A multiplexer packetizes only one raw AC-4 frame as an MP4 sample.
About this task
A seamless switch can be accomplished only at a random access point. The first sample
of each segment must contain a random access
point. A Dolby AC-4 elementary stream contains I‑frames to indicate random access points. Similar to the length of
GOP for video streams, the I-frame interval for a Dolby AC-4 stream indicates the
number of frames between two I‑frames.
The multiplexer must ensure that the first MP4 sample of each Dolby AC-4
media segment is an I-frame. A Dolby AC-4 media segment may contain one
or more random access points.
Procedure
-
The multiplexer scans the bitstream to find the first I‑frame (a frame with the
b_iframe_global value
of 1). This I‑frame begins a
Dolby AC-4 media segment.
-
The multiplexer continues to scan the bitstream for preceding I‑frames and keeps track of the number of frames between two
I-frames( I-frame interval).
The I‑frame interval should be constant.
-
When the multiplexer reaches the end of an I‑frame interval and accumulates a media segment size worth of AC-4 frames, the multiplexer adds the AC-4 frames to an MP4 segment.
The media segment size is determined by packager settings and A/V alignment
and segmentation rules.
-
The multiplexer begins the next media segment with the next I‑frame.