The missus was given some mp4 files of an old show she wanted to watch. Sadly the 4:3 original programming was recorded/broadcast in 16:9.
I know that we could just change the aspect ratio in our player (MythTV) but the acceptance factor would be much higher if the videos could be changed to 4:3 so they play in the right aspect ratio without futzing with the playback menus.
When this happened with some OTA subchannels broadcasting in the wrong ratio, I just used a Windows program called Restream to change the aspect ratio flag in the file header to 4:3, so there was no re-encoding loss but that was for mpeg2 files. I Googled around and found some threads here about doing the same to an .mp4 file. There was a mention of MP4Box. I downloaded the gpac installer, but despite saying to install the whole package, I couldn't locate MP$Box anywhere in the install.
Any suggestion for how to losslessly change an .mp4 file to 4:3?
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Assuming you have h.264 video, you can try using H264 AR Changer. I think you have to demux the video stream, change the AR flags, then remux. But you may still have a problem because not all players will respect the flagged aspect ratio. Most media player software on a PC will respect the flag, but many standalone devices will no (eg, a media player built into a TV).
Note that H264 AR Changer changes/adds the AR flags within the h.264 data stream, not just the header. So it works with more players.
Look at these recordings very carefully before attempting to change their AR. I have noticed an atrocious format change sweep across the "retro tv" subchannels recently: the old 4:3 shows are not being "stretched" to 16:9, they are being hard cropped at the top and bottom to create a pseudo 16:11 semi-wide format. If you change this messed-up butchery to 4:3 you'll end up with distorted skinny actors squeezed into an anamorphic compressed frame that is actually thinner than 4:3. This type of broadcast requires the 16:9 "unsqueeze" flag to eliminate distortion, since it is technically no longer a true 4:3 source.
The tell-tale sign that 4:3 material has been hacked by this now-trendy top/bottom crop is black bars at the sides of the 16:9 screen. They will be thinner than when accurate 4:3 is displayed, but still visible. The older hack of simply stretching 4:3 to 16:9 fills the 16:9 screen completely, with no black side bars at all. This can sometimes (but not always, depends on the stretch algorithm) be repaired by changing the AR code to 4:3.
ffmpeg -i <INPUT_FILE> -aspect 4:3 -c copy [OUTPUT_FILE]
or use my smart FFMpeg gui
Last edited by ProWo; 15th Feb 2020 at 04:26.
Looks like a lot of those old shows is going to be forged, butchered and fried. They are broadcasting wide formats now and backing off from letterbox. Those original 4:3 formats, without any crop will be a valuable commodity soon. . It is against all logic though. Not enough pixels and it is even further cut off and blown up. Why would it even matter if folks have 50'' TV's home.
There was a mention of MP4Box. I downloaded the gpac installer, but despite saying to install the whole package, I couldn't locate MP$Box anywhere in the install.
If you download MeGUI, there's a working mp4box.exe executable with all required dependencies under \tools\mp4box.
If the syntax hasn't changed (found this in an old .bat file) :
mp4box -add "source.mp4#video:par=4:3" -add "source.mp4#audio" -new "source_fixed.mp4"
Then colors. (GSN still does this.)
Or logos (ESPN).
Then partial cropping (lessen black bars).
Then because too much picture was lost, they're basically raping the image now (WGN, TBS) with weird AR/distortion.
Just leave it alone, would you? 4x3 is old content. It has "black bars" on wide modern HDTVs. Watch it as intended. The end.
I've had a 55" HDTV for 10+ years now, mind 2000s, and indeed have no problems viewing 4x3 as 4x3. The image is huge, even if it doesn't fill the screen.
Same problem from VHS days, P&S "full screen" vs. matted widescreen (original theatrical AR).
When people insist that they must fill the screen of the TV, I ask to see the trunk of their car, bed of the truck, or back of the SUV. My question: Why didn't you fill this up? If you're supposed to fill a TV screen, then you're also supposed to fill every container you own.