? Plus, during the transcoding, you are going to lose some of the quality (although it may be minimal). Which brings me to: why would you want to transcode in any situation? Is it mainly for users without a FX5200 (mpeg encoder card)? Or is it so the new file will be made with the cutpoints (if you made any) already removed for you? Both?

Basically, what is transcoding and how is it useful? In what situations would you transcode a file?

Thanks (ahead of time) for your responses.
Author:  Xsecrets [ Sat Oct 01, 2005 7:48 pm ]
Post subject: 
first thing the fx5200 doesn't record at all that is handled by your tvcard.

And the reason people transcode to mpeg4 is twofold it creates much smaller files, and playback of mpeg4 files takes less processing power than mpeg2. and you can have it actually remove the comercials.

If you have the space and the processing power you would not want to transcode at all that way your files are ready to burn to dvd if that is something you are interested in.
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