I am running 3840x1600 in 1CH mode at 53Mbps and that seems plenty to me. It saves a little bit on memory card space and heat generation compared to 58Mbps.
Taking a quick look at the numbers, 1600/2560 = 0.625. So the 1600p video has 62.5% number of pixels compared to 3840x2560.
It looks like the Viofo engineers started with 44.8Mbps as the max bitrate they could run in combination with 16Mbps on the rear camera, then reduced the 1600p video bitrate in proportion to the change in pixel area: 0.625 x 44.8 = 28.0Mbps
I have tried the A139 Pro in 1CH, 2CH and 3CH configurations. Personally, I didn't like the effects of the reduction in bitrate, but then I am used to having 4K cameras at around 60Mbps.
I think you made a mistake somewhere in your calculation?
3840x
2100 = 8,064,000 pixels
3840x
1600 = 6,144,000 pixels
The difference between both resolutions is a ratio of 1.3125:1. (meaning 2100P has 31.25% more pixels than 1600P)
The bitrate posted by
@rcg530 for the front camera recording of the 2CH version (front/rear) version (unless if he mistakenly transposed one of the numbers somewhere which I doubt because I see the same thing when I compare front/interior?):
3840x
2100 = 44.86Mbps bitrate
3840x
1600 = 28.06Mbps bitrate
The difference between both bitrates is a ratio of 1.5987:1. (meaning 2100P has 59.87% more bitrate than 1600P)
It is better for power use, yes it is better...but the image/video quality seems to be less per usable area because there is a major reduction in bitrate to accompany the stepdown to 1600P resolution.
In this case, there is a
23.81% reduction in resolution (going from 2100P to 1600P), but there is a
37.45% reduction in bitrate (going from 44.86Mbps to 28.06Mbps) to accompany this resolution reduction.
Or to explain it another way...
There is a
31.25% increase in resolution (going from 1600P to 2100P), but there is a
59.87% increase in bitrate (going from 44.86Mbps to 28.06Mbps) to accompany this resolution increase.
Conclusion: In 2CH mode, 2100P has
more bitrate per pixel area than 1600P and therefore will result in
higher image quality.
The 16:9 has 35% more pixels, so needs 35% more bitrate to achieve the same image quality.
The only reason to run the 1600 at the same or higher bitrate is to achieve significantly higher image quality, but the engineers were not necessarily choosing the bitrates for maximum quality. There is always a compromise between image quality and other things, such as temperature, reliability, hours of video per card, power consumption (especially if running on battery in parking mode), etc.
2100P (AKA 16:9) has 31.25% more pixels, so needs 31.25% more bitrate to achieve the same quality in 2CH mode...but Viofo gave it a whopping
59.87% increase in bitrate.
The interesting thing is if you do the same exact calculations that I did, except in 3CH mode; the equation goes the other way on the other side of the spectrum (running at 1600P in 3CH mode is much more superior to running at 2100P in 3CH mode)
3840x
2100 = 8,064,000 pixels
3840x
1600 = 6,144,000 pixels
3840x
2100 = 26.74Mbps bitrate
3840x
1600 = 26.46Mbps bitrate
There is a
31.25% increase in resolution (going from 1600P to 2100P), but there is only a
1.06% increase in bitrate (going from 26.46Mbps to 26.74Mbps) to accompany this resolution increase.
There is practically almost zero benefit at all of running 2100P in 3CH mode configuration.
Conclusion: In 3CH mode, 2100P has
less bitrate per pixel area than 1600P and therefore will result in
lower image quality.
The question for Viofo engineers is what exactly are they optimizing? and why does it seem like they are optimizing one thing in 2CH 1600P mode and the complete opposite of that in 3CH 1600P mode?
Consumption during parking mode is not as relevant because in low bitrate recording, the camera will not record anywhere near that bitrate regardless of the number of channels.
Low bitrate parking mode bitrate is 4.06Mbps (front) and 3.90Mbps (rear).