Dahua and the word "Reference" as it refers to bit rate seems mislabeled

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I noticed that in the video settings of my Dahua cameras there is what is called "Reference" bit rate that includes a range with a lower and upper end. It my mind, "Reference" means ultimate or as good as it gets so if you set it to the upper end, that should be the max bit rate setting to give the best picture possible for the given resolution, codec, and frame rate. However, I think this is erroneously labeled. If you go to the Dahua wiki and go about 3/4 down the page, the wiki calls it "Recommended" bit rate and not "Reference". So this upper end is simply what Dahua thinks should be "good enough" for most people for general viewing but its not "Reference".

In the actual field when you set your bit rate (I'm using CBR) there is another range of numbers (to the side) that include a low and high. That high number is, in fact, the reference bit rate for the given frame rate, resolution, and codec. So, for example, if running 1080 @20fps and using H.265 as the codec, the high end "reference" bit rate is actually 20480kbps.

So I've done some testing at night on my LPR and I cranked the CBR bitrate up to 17000kbps (17Mbps) which is ~80% of the true reference bit rate for my settings. OpenALPR is now reading more plates at night than it had before. Its still not as good as day time reading but I'd say I'm above 90% successful plate reading at night now and I was much lower than that before when I was running a CBR of 8192kbps. Plates that were dim at night before and OpenALPR couldn't pick up have much more bit rate information that it can dig the numbers out and read them. I'm also getting ridiculously clear day plates. I can read the actual registration expiration years on the registration sticker and the words on dealer license plate frames.

Of course running this high of a CBR means more storage space (double 8192kbps). Also, the computer that OpenALPR runs on has to work a bit harder. But so far I'm pleased with the increase in bit rate going from 8192 to 17000. Just wanted to relate my experiences thus far. It may be overkill but missing plates at night because they were too dim has really bothered me. I think I’ll take the trade off of increased space if it means I get better captures at night.

 

[wittaj]

Generally the higher the bitrate, the better the quality (but there are exceptions). Run a 4K camera at 256 bitrate and then a D1substream at a bitrate of 8192 will look better LOL.

But as you said, it does come at a cost to storage capacities.

It also comes down to the eyes of the user LOL. Some people above a certain bitrate do not notice an increase in quality, while others will.

It is also dependent on the camera, the scene it sees, what you are trying to accomplish, etc.

If the camera image has a lot going on, it may need a higher bitrate. A camera without a lot going on can get by with less.

I have one camera that gets an equal split of dark green grass and almost blinding white concrete in the day. Anything below 10,000 or any bitrate on VBR and it is a pixelated mess.

I like you have played with trying to find that ultimate setting for LPR. At the angles and zoom I am at, almost my entire image is weathered blacktop that has lots of angles/voids/etc. what ever you would call not a flat surface. I believe that this was messing with mine at higher bitrates as I think it was picking up all these little nuances in the pavement as noise and wreaked havoc. My LPR runs much better and more plates capatured at VBR with 4192 bitrate. The bitrate drops to almost zero at night, but can ramp up quickly enough to capture a plate that is on the screen for a split second. But I know many experience just the opposite with LPR.

MY PTZ that has that same weathered background when people are walking on the sidewalk was constantly trying to focus as they walked by - again I think it was related to all the nuances in the pavement reflecting back what the camera was taking as different focus differences. I kept upping the bitrate expecting to see it improve and it got worse, so when I back down to 12,384 was when I found the balance between quality and not constantly trying to refocus on a subject.

None of my cameras run the same bitrate and some run VBR just fine while others need CBR, not even the same model pointed at different things.

 

[biggen]

Interesting. Thanks for that info! Its basically comes down to playing with the settings and find what works for each camera.

I haven't tried VBR yet. I may give that a try now and see if the camera can "ramp up" fast enough to keep up with a moving car but still have a good quality plate.