Thanks Cliff, really interesting!
-- There are some fine points to add in order to get the best results. I'll try and keep this simple but it is a very complex topic at the level you need to operate at to get the best results and there are several important pitfalls to avoid. If the source video is from a UK VHS tape, the visible picture size after ingest is more likely 768 pixels across x 576 lines high at 25 frames per second. That picture aspect ratio is 4:3 unless the image is stretched to fit 16:9 widescreen (1024 x 576). Wide screen is still digitised at 768 pixels wide. That (anamorphic) stretching is labelled as the pixel aspect ratio on some encoding tools. The picture is split into two separate scans with odd and even lines. We call this interlacing. These 'half' frames exist at distinctly different times on a 50 fields per second timebase. Any movement in the view will have changed from field to field. Moving objects exhibit a horrible combing effect if you just slam them into a progressive single field format. They need to be de-interlaced carefully with smart motion compensation applied. Sam was absolutely right about VHS having very low horizontal resolution. It is limited by the highest recordable frequency and is theoretically 333 pixels across at best. Oddly we can tolerate lower resolution in the horizontal axis. This is why VHS looks so soft compared with DVDs. You should also make sure the heads are clean and the VHS player's tracking is correct because that can cause issues with syncing and source picture quality. The framing is 625 lines for the full picture but you lose some lines for ancillary data services (teletext/copy protection) and flyback. So, stretching the image to 720 lines in the vertical axis is scaling the visible 576 lines up by 1.25. Because that's a non-integer ratio, some lines will get interpolated. Scaling up won't improve your picture quality but it will result in a bigger output file and it will look very nasty if you don't de-interlace it first. TVs have pretty good scaling software so this is best done at playback. The p in 720p means progressive single field vertical scanning and VHS is ALWAYS interlaced, two fields. So are DVDs. If you see a combing artefact on fast moving objects when you pause the video, it hasn't been properly de-interlaced. DVDs have a native resolution of 720 x 576 interlaced SD (Standard Definition) encoded with MPEG-2. That's 720 pixels wide not 720 lines high. They cannot carry 720p video. By convention, we specify horizontal resolution first. Encoding tools describe the same video format in different ways. H.264 is also called MPEG-4-Part 10 or Advanced Video Coding (AVC). Ignore MPEG-4-Part 2. It was an earlier codec that wasn't popular and not as good. H.265 is also known as HEVC and MPEG-H-Part 2. It should deliver files that are 50% smaller than AVC and works up to much higher resolutions so it's good for 2K (Bluray), 4K and 8K. Transcoding upwards from MPEG-2 into H.264 should be OK if you choose a high enough output bit rate. Transcoding is not the same as completely decompressing and recompressing, which might introduce undesirable artefacts and blockiness. Transcoding does not fully decode the video but just rearranges the blocks in the new output format. This is possible because the codecs are essentially working the same way. You will get better quality if you invoke a de-interlace filter and keep the image size to what's on the tape or DVD. A small amount of sharpening might help visually but the codec won't work as efficiently and you will get bigger files. Also bear in mind that only the black and white image is handled at full resolution. The colour is reduced to half the resolution because our eyes can tolerate it. This is a legacy from the way Analog video was transmitted and digital encoders have inherited the same idea to make the files smaller. There is a really good (free) tool called ffmpeg but you need to use that from the terminal command line. There are graphical UI wrappers for that which make it easier to use but the command line gives you more control. It may even be buried inside your preferred tools. If you tweak the output size to be the same as the source video and do some pre-processing before compression and code with HEVC it might look quite good and come out at a smaller size. On Friday, 6 December 2024 at 11:31:20 UTC Paul R Owen wrote:
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- Re: [sussex_mug] VHS Decodin... Sam - MacAmbulance
- Re: [sussex_mug] VHS Decodin... 'Steve Davies' via Sussex Mac User Group
- Re: [sussex_mug] VHS Decodin... Sam - MacAmbulance
- Re: [sussex_mug] VHS Decodin... 'Steve Davies' via Sussex Mac User Group
- [sussex_mug] Re: VHS Decodin... Cliff Wootton
- Re: [sussex_mug] VHS Decodin... 'Paul R Owen' via Sussex Mac User Group
- Re: [sussex_mug] VHS Decodin... 'Paul R Owen' via Sussex Mac User Group
- [sussex_mug] Ingesting Analo... Cliff Wootton
- Re: [sussex_mug] Ingesting A... 'Jason Davies' via Sussex Mac User Group
- Re: [sussex_mug] Ingesting A... 'Steve Davies' via Sussex Mac User Group
- Re: [sussex_mug] Ingesting A... Sam - MacAmbulance
- Re: [sussex_mug] Re: VHS Dec... Martin
- Re: [sussex_mug] Re: VHS Dec... Cliff Wootton
- Re: [sussex_mug] VHS Decoding and... 'Jason Davies' via Sussex Mac User Group
- Re: [sussex_mug] VHS Decodin... 'Steve Davies' via Sussex Mac User Group
