Delivery of Multimedia Data

The delievry of multimedia data is dependeny on the following factors:

• Bandwidth - hundreds of Mbps

• Latency (maximum frame/cell delay) - important for Interactive TV, audio/video synch, etc.

• Frame/Cell Loss or Error

• Jitter - Playback Jitter is standard deviation between frame playback (Fig. 25.16).
  

  Jitter

  • Intuitively, it is a measure of smoothness.

  • Video prioritization algorithms set priorities to minimize playback jitter in the event of loss.

      Application          Avg Delay Tolerance    Avg Jitter Tolerance
                                  (msec)                (msec)
-----------------------   ---------------------  ----------------------
64 kbps video                      300                   130
16 kbps audio                       30                   130
MPEG-1 video (1.2 Mbps)              5                     7
MPEG-1 audio (256 kbps)              7                     9
HDTV video (20 Mbps)               0.8                     1

The delivery of multimedia data may have to be prioritized in some manner:

• Transmission algorithms provide prioritized delivery

  e.g. higher priority to audio than to video

• Different media formats can lend them selves to different prioritisation, For example:
  • Prioritisation for MPEG:
  • Highest priority - I frame, lowest - B frame.

  • Choose proper levels in Progressive mode or hierarchical mode.
• Prioritisation for Audio

  • Break into groups of every nth sample, prioritize and send groups. Receiver interpolates lost groups (Fig 25.17).
    

    Prioritisation for Audio

  • Example: if groups 3 and 4 are lost, effective sampling rate is 22 kHz instead of 44 kHz.

    Loss is perceived as change in sampling rate, not dropouts.