Dash Cam file sizes has me puzzled

[AJMike]

I assume there are some people on this forum familiar with video recording formats, so I am posting this question here. I tried on the Rexing forum but got no answers, at least so far, and that forum does not seem very active.

I recently bought a Rexing dash cam (V1P Pro) for our car and I chose this model because it had both front and rear cameras, an assortment of recording resolutions and a built-in gps logger. Being the geek that I am one of the first things I did was determine how much space each recording setting took on a video card so I would know what size card I needed. The device has 6 single camera video resolutions (3840x2160 P24, 2560x1440 P30, 1980x1024 P 60, 1920x1024 P 30, 1280x720 P60 and 1280x720 P30) and 2 dual camera formats (1920x1080 P30 and 1280x720 P30).

OK. All of that makes good sense, and I tested all of the single camera formats to see what I liked best and determine how much space each took, but there is not much choice for dual camera since the 1280x720 seems a bit too low a resolution. Still, I did the same thing for the 1920x1080 dual camera, which produces 2 files, each 1920x1080, helpfully suffixed 'A' and 'B'.

So far so good. But when I looked carefully at the file sizes something struck me as not making any sense. The single camera 1920x1080 P30 video file, for a 3 minute segment, took approx 344MB, or 6.9GB for an hour recording. The dual 1920x1080 P30 video files from the front and rear 1920x1080 P30 recording took 232MB each or approx 9.3GB for an hour recording for both. And that is what I don't understand.

I don't know much about video recording and the associated codexes, but it seems to me that a 3 minute 1920x1080 P30 recording should be pretty much like any other 1920x1080 P30 recording, but the single camera one takes a lot more space than a single one of the dual camera recordings does. Why? What am I not seeing here?

Since the dual recording formats, even for the same resolution, are smaller it means that I can record more video on a given card size than I would have thought, and I suppose it was engineered to be that way, but I don't know enough about video file formats to understand why the two files with identical resolutions for identical times should vary in size that much.

[NXR]

The dual camera one could be using a more aggressive compression to reduce the disk space needed.

Ray

[AJMike]

Originally Posted by NXR

The dual camera one could be using a more aggressive compression to reduce the disk space needed.

I'm embarrassed to admit that I never even thought about the file compression. I guess I need to Google the mp4 file format and see what I can see.

Thanks for that.

[tcg]

I looked at the manual for your camera, there is a chart with card size capacity. Get the biggest card they recommend and use the best resolution.

But if you're really curious, make some recordings at each resolution setting and then open them in a video player on your computer. Look for the file information under codec and note the bitrate. My guess is you'll see different bitrates for each file. The bitrate is the bigger determining factor than resolution. It's also the one thing you can't change in the settings, at least not for your camera.

[AJMike]

Originally Posted by tcg

I looked at the manual for your camera, there is a chart with card size capacity. Get the biggest card they recommend and use the best resolution.

Yes. I have seen that chart, but that is only for the dual camera setup. My RV has no rear window so I have no place to set up the rear camera (unless I install it outside and to the best of my knowledge it is not weatherproof) so I will be using the camera in single mode.

The problem using it in single camera mode with the largest card (256GB) and the highest resolution (UHQ - 3840x2160 P24) I only get 22.4 hours of recording time. That may sound like a lot, but I want to be able to go on a trip and return with the entire trip video intact so that I can hold on to it in case I have a need, and for that 22.4 hours is not long enough. It would be nice if the camera could use a 512GB card but it can not and my alternatives are using a lower resolution or changing cards.

Originally Posted by tcg

But if you're really curious, make some recordings at each resolution setting and then open them in a video player on your computer. Look for the file information under codec and note the bitrate. My guess is you'll see different bitrates for each file. The bitrate is the bigger determining factor than resolution. It's also the one thing you can't change in the settings, at least not for your camera.

I have done that, and you are right. The bitrate differs between the 2 1920x1080P30 recordings with the single mode having a bitrate of almost 15 Mb/s and the dual mode, at the same resolution and frame rate, have a bitrate of about 10 Mb/s. I am guessing that means that although the two videos look the same to me when I view them, the single mode has more information.

Thank you for explaining this to me.