On Tue, 2005-09-20 at 15:58 -0400, Brian McEntire wrote: > When resizing, is it possible to accidentally stretch the image? > > I saw someone say 480x480 wouldn't be a noticeable difference from > 720x480... is the because of the way a TV uses scan lines? On a > monitor it seems like it would be very different.
To be fair there are two issues here. First of all you have the immediate source of the signal. In general they will be 1) Pure analog 2) Some form of mpeg compression If you have dish network afaik they encode normal channels with 480x480 mpeg 2 compression. In general this will limit the frequency content in the signal so that it is not useful to sample it at a higher resolution than 480x480 again. Of course with the case of the number of lines, that is a fixed quantity and you simple sample at 480 lines and be done with it. In the case of the horizontal resolution this is not 100% true since, unfortunately, you can't sample the exact same positions as were originally encoded, and there is always the case of noise being added due to the digital to analog conversion. (The noise may increase the frequency content of the signal, and thus require a higher sampling frequency to preserve the information content. Of course, it is quite possible the capture card will filter out high frequency noise before capturing data anyway.) In the case of a typical myth setup, you will likely see some black bars on the side of your encodings meaning your capture card is possibly sampling parts of the tv signal that has no data and dish network never encoding anything into, so that to, be paranoid, and assure you are not losing quality you might have to sample at a bit more than 480 pixels horizontally. Perhaps 512x480 or 640x480 for instance. Of course things are not that simple either. As you increase the sampling resolution, you must also increase the bit rate of the encoding to avoid degradation. This is why a 480x480 encoding may look better than a 640x480 encoding if the bit rates are the same. If in doubt, try various settings and compare them. There are a lot of factors here, and the vague arguments I made only make a case for not going too much beyond what the signal was originally encoded as. In the case of a pure analog signal that was received by an aerial, things are different and it may indeed help to go 720x480 or 640x480, provided you also give the codec a large enough datarate to handle the video. Finally, if you absolutely know you are going to transcode something anyway, you generally want to have the capture size set to whatever your final size will be, and set the initial capture datarate a bit on the high side so you preserve as much quality as possible prior to the final transcoding to mpeg4 or whatever. I used to use huffyuv for captures in windows, even though it used a very high datarate, because I wanted the initial encoding to be lossless, before I edited the system and converted it to mpeg. (Huffyuv is probably too high of data rate for myth and useless with cards designed to capture in mpeg2 anyway since you would have to add a pointless recompression.) > > I'm curious because I want to transcode HDTV (1920x1080, 1280x720, > and 850x480) down to 850x480. > > For HDTV pulled off of QAM/Cable, I'm seeing MPEG2 file sizes of about > 6.2 GB/hr. > _______________________________________________ > mythtv-users mailing list > mythtv-users@mythtv.org > http://mythtv.org/cgi-bin/mailman/listinfo/mythtv-users
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