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A playback stream carries audio data over an isochronous OUT endpoint. There is a one-to-one relation between an isochronous OUT endpoint, an AudioStreaming interface and a Terminal. 101482827 presents the audio data flow implemented inside the Audio Processing module. The playback path relies on a ring buffer queue to synchronize the playback task, the core task and the codec ISR.

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titleFigure - Playback Ring Buffers Queue Monitoring


Figure - Adding a Sample in Case of Underrun illustrates the algorithm to add an audio sample in case of underrun situation. 

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Warning

This stream correction is convenient for low-cost audio design. It will give good results as long as the incoming USB audio sampling frequency is very close to the DAC input clock frequency. However, if the difference between the two frequencies is important, this will add audio distortion.

Figure - Removing a Sample in Case of Overrun illustrates the algorithm to remove an audio sample in case of overrun situation. 

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The playback stream correction offers the possibility to apply your own correction algorithm. If an underrun or overrun situation is detected, an application callback is called. Listing - Example of Playback Correction Callback Provided by the Application shows an example of playback correction callback prototype and definition provided by the application. 

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(1) Prototype of your playback correction callback.

(2) Upon configuration of an AudioStreaming interface with the function USBD_Audio_AS_IF_Cfg , the callback function name is passed to the function. You have the possibility to define a different correction callback for each playback AudioStreaming interface composing your audio topology.

(3) Definition of your playback correction callback. Once the playback is open by the host and the built-in playback correction is enabled (USBD_AUDIO_CFG_PLAYBACK_CORR_EN set to DEF_ENABLED), if an overrun or underrun situation is detected by the Audio Processing module, your callback will be called. You will have access to the structure USBD_AUDIO_AS_ALT_CFG associated to this playback stream through the pointer p_as_alt_cfg. Among the fields, you may be interested in:

  • TerminalID: ID of terminal associated to the playback stream.
  • NbrCh: Number of channels supported by the stream.
  • SubframeSize: Number of bytes occupied by one audio sample.
  • BitRes: Effectively used bits in an audio sample.

Beside the AudioStreaming alternate setting configuration structure, you will know the situation type (underrun or overrun via underrun_flag), the current buffer length (buf_len_cur) and the total buffer length ( buf_len_total ). Then you can apply your own correction algorithm to the buffer referenced by p_buf. If some samples are removed or added to the buffer, you will have to return to the Audio Processing module the adjusted buffer length. Note that you can add or remove only one sample at a time. You can also specify an error code if something went wrong while applying your correction.

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