Ok, after some research , it seems clear but haven’t prove it.
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After reading the article ARI and Bridges found the section:
Multi-party mixing - when there are more than two channels in a mixing bridge, Asterisk will **transcode** the media from each participant into signed linear, mix the media from all participants together into a new media frame, then write the media back out to all participants.
So when i add the third channel into the bridge , it automatically does the transcode stuff. -
Knowing this, i searched why asterisk cant transcode the opus codec! Then i found this article asterisk audio Capabilities and seems that asterisk not handle opus codec by default.
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then i found this :[Asterisk14 OPUS transcoding](http://asterisk enable opus transcode)
Now i’m trying install codec_opus