HDV to Blu-ray: why is it recompressing?


John_Cline wrote on 12/31/2011, 1:28 PM
8 mbps is way too low. I just usually encode at whatever bitrate will fit (up to about 35 mbps.)
PeterDuke wrote on 12/31/2011, 7:04 PM
My rule of thumb is that MPEG4 AVC should be about half the bit rate of MPEG2 for the same quality. Just a rule of thumb, of course, not exact.
athomasl wrote on 1/1/2012, 6:12 PM
Thanks. For this particular 90 minute recording of a play, I got these results:

Input: HDV 1440x1080i
Blu-Ray .iso file: MPEG 25Mbps => 14gb, Compression time about 5 hours
Blu-Ray .iso file: MPEG 18Mbps => 11.5gb, Compression time about 5 hours
Blu-Ray .iso file: AVC 18Mbps => 11.5gb, Compression time about 14 hours
Blu-Ray .iso file: AVC 8Mbps => 5.3gb, Compression time about 14 hours
All had AC3 audio.

I would have expected more difference in file size from 25 to 18, but maybe this is normal with relatively static scenes and not much action. Overall, I could not easily detect much difference in visual quality in any of the results. OK, maybe the 8mbps had slightly more blurriness/shakiness in the dark backgrounds, but I bet few people watching it would say that the overall result looks blurry, especially coming from having seen a standard DVD. I focused in on a scene that had a fast-moving object moving across the screen, pausing and advancing one frame at a time. Between the 25 and 18 mbps MPEG files, I saw little difference. There was some blockiness around the edges of the object as it moved. This blockiness is in the original HDV. Between the MPEG and AVC files, I saw that the edges of the object were more blurred instead of blocky. I'm not sure if it is worth the extra processing time to put my files in AVC format, but I'll probably go to AVC anyway. I will just schedule the conversions overnight. I couldn't see a difference when the scene was played at full speed.

I think I'll try aiming to burn my projects with two play-length movies per Blu-Ray, thus I'll use a bitrate calculator to convert my HDV .m2t files to AVC .m2ts with a target size less than 12gb each.

I'll keep the original HDV files around as long as I can. Disk space is cheap, I guess.
jmpatrick wrote on 1/13/2012, 6:49 AM
Tried this last night. Works great. Thanks!
athomasl wrote on 1/27/2012, 3:16 PM
Has anyone tried the magic workaround with long videos, like 2 hours in length? When I do it, the audio and video get horribly out of sync when played back.