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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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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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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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The frequency at which the record stream correction is evaluated is configurable via the field CorrPeriodMs of the structure USBD_AUDIO_STREAM_CFG .
Playback Feedback Correction
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The feedback value update to the host is evaluated every refresh period. The refresh period is configurable via the field CorrPeriodMs of the structure USBD_AUDIO_STREAM_CFG . When the refresh period is reached, if there is a correction to apply, the feedback value update is sent to the host by calling the function USBD_IsocTxAsync() . If there is no correction necessary, the audio class does not prepare an isochronous IN transfer. Thus when the host sends an IN token, a zero-length packet is sent by the device. The host interprets this zero-length packet as "continue to apply the previous valid feedback value". The feedback value is sent in 10.14 format.
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