Testing Bandwidth vs Low Light

Ethan,

The tests confirm what I've been saying for years: while AGC may be necessary to obtain usable images in low light, it has extreme downsides - mostly in the form of bandwidth requirements and noise.

The other side of the coin regarding AGC is that it often "muddies" the images, especially of brighter objects in darker scenes. For that reason, we try to either avoid it altogether by turning it off or minimize its effect by lowering AGC level substantially during camera setup.

I strongly believe that manufacturers rely on AGC way too much to inrease their cameras' low light capabilities. I've often found that limiting AGC to 10db or 15db is a good compromise. The image may be slightly dark if the light level is very low but most of the noise, muddiness and excessive bit rates disappear.

Tedor,

Excellent example. I note that high AGC levels show just as poor or poorer picture as very low AGC levels, with 9db to 18db appearing to be the sweet spot. That pretty much jibes with my findings in our application. I've found that in general, manufacturers tend to use higher-than-necessary AGC levels when AGC is set to "Auto".

Of note: AGC compensates for the overall illumination, often sacrificing image quality in brightly lit areas to compensate for less well lit areas.

Ethan,

Interesting you recommend VBR with cap to provide 'spike protection' in fact in most current implementations it does not provide any protection from big data spikes, however it can and should shorten the duration of the spike, as it simply responds to spikes by increasing compression ratio after the event. VBR-with-cap implementations that provide effective 'spike protection', have to drop frames to limit the data size, which is the same as pushing 12mbps over a 2mbps pipe, you lose the data.

The main benefit of VBR-with-cap is reducing storage requirement. It has minimal benefit on network usage/planning as the average 100BaseTX network will easily be saturated with very few cameras when peaks occur, which means packet/data loss. For site to site applications or where bandwidth is truely restricted (Wi-Fi, Mesh, DSL links) VBR with cap is only useful if you are happy drop frames whenever there is significant change or significant activity in the image, unfortunately it is often the time when you need every frame for evidence.

If you have a 100BaseTX network how many cameras can you hang on it with bit rate set to 4mbps? In theory allowing for 20% protocol overhead, and some wriggle room you can say there is approximately 60mbps capacity which at 4mbps per camera would support 15 cameras. This is true with a good CBR encoder, but with most VBR encoders peaking at 16-30mbps (depending on quality setting, you limit yourself to 2-4 cameras on the network to avoid data loss. With VBR-with-cap limit set to 4mbps, since the peaks are not sustained you could argue that more cameras can be supported, but how do you plan for them? How many would you use? 7 - 8 cameras? This is not a big impact on a small system but as the system scales up to a few hundred cameras, it quickly becomes a network planning issue, not just on the camera to VMS link but also for monitoring side.

By their nature, all encoding systems have their limitations, I think VBR-with-cap is mis-understood and often hyped as a cure-all, but it too has limitations.

Thanks for the report Ethan. I am curious if you enabled DNR on the Hikvision cameras? By default, Hikvision has the DNR turned off. We turn that configuration item ON and set the Noise Reduction Level to 50 (the default).

In addition I am curious whether True Wide Dynamic Range (WDR) might also have a reduction effect on the nighttime bandwidth. We also enable that feature on the Hikvision cameras with a Wide Dynamic Level setting of 50 (the default).

We use VBR with a cap on these cameras. With configure the stream for 30 fps and 1080p resolution with a 4096 kbps cap and very rarely see any of the cameras even come close to that bandwidth cap. Note that we are also using the H.264+ compression provided by the Hikvision cameras.

Note also all of our Hikvision cameras have integrated IR and most of them have the Smart IR as well so that as you identified that may help with nighttime bandwidth especially indoors.

With all of those camera features enabled, I just don't see that explosion of bandwidth at night on our Hikvision cameras. We might see a 50% increase but not orders of magnitude.

You have piqued my curiosity so the next time I am on site I'll see if I can get some real-world hard numbers on nighttime vs. daytime bandwidth. I struggle with establishing an appropriate VBR cap value. Generally I use 4000 kpbs for 1080p and 8000 kbps for 4K camera resolution. These are just some educated guesses but I don't know what the end effect is on stream quality at night when it might approach some of these cap limits. Another good metric you could collect, if possible, is the quality degradation when cameras hit their bandwidth caps. If the default quality is set to 60 on Hikvision cameras, how much is the quality reduced when it approaches the bandwidth caps (50, 40, ...)?