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MIPI Soundwire: Digital Audio Streams and Channels

In MIPI Soundwire: Digital Audio Simplified, we mentioned that digital audio formats including Pulse Code Modulation (PCM) and Pulse Density Modulation (PDM) are target applications for MIPI Soundwire. Here we will discuss Digital Audio Streams and Channels.

For more information on MIPI Soundwire, you can download our whitepaper.

The figure below illustrates how Digital Audio is transferred between Codecs and System memory via Digital Audio Streams. A Codec (short for encoder-decoder) converts analog signals to digital streams, or vice versa. Digital Audio Stream is a logical or virtual connection, which may have one or many channels. For example: Stream-3 has two channels (Stereo) which are decoded by both Codec-A and Codec-C, while Stream-2 has a single channel (mono) – the input side of  the modem.

Soundwire-DigitalAudioStreams

Digital Audio Streams

The figure below shows a conceptual audio frame, and how a digital audio stream and its channels are transferred on the link. Each input or output signal in the link transmits a series of frames. A new frame starts every 20.83us (48KHz). A typical frame consists of command (control information) and payload (audio data). The audio data could be single stream or multiple streams together. Since the frame occurs at a fixed rate, streams can occupy more or less than one sample block every frame. In Figure 7, stream S-2 occupies two sample blocks. This implies that the sample rate for S-2 is 96 KHz. Moreover, S-2 has 4 channels of 20 bits each. So the stream sample block uses 80bits. The audio bit rate would be (80 bits) * (96KHz) = 7.68Mbps.

Soundwire-ConceptualAudioFrameComposition

Conceptual Audio Frame Composition

The total number of streams supported would depend on frame size and streams. Any unused space is filled with Filler/Null data.

For more information on MIPI Soundwire, you can download our whitepaper.