Video Quality Problems and Impairments

Video quality problems

Video quality can be degraded by the encoding process, transmission impairments and the decoding / playback process.

There are various encoding / transmitting system parameters that can be adjusted to trade off bandwidth against robustness. For example:

Using a larger number of P and B frames will reduce bandwidth however will lead to errors being propagated for longer periods of time.

DCT coefficients can be quantized more coarsely, which increases image distortion and quantization noise.

Typical frame rates are 30-60 frames per second.  It is possible to reduce this frame rate however this will lead to quality degradation in video sequences that have significant motion.

Problems due to transmission impairments

Lost packets lead to missing blocks within the decoded image - causing "blockiness" and if a large proportion of blocks are missing - frame freeze. 

The impact of a lost packet will vary considerably, depending on the type of frame impacted.  If an I or P frame is corrupted then the resulting image degradation will affect all the following P and B frames until the next I frame is received.  As a P frame generally represents a smaller region of the image than an I frame, the effect of packet loss on P frames will be slightly less than on an I frame.  B frames are not used as reference frames and hence a lost B frame packet will only affect that frame.

Problems due to encoder

The encoding process introduces some distortion, which can vary considerably with the amount of motion in the image.  Examples of problems are:

Block distortion can be caused by coarse quantization of the spatial frequency components of an image during encoding, and is due to the block structure of MPEG images. 

Blurring is a reduction in the sharpness of edges, and will be more widely observed in lower bit rate or lower frame rate algorithms or on video sequences with high rates of motion.

Edge busyness is caused by quantization of the image at the boundaries between areas with a significantly different color or brightness level. 

Mosquito noise is a form of edge busyness distortion that is associated with movement within the image which results in moving artifacts or noise patterns superimposed over the moving object. 

Ringing occurs on the boundaries between high contrast areas and is due to the use of DCT or Fourier transforms as part of the encoding process.  This leads to ripples being introduced on either side of the edge. 

Quantization noise typically occurs as visible “noise” (snow) over most of the image and will not necessarily be uniform.

Jerkiness is typically associated with low bit rate encoding of video sequences with motion.  Motion that was originally smooth appears as discontinuous “jumps”.

Blocks of colored pixels are typically due to errors that occur during the transcoding process when a digital image is converted from one format to another.


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