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Digital technology is changing the way we experience high-quality sound in everything from smartphones to automotive systems. Digital interfaces offer the potential to achieve high performance, power-efficiency, and flawless clarity. In this article we explore how the MIPI SoundWire® protocol sets a new standard in digital audio transport and how it’s shaping the future of audio devices.

What is SoundWire?

MIPI SoundWire an audio interface that has been developed by the MIPI Alliance Audio Working Group to meet the market need for a comprehensive, scalable, low-power audio transport solution. It offers a digital protocol for audio devices that are embedded in a wide range of applications such as mobile devices or PCs. In addition, SoundWire is a compact solution that transports both the control information and audio data on a common interface. This improves on traditional fragmented solutions, such as PDM, and I2S, which require external pins to support an out-of-band control bus.

The architecture of SoundWire is based on a 2-pin scalable design that can connect up to 11 audio peripherals on the same bus. There are mainly 2 types of devices supported on a SoundWire bus, a Manager that is responsible for managing bus activity and a Peripheral, which could be any kind of audio device, such as a speaker. The bus consists of a clock and a bi-directional data line. While the clock line is solely driven by the Manager in order to ensure the clock quality during audio transport, the data line can be driven by any of the devices. Being able to support high performance is a big part of SoundWire’s power-efficiency, and this is demonstrated by the Manager’s ability to drive the clock as fast as 13MHz. This is equivalent to 26Mbps because SoundWire is always operated at Double Data Rate (DDR).

Figure 1: The architecture of the SoundWire bus.

Main Features of SoundWire

Power-Saving Mode

To facilitate power-efficiency, SoundWire allows clock stopping during periods of inactivity in order to save power, and the protocol provides a methodology through which the Manager can inform the peripherals that it is going to stop. This methodology facilitates seamless clock stopping without having the peripherals lose synchronization. SoundWire also supports a feature where a peripheral device can send a wake-up signal, requesting from the Manager to restart the clock again and continue normal operation.

In-Band Control and Command

One of the main advantages of SoundWire is that it is a comprehensive interface that can transport both audio payload and control information in-band without requiring external pins or an independent control bus. This gives the Manager device the ability to control bus operations and communicate with multiple devices using the same shared pins. For example, the Manager can utilize the control word to assign dynamic addresses to every one of the audio devices using a standardized process, called Device Enumeration.

Control Info and Audio Data Frame Structure

The Manager encapsulates the control information and audio data in a 2-dimensional frame structure of a configurable size. As shown in the figure below, the control information is encoded in the 48-bit control word, while the rest of the frame is used to carry payload data. This 2D structure gets serialized and transported across the shared SoundWire data line. Its shape can be configured in order to reduce overhead, unlike in SlimBus where this shape was of fixed dimensions. For example, the smallest frame shape is used during initialization, and as soon as the corresponding devices get ready to receive data, bigger frame shapes can be used to transport valid audio data. Synchronization patterns are embedded in the control word, which the audio peripherals can decode to infer the frame dimensions and reconstruct the frame shape.

Figure 2: Frame structure of a typical SoundWire operation.
Figure 2: Frame structure of a typical SoundWire operation.

 

Neat Bus Management

Bus management is one of SoundWire’s major advantages, and this is largely represented by the neat status monitoring framework provided by the specification. SoundWire introduces a specialized command, called ‘Ping’, where the Manager can enquire about the status of all the peripherals attached on the bus to track their synchronization. Through the ping command, the peripherals can also send an ‘ALERT’, indicating that it captured an error, such as parity error or a bus clash. It could also indicate that the peripheral has captured some information that it wants to share. As soon as the Manager receives an ALERT message, it can follow up with a series of register writes and reads to get more information about the nature of the ALERT.

Testing SoundWire with the SV6E-X

With SoundWire and its next generation specification, SWI3S, engineers need a comprehensive solution for design validation and characterization. Introspect Technology’s SV6E-X supports a multi-channel configuration that can emulate up to 4 independent SoundWire devices, either as a Manager or Peripheral. In addition, this versatile tool provides 4 independent SoundWire Protocol Analyzer cores with an embedded real-time oscilloscope that can probe a bus without being involved in the bus’ activity. This is all enabled by our PinetreeTM software, a Python based test environment, which you can use to send SoundWire python commands to test your chip.

Figure 3: Example test configuration of our SV6E-X simultaneously driving two microphones on channels 1 and 2 using SoundWire.

 

For any inquiries on SoundWire applications or the SV6E-X, reach out to us at info@introspect.ca.

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