# Multimedia pass-through audio channel operation

The pass-through audio channel consists of the components shown in the following illustration:

The pass-through audio channel operates as follows:

Calling... | Causes the following action...
ctaOpenServices | Creates the ADI port. A DSP resource is used if a timeslot is specified, for example, when silence detection or audio transcoding is needed.
mspCreateEndpoint | Creates the RTP endpoint (AMR or G.723).
mspGetFilterHandle | Translates the MSPP endpoint handle into a CG board endpoint handle that can be used in the ADI function calls.
adiPlayMMFromMemory or adiPlayMMAsync | Creates the pass-through audio play filter. No DSP resource is used. Connects the ADI port, the pass-through audio play filter, and the RTP endpoint together. Starts play of audio buffer onto the ADI port.
adiStopPlaying | Stops the audio play. Disconnects all components. Destroys the pass-through audio play filter.
adiRecordMMToMemory or adiRecordMMAsync | Creates the pass-through audio record filter. Connects the ADI port, the pass-through audio record filter, and the RTP endpoint. Starts recording from the ADI port in the audio buffer. Establishes inter-dependence between video and audio channels due to the audio board endpoint handle provided with the video board endpoint handle.
adiStopRecording | Stops the audio record. Disconnects all components. Destroys the pass-through audio record filter.

Typically, the ADI port and the RTP endpoint exist for the life of the channel. The pass-through audio play filter exists for the life of adiPlayMMFromMemory or adiPlayMMAsync, while the pass-through audio record filter exists for the life of adiRecordMMToMemory or adiRecordMMAsync.

It is not necessary for the application to create or destroy the pass-through audio filters or for the application to create filter connections or disconnections. These tasks are performed automatically when corresponding ADI service functions are called.

See Video enhancements to the ADI service and Video enhancements to the MSPP service for more information.