Thomas Smet
Veteran
Anybody else finding the down converted HDMI signal and recording it to a external recorder a bit too sharp? I finally got around to comparing a 10bit external recording 1080p ProRes422 HQ to internal 4k down converted to 1080p. What I found was that not only did the ProRes version have more contrast but it was sharper as well. The contrast I found out is because I am shooting 0-255 and some external recorders truncate that to 16-235. The sharpness brings up an interesting topic however.
Just how good is the on the fly HDMI down conversion on the GH4? Its a nice looking signal but just how good is it compared to a software down conversion? In some ways we expect it to be better because it does appear sharper but a sharper down conversion isn't always the cleanest down conversion. I can't tell exactly what the GH4 is doing to get a 4k sensor down to a 4:2:2 10bit HD HDMI signal but it may not be 100% perfect.
What I'm now seeing is the HDMI recorded version has more texture and sharp edges. This could partially be from ProRes handling noise in a video a lot better but it could also be from a simplified down scaling method. We know the GH4 when recording HD has to use a binning process to get that sensor down to HD size. This camera doesn't have the processing power to do a bicubic or better down conversion on the fly 30 or 60 times a second. So then how is the GH4 doing its down conversion? Could it be using the same pixel binning process but with cleaner results since its down converting a perfect 2:1 ratio? I doubt its anything crude like nearest neighbor because that would have horrible artifacts but I really don't think its anything like bicubic or even bilinear. Its just way too sharp compared to a down conversion in After Effects, FCPX or Premiere and I know the camera doesn't have the processing power for Lanczos or anything more complex.
ProRes422 HQ crop on the left and 4k internal to HD post on the right.
Here is a video another GH4 user did with a similar test which shows sharper fine details and more pixel chatter in the trees. You can really see it if you pause 20 seconds into the video.
https://vimeo.com/101350338
Finally we have known for a long time about the benefits of down converting 4k to get 4:4:4 but here is a crop of just a single chroma channel to see how external HD 4:2:2 compares against down converted to HD 4:4:4. Blown up to 400% so you can see whats going on. Under most circumstance you will not be able to see the chroma difference at HD resolution but it is there and under the right conditions can show up. The 4:2:2 from external HD recording may suffer even more with the sharper edges. The sharper the video the more chroma subsampling is noticed.
Of course this is all kind of splitting hairs but if we are striving for the best possible image and don't need original 10bit for V-log then down converting in post may deliver better results than capturing to an external recorder for HD. If you have a 4k external recorder ignore all of this since the camera doesn't need to down convert anything. I should point out that the HDMI HD capture does look great and is still a solid solution and workflow. Just be aware that it may not be the best down conversion possible. I have yet to determine if the sharper edges do cause any added artifacts or if the sharpened details are whats natural from the camera and the H264 is what kills the detail in the other versions. Honestly I find it hard to believe the GH4 can do a down conversion that good with the limited processing power it has.
Anybody know for sure whats going on or has done a similar test with an external 4k recorder to see if the video is sharper than internal 4k? I only have the means to record externally in HD right now.
Just how good is the on the fly HDMI down conversion on the GH4? Its a nice looking signal but just how good is it compared to a software down conversion? In some ways we expect it to be better because it does appear sharper but a sharper down conversion isn't always the cleanest down conversion. I can't tell exactly what the GH4 is doing to get a 4k sensor down to a 4:2:2 10bit HD HDMI signal but it may not be 100% perfect.
What I'm now seeing is the HDMI recorded version has more texture and sharp edges. This could partially be from ProRes handling noise in a video a lot better but it could also be from a simplified down scaling method. We know the GH4 when recording HD has to use a binning process to get that sensor down to HD size. This camera doesn't have the processing power to do a bicubic or better down conversion on the fly 30 or 60 times a second. So then how is the GH4 doing its down conversion? Could it be using the same pixel binning process but with cleaner results since its down converting a perfect 2:1 ratio? I doubt its anything crude like nearest neighbor because that would have horrible artifacts but I really don't think its anything like bicubic or even bilinear. Its just way too sharp compared to a down conversion in After Effects, FCPX or Premiere and I know the camera doesn't have the processing power for Lanczos or anything more complex.
ProRes422 HQ crop on the left and 4k internal to HD post on the right.
Here is a video another GH4 user did with a similar test which shows sharper fine details and more pixel chatter in the trees. You can really see it if you pause 20 seconds into the video.
https://vimeo.com/101350338
Finally we have known for a long time about the benefits of down converting 4k to get 4:4:4 but here is a crop of just a single chroma channel to see how external HD 4:2:2 compares against down converted to HD 4:4:4. Blown up to 400% so you can see whats going on. Under most circumstance you will not be able to see the chroma difference at HD resolution but it is there and under the right conditions can show up. The 4:2:2 from external HD recording may suffer even more with the sharper edges. The sharper the video the more chroma subsampling is noticed.
Of course this is all kind of splitting hairs but if we are striving for the best possible image and don't need original 10bit for V-log then down converting in post may deliver better results than capturing to an external recorder for HD. If you have a 4k external recorder ignore all of this since the camera doesn't need to down convert anything. I should point out that the HDMI HD capture does look great and is still a solid solution and workflow. Just be aware that it may not be the best down conversion possible. I have yet to determine if the sharper edges do cause any added artifacts or if the sharpened details are whats natural from the camera and the H264 is what kills the detail in the other versions. Honestly I find it hard to believe the GH4 can do a down conversion that good with the limited processing power it has.
Anybody know for sure whats going on or has done a similar test with an external 4k recorder to see if the video is sharper than internal 4k? I only have the means to record externally in HD right now.