LFE channel should always be counted for AC-4 object-based content on MPEG-4 layer #1943
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Merge from androidx/media main branch
ybai001
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Dec 18, 2024
PiperOrigin-RevId: 702281314 (cherry picked from commit e499377)
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A limitation has been identified in Annex E of the AC-4 specification (ETSI TS 103 190-2 V1.2.1, February 2018).
The specification defines the parameter n_umx_objects_minus1 in Annex E (E.11.11) to indicate the number of FULLBAND objects. However, it does not provide a mechanism to determine the presence of an LFE (Low-Frequency Effects) channel.
At the elementary stream level, the presence of an LFE channel can be determined from the "b_lfe" property defined in section 6.3.2.8.1. Unfortunately, this property is not exposed at the MPEG-4 layer. Following internal discussions, it has been decided that the LFE channel should always be included in the total channel count. This decision aligns with current implementation practices, which consistently include the LFE channel when creating an A-JOC (Advanced Joint Object Coding) substream.