1From Gerhard Monzel AT sap.com 2Date: Mon, 21 Jan 2002 09:36:34 +0100 3 4While producing mpeg videos, you have to give special attention to the 5desired image size of your output video (this is a big difference to 6avi-encoding). Because mpegs, especially VCD and SVCD can be played in 7consumer players, there are some restrictions in "resizing" ! 8Usually image sizes for VCDs are 352x240 (NTSC) or 352x288 (PAL), for 9SVCD 480x40 (NTSC) or 480x576 (PAL) -> some consumer players can play VCD 10sized videos as SVCDs too and vice versa. 11 12As a further difference to AVIs, the aspect ratio of the video is stored 13in header of the mpeg-videos. Therefore most players (pc and consumer 14players) will display the right image proportions no matter what X/Y dimensions 15your video is encoded. 16 17In your case of a 720x576 vob in 16:9 aspect ratio, you should use 18480x576 as destination image format (PAL) and (force) apsect ratio to 16:9. 19 20The resulting video will look like expected on PCs with e.g. mplayer, 21which will honore the aspect ratio, but many consumer players have problems 22switching to 16:9 format in VCD/SVCD mode -> in this case you need a resizer 23capable of generating black bars on top/bottom of your video. 24 25This can't be done with the trancode resizer, but with the internal mpeg- 26module resizer (I mean -y mpeg not -y mpeg2enc). You can also mix the resizer 27of transcode with the resizer of the mpeg-module too. 28 29On base of this knoweledge you will concluse, that following options 30should produce a "original" looking picture on a 4:3 aspect ratio: 31 32"-B 5 -y mpeg -F s,3 ..." 33 34"-B 5" => 720x416 (formats image near to 16:9), 35 36"-F s,3" => 480x576 (formats image to SVCD size with black bars on top 37(80 pixels) and botton (80 pixels), so it will look well on 4:3 display. 38 39Because of using only high speed resizing options, the transcoding 40process will run in better speed than using zoom or other slow rescaling 41options. Unfortunately transcode itself can't handle black bars for output 42video (in AVI world there is no need of this feature and transcode begans as 43AVI transcoding tool). Of cause this described solution won't work 44with other export modules e.g. "mpeg2enc" (it lacks of internal black bar 45resizer). 46 47There is no suggestion to calculate filesize for mpegs with variable bitrate 48(usually used in SVCD mode), because filesize depends on kind of your 49source video (luminances, motion etc.) and the quality value (quant_value) 50you are using while encoding. Only the max. possible size is known by the 51max. bitrate restriction for SVCD and the count of frames. 52to avoid problems with oversized mpeg-files (won't fit on CD), the mpeg- 53encoder module can forced to split the encoded file in several parts. 54The size per part in MB is given by the "max_file_size"-Parameter used in 55conjunction with a user-profile 56(-> remember option -F s,3,<name of user-profile>, where this profile may 57look like: 58max_file_size = 700 59... 60 61The encoding speed in SVCD-mode varies in a wide range, because of the 62big influence of bitrate control method: the encoder calculates the actual 63bitrate for all frames of one GOP (group of pictures) and if the max. 64bitrate limit is exceeded, the whole GOP will be encoded again with a higher 65quantizer value until bitrate fit into limit. Bitrate (for SVCDS) depends 66mainly from "quant_value" (lower value / better quality -> higher 67bitrate). In conclusion higher quality leads often to lower speed, so it's 68your decision what is more worth for you :-) 69But there there are some tricks to reduce bitrate for mpeg-2 typed streams 70without loosing much quality: 71- use of parameter "qscale_type = 0" in your own user-profile (worth to try) 72- use image size 352x288 instead 480x576, but with smaller "quant_value" 73 (3-5) in your user-profile 74 (most consumer SVCD players have no problem to play such sized streams) 75maximum effect results in combination of both hints. 76 77To observe actual bitrate during encoding you should set transcode option 78"-q 2", so you can run short encoding turns to find out the best quant_value 79settings. 80 81To test the maximum encoding speed you should use option -F 1,3 82so you'll see how much the SVCD bitrate control will drop speed. 83 84Common problems: 85---------------- 86Green artefacts may be caused from wrong resizing too, please try with 87options of the above description. 88 89Using "-V" option (needed to encode mpegs) will force import modules to 90use internal YUV-format instead of RGB. 91 92But there are several different YUV-Formats and for example some of it 93has swapped chroma (UV-) components. 94 95Weird color on encoded mpegs is not a bug, therefore you can use the "-k" 96option to "reswap" the croma components. 97 98 99_______________________________________________ 100transcode-users mailing list 101transcode-users@theorie.physik.uni-goettingen.de 102http://www.theorie.physik.uni-goettingen.de/mailman/listinfo/transcode-users 103 104 105