iFrame Interval: Only With Variable-Rate?

[PeteCress 0]

On all my cams, I have selected Constant Bit Rate - under the impression that it maximizes quality over Variable.

 

On my HikVision cams, I have been fooling around with iFrame Interval, trying to clean up the clips.

 

But when I went into Setup on another brand of cam (ACTi) I noticed that the iFrame setting goes away when Constant Bit Rate is selected and only shows up when Variable Bit Rate is in effect.

 

The ACTi user's manual seens to imply (page 5) that Constant Bit Rate gives better quality than Variable only when the image is still and that, with "Large" amounts of motion or "Complex" scenes the image will be better with Variable Bit Rate.

 

Seems to imply that iFrame interval is only relevant to Variable bit rate and that the cam is sending a complete image for each frame when it is doing Constant bit rate.

 

Bottom line, it sounds like I should be using Variable instead of Constant bit rate for cams whose job is to record the movements of people.

 

Have I got it right?

[Spark 0]

Another thread that may be of interest for you: viewtopic.php?f=19&t=42680

[PeteCress 0]

Another thread that may be of interest for you: viewtopic.php?f=19&t=42680

That was a good thread for me. Thanks.

 

My takeaways from the first read:

 

- There is no simple hard-and-fast answer to CBR-vs-VBR

 

- Which works better depends, in part, on the make/model of the camera

[thewireguys 3]

I recommend VBR with a cap as the best option.

 

Say you set the cap at 5Mbps. The camera will use less bandwidth when there is not much movement or complex scenes. When there is alot of movement the cameras will then use more bandwidth up to 5Mbps.

[PeteCress 0]

I recommend VBR with a cap as the best option.

 

Say you set the cap at 5Mbps. The camera will use less bandwidth when there is not much movement or complex scenes. When there is alot of movement the cameras will then use more bandwidth up to 5Mbps.

Suppose bandwidth were unlimited.

 

Would you change the recommendation?

Edited November 13, 2014 by Guest

[Don Stephens 0]

I recommend VBR with a cap as the best option.

 

Say you set the cap at 5Mbps. The camera will use less bandwidth when there is not much movement or complex scenes. When there is alot of movement the cameras will then use more bandwidth up to 5Mbps.

Suppose bandwidth was unlimited.

 

Would you change the recommendation?

 

I would say CBR in that case, but I would also tell you to do whatever thewireguys tells you to do (haha).

[thewireguys 3]

Bandwidth is never unlimited

 

More importantly you are wasting hard drive space

[devsec 0]

To answer your questions about i-frame interval in CBR, you have to understand that when a camera sends an i-frame, the bit rate will spike because this frame is a lot larger than p-frames. As the interval between i-frames becomes bigger and bigger, it's harder to maintain a constant bit rate because the spikes are further and further apart which has an impact on bandwidth calculations.

 

Furthermore, changing the interval means that, for a given bandwidth, you cannot keep the same quality and frame rate. Each manufacturer deals with this in their own way. Some might decide that it's better to increase/decrease the compression to compensate, others might drop frames or do some other crazy ****. It would appear that ACTi's decision was to remove the setting entirely in favor of a fixed interval.

[PeteCress 0]

Furthermore, changing the interval means that, for a given bandwidth, you cannot keep the same quality and frame rate. Each manufacturer deals with this in their own way. Some might decide that it's better to increase/decrease the compression to compensate, others might drop frames or do some other crazy ****. It would appear that ACTi's decision was to remove the setting entirely in favor of a fixed interval.

Sounds like we are back to there being no hard-and-fast answer and evaluating each camera/situation.

 

Bandwidth is never unlimited

 

More importantly you are wasting hard drive space

The use of "Unlimited" bandwith was sloppy on my part.

 

Instead of saying "Unlimited" I should have described a situation where:

 

• The radio link between server and cameras had a bandwidth of, say, 55 mB/s
  .
  
• The sum/total of all the cams running at Constant Bit Rate was, say, 35 mB/s
  .
  
• There was no other traffic on the link
  .
  
• The server mediates between people viewing and the cameras. i.e. a camera's bit rate it unrelated to the bit rate delivered by the server to the users over Internet connections.
  .
  
• We have almost a TB of drive space dedicated to clips and the server manages that: dropping old clips as space is required for new clips - like a dash cam in a car. This gives us several days of 5-minute clips back-to-back 24-7.... which is way more than anybody needs or wants in this application - which is basically to let windsurfers see how conditions are without calling the windsurfing shop's phone every five minutes on a windy day..... Although we *did* manage to save the hotel owner some court costs when we caught somebody in a lie vis-a-vis their injuring themselves stepping off the deck into the parking lot... -)