Variable bitrates

[Module 79L]

From day 1, occasionally and rather randomly, the AT11DA produces clips with much higher bitrates than the normal 12,40Mbps average. It usually happens to the full 5 min clips but there had been instances when shorter clips also had higher bitrates.
Here's the latest example:


These clips are from a roundtrip which took a little over 25 mins in each direction (the red line divides the two partes of the trip).

As you can see, the bitrate of the first clip is the normal one, there's a slight increase in the 2nd clip, the 3rd clip went back to normal but then the 4th and the 5th had a huge increase. When I returned, at 10:41h, the first 2 clips also had a higher bitrate, although not as high as the last ones of the away trip, and the 5th clip had the lowest bitrate I've ever seen the camera produce. In this camera, a typical 5 min clip is usually between 480 and 482MB in size and I don't recall ever seeing any 5 min clip smaller than that.

It's not that I think there's a problem with the camera, I'm just curious about why this happens.

Edit: the weather conditions were sunny with clear sky.

 

[Nigel]

It should depend on how much detail there is to store, maybe the end of the first trip and thus start of the return where under trees with loads of detail while the start/end where open desert and you arrived home after dark reducing detail even more.

From what I have seen, most of the dashcam codecs are very poor at deciding on a good bitrate for the situation and are also poor at deciding what to throw away in compression and so the better cameras have either fixed or close to fixed bitrates. Same for the action cameras.

 

[jokiin]

From what I have seen, most of the dashcam codecs are very poor at deciding on a good bitrate for the situation and are also poor at deciding what to throw away in compression and so the better cameras have either fixed or close to fixed bitrates. Same for the action cameras.

have to say I much prefer fixed bitrate rather than letting the camera decide what bitrate/level of compression to use, most developers seem to justify this due to saving memory space, maybe a valid idea years ago when memory was pricey but I'll gladly waste a bit of space and have a known output

 

[Module 79L]

(...) maybe the end of the first trip and thus start of the return where under trees with loads of detail while the start/end where open desert and you arrived home after dark reducing detail even more.

That probably answers the question of clip 0004 (it indeed started in a stretch of road surrounded by trees) but not the huge increase to 15539kbps in clip 0005, which started in an open area with very little trees. Interestingly, clip 0007 started where clip 0005 ended and it also has the higher bitrate of the return trip clips. I don't know if it's useful information or not, but on the away trip the sun was behind me and on the return trip the sun was in front of me.
About clip 0012 (when I arrived home): it was 11:05AM.

 

[Nigel]

have to say I much prefer fixed bitrate rather than letting the camera decide what bitrate/level of compression to use, most developers seem to justify this due to saving memory space, maybe a valid idea years ago when memory was pricey but I'll gladly waste a bit of space and have a known output

That leaves you recording the car park at 32 mbps when 2 mbps is more than adequate for 95% of the time, which means you can't record a full 24 hours on a reasonable price card when could get a full week of parked footage with a good variable rate implementation.

I don't think the codec developers currently have much incentive to develop software that actually works well for what we use it for though, so for now fixed bitrate is preferable, and at least with fixed bitrate you know how much recording you will get, your parking footage is not going to be reduced to 1/4 record time just because the wind is blowing a tree about!

...on the return trip the sun was in front of me.
About clip 0012 (when I arrived home): it was 11:05AM.

Given those 2 facts and the fact that this is a low bitrate and thus presumably a cheap camera, I guess that the lens went slightly out of focus due to the heat of the midday sun thus reducing the detail in the image?

 

[jokiin]

That leaves you recording the car park at 32 mbps when 2 mbps is more than adequate for 95% of the time, which means you can't record a full 24 hours on a reasonable price card when could get a full week of parked footage with a good variable rate implementation.

that's a different scenario, using a lower bitrate selectively for situations such as when parked has merit but that should be part of an automated change as a part of parking mode, not what I was referring to, I don't like VBR of regular recording when the camera has a greater chance of getting it wrong

 

[Module 79L]

Given those 2 facts and the fact that this is a low bitrate and thus presumably a cheap camera, I guess that the lens went slightly out of focus due to the heat of the midday sun thus reducing the detail in the image?

It wasn't that hot (around 26ºC, if I remember it correctly) and the sun was high, as expected at that time of the day, thus not directly in front of the lens like between 6 o'clock and 8 o'clock in the evening. Anyway, I reviewed clip 0012 again and I didn't notice anything out of focus, so was it probably due to the fact that everything the camera was capturing was super illuminated and it didn't have to work so hard to capture details?

 

[Nigel]

that's a different scenario, using a lower bitrate selectively for situations such as when parked has merit but that should be part of an automated change as a part of parking mode, not what I was referring to, I don't like VBR of regular recording when the camera has a greater chance of getting it wrong

If the VBR worked really well then there would be no need to have an automated change of bitrate or frame rate when entering parking mode, after all when a car races into the car park, does a handbrake turn and power slide and clips your parked car in the process you will want it recording at maximum bitrate and maximum frame rate, when there is nothing moving you want it to drop back to 2 mbps so that the power slide is not overwritten before you see your car next morning.

Doesn't come anywhere close to working like that with current hardware/software though!

...so was it probably due to the fact that everything the camera was capturing was super illuminated and it didn't have to work so hard to capture details?

High contrast = low detail as most of the detail is out of range?
I'm sure the camera had it's reasons, even if they are hard to understand.

 

[jokiin]

The cameras aren't that smart with their implementation of VBR so not really a consideration at this point in time