Bandwidth vs. Image Quality Shootoutby Antony Look, posted on Jul 24, 2011
When thinking about maximizing image quality, resolution is usually the first thing that comes to mind. However, resolution is not the only factor that impacts quality. The amount of bandwidth available and used can have a dramatic impact on image quality. In this report, we examine bandwidth and the effect that it has on quality across numerous cameras.
Which Image Quality is Better?
To better understand image quality, let's start by examining two samples of the same scene side by side:
Consider two questions:
- Which camera has higher resolution? A or B?
- Which camera is better? A or B?
It is pretty obvious that the image from Camera B is better so this should be a simple case.
The reality is that those images are from the same camera at the same resolution and frame rate (720p/30). All that was done to the camera was changing the Constant Bit Rate target from 512 Kb/s to 8 Mb/s.
Factors Impacting Quality
Even with the same resolution, two common settings impact quality:
- Bit Rate: Most cameras can have their bit rate adjusted to specific levels (e.g., 512 Kb/s, 2 Mb/s, 8Mb/s, etc.)
- Quantization Level: Most cameras can have the level of compression adjusted (often called a quality or compression setting with options from 1-10 or 0-100)
Typically, these are mutually exclusive. If you lock in bit rate, the camera will automatically adjust the quantization level to not exceed the bandwidth set. Vice versa, if you set the quantization level, the camera will automatically change the bandwidth consumed to make sure the quality / compression always stays at the same level.
Our Test Process
We wanted to better understand how changes in these two factors impact video quality. To do so, we did a series of tests with three HD cameras: the Axis P1344, the Sony CH140 and the Bosch NBN-921.
For the bandwidth tests, we tested each camera at the following levels:
- 512 Kb/s
- 1 Mb/s
- 2 Mb/s
- 4 Mb/s
- 8 Mb/s
We did this across a series of scenes to see how quality would vary in different conditions:
- Daytime Indoors (300 lux)
- Nighttime Indoors (.5 lux)
- Daytime Intersection
Finally, we did a similar series of tests varying the quality level of a VBR camera (the Axis across 0, 30, 60 and 100 levels) to better understand changes in quality and bandwidth consumption.
Questions We Considered
Here are the questions we considered and answered inside the report:
- When changing bit rates, how much are image details impacted?
- How much quality differences exist among manufacturers at the same bit rates?
- How do manufacturer streaming methods differ? How does that impact quality and performance?
- What is the 'optimal' bit rate to use?
- When should you consider using higher bit rates?
- When adjusting compression levels, how much does bandwidth consumption and image quality vary?
- What is the 'right' compression level to use?
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