TopTechnical DictionaryBit rate

Bit rate

(bit rate) the amount of data saved or transferred by a given transmission channel within 1 s. In IP video cameras or recorders, a bit rate is usually measured in megabits per second (Mb/s) and the higher its value is (number of bits saved/transferred within one second), the higher quality of the material saved is.

 

A bit rate is often confused with throughput because they are both expressed by the same unit. A bit rate determines transmission capabilities of a given communication channel and throughput determines temporary intensity of a data stream.

 

A bit rate may be calculated using this formula:

 

K = V log2(n)

 

Explanations:
V – rate of generating signs in bodies (number of changes within transmission medium per second)
n – signal valence (binary = 2)

 

In IP video cameras, especially those with the resolution of 1920x1080 px (Full HD) or more, the recording quality may differ between respective models due to the maximum available (bit rate). Even if two video cameras make use of the same encoder with compression (e.g. H.264), the image quality will be visibly higher in the model with a higher bit rate; which at the same time results in higher available connector occupancy and higher consumption of a recorder mass memory in a given time unit.

 

Due to techniques used in data transmission, we can differentiate CBR – Constant Bit Rate and VBR – Variable Bit Rate.

 

Constant Bit Rate means the use of the same number of bits in the data stream in the transmission or recording, regardless e.g. of the current quality of a video or image sequence. Such a solution is reliable with the throughput limited, e.g. in stream media during multimedia data transmission via Internet. However, this method is disadvantageous while recording multimedia data, where CBR may contain excessive information, e.g. for uncomplicated video sequences characteristic for low variability in time and not sufficient for very dynamic video sequences (recording quality loss).

 

Variable bit rate consists in the varied number of bits in a given time segment. In IP video cameras, the aim of using this method is to maintain permanent quality of the output image regardless of input data (e.g. image dynamics, colour difference, contrast) and not the constant number of bits per given time unit. This technique is also used while saving multimedia data in mass memories, also due to more optimal use of available space.

 


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