Exfo EXpert IPTV Test Tools (FTB-1 / 2 Pro) Manual do Utilizador descarregar pdf (Página 46)

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

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Exfo EXpert IPTV Test Tools (FTB-1 / 2 Pro) Manual do Utilizador Página 46