[DAHUA] "Bit Rate" setting in VBR mode?

[kaon 0]

This is the ENCODE page of the Dahua DVR1604HF-A.

 

 

Does the "Bit Rate" setting have any effect?

[kaon 0]

After 20 minutes on the phone with Bruce of Dahua, (With him looking at the same screenshot above), I still wasn't hearing an answer to my question. He repeated the same old generalities about CBR and VBR a few times, and asked me to repeat my question a few times. I said I wanted to understand in more precise terms, how the "Quality" setting and the "Bit Rate" setting, both affect the VBR behavior. He told me he would email me a more complete explanation later, and I said OK.

 

I got the email... here's what he wrote.

For the VBR and CBR, VBR means the bit rate will be changed with the surroundings. it will change a lot if there is not too much movement. and if you use CBR, the bit rate almost similar whatever the surroundings change or not. For example, if you set bit rate as 1024, in CBR, the bit rate will be always nearby.

 

And for picture quality, the higher the better. For example, if you set bit rate as 1024 VBR, compare the picture quality, 6 will be better than 1. In lower quality settting, the bit rate will be lower then higher quality settings.

 

After some experimentation, using the "MENU > INFO > BPS" page, and a 1to3 video splitter, allowing me to feed the same analog signal into 3 input ports, my findings are:

- On static scenes, higher "Quality" produces higher actual bitrate.

- On static scenes, (esp. with low detail), the "Bit Rate" setting has almost no effect on actual bitrate.

- On high-motion / high-detail scenes, the "Bit Rate" setting works like a maximum cap. With corresponding blockiness when the VBR is constrained by a low cap.

- The effect of "Quality" on actual bitrate can sometimes be hard to see.

 

UPDATE: the behaviour described above holds when RECORD mode (in SETTING > SCHEDULE) is set to REGULAR. BitRate behaviour is completely different when RECORD mode is set to MD (Motion Detect).

Edited October 16, 2012 by Guest

[kaon 0]

Bruce of Dahua also wrote back with similar findings, if I may rephrase and simplify:

- higher "Quality" setting gives clearer picture in stationary scenes.

- higher "Bit Rate" setting gives clearer picture in high-motion scenes.

 

For example:

Config A: VBR, Quality=1, BitRate=2048

Config B: VBR, Quality=6, BitRate=1024

If given the same camera input, which setting (A or B?) will have clearer picture?

 

In the above situation, the logic how DVR works is like below:

If there is few movement(I put the camera in the desk, then focus on the wall) where the camera installation. The Quality settings is very important for picture quality, in this time, config A picture should get worse picture than config B. If there is a lot movement(move your hangs always in front of the camera), you can see the picture quality(real BPS of this camera), Config A should be better than congfig B. But in project when you use DVR, we recommend you use Quality 3 or above, and the bitrate 2048 if you use all channel D1 DVR.

You can test in your side, in the DVR--menu-infor--bps, you can see the real bitrate after change the settings, it will be easy for you to find the difference between each settings.

 

However, from further tests, I find that the record mode (in SETTING > SCHEDULE) (REGULAR/continuous vs MOTION DETECT/MD) has an unexpected effect on how the VBR bitrate (according to INFO > BPS) changes.

The above descriptions of how Quality and BitRate affect VBR behavior is true when RECORD mode is "REGULAR".

 

When RECORD mode is MD, then the VBR "Bit Rate" setting seems to only affect how low the VBR bitrate is allowed to drop to during stationary scenes.

It does not act like a maximum cap.

It acts more like a minimum.

Edited October 16, 2012 by Guest

[yakky 0]

Thanks for posting your findings, it was on my list as a todo. I wonder why motion vs record are different as well.

[kaon 0]

Here is video showing that the lowest (768 kbps) and highest (4096 kbps) VBR "Bit Rate" settings do not appear to make a difference to the INFO>BPS during high-motion scenery. In both cases, BPS maxes out at 800~900 MB/hour, when in Record type: MD.

7Yn-ArQNHOA

 

Again, if recording type is set to REGULAR, then the "Bit Rate" setting seems to act like a maximum cap. Interestingly, the INFO>BPS can exceed 1000~1200 MB/hour when Record type is REGULAR. But if Record type is MD, then the INFO>BPS never exceeds 900MB or so, (in high-motion scenery).

 

All the above is with DVR1604HF-S, firmware build-date: 2012-05-30, PAL, D1, 25 FPS (full frame rate), VBR, Quality=3.

[vector18 1]

The way I see it is if your looking to save HDD space and your on 24 hour record, than you should set to VBR so when there is no action in the camera, the bit

rate is lower and less space for better quality is taken from HDD. But, if you record in Motion, which is my preference, than set to CBR with the highest bit rate possible

so when the camera does start to record from motion, at least it has the best picture possible.