Random access point

The Dolby AC-4 decoder features a seamless switch between Dolby AC-4 elementary streams of the same media content and different configuration options.

Configuration changes can include:

  • Bit-rate changes

  • Channel-mode changes

  • Sampling-rate changes where the higher sampling rate is an integer multiple of the lower sampling rate (for example, from 96 to 48 kHz)

  • Frame-rate changes where the higher frame rate is a factor of two or four times the lower frame rate (for example, from 48 to 24 fps and vice versa)

  • Any combinations of these

A seamless switch means that the audio output has no audible artifacts or distortion introduced by audio gaps or additional audio during and after switching.

A seamless switch can be accomplished only at a random access point. Comparable to random access frames in H.264 video streams, the Dolby AC-4 elementary stream contains I-frames to indicate random access points. The b_iframe_global flag inside the table of contents indicates an I-frame. Unlike video I-frames, audio I-frames are for signaling only. Audio can still be decoded if an audio I-frame is not present at a random access point. When a switch is not immediately followed by an I-frame, the Dolby AC-4 decoder performs partial decoding until it receives the next I-frame. Partial decoding may lead to degraded audio quality, including short periods of silence, limited audio bandwidth, or a downmix of the content.