EXFO EXpert IPTV Test Tools (FTB-200v2) - Manual (Page 48)

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cause obvious distortion or may not even be noticed by the end user, depending on which
video frame types are impaired.
In addition, impairments can be introduced during the encoding/decoding process, by the
codec itself or an inappropriately low bitrates. The video content (e.g., level of detail and
motion onscreen) can also have a significant impact on the visibility of problems.
Furthermore, perceptual quality is affected by subjective factors including human reaction
time and the ‘recency effect’. Coupled with the type of content, e.g., fast motion, high detail,
or frequent scene changes, the quality of experience for the viewer will vary even under the
same impairment conditions.
Each of these objective and subjective factors must be taken into consideration in order to
accurately estimate IPTV video perceptual quality.
A.1.1 Transmission‐Related Impairments
Packet‐based video can be very sensitive to network impairments. Packet loss can
cause sections of frames or complete frames to be corrupted or deleted. For example, the
MPEG compression algorithm uses block‐based motion compensation for the reduction of
temporal redundancy and Discrete Cosine Transform (DCT)‐based compression for the
reduction of spatial redundancy. An MPEG encoder may generate three types of frame: Intra‐
coded (I),
Predictive (P), and Bi‐directional (B) frames.
Figure2‐1. Packet
vs. Frame Loss
Rates for MPEG