Introduction to Audio System Communication Interface Standards and Configurations

Introduction

Audio signal transmission has been standardized at both the protocol and interface levels. This article explores commonly used analog and digital audio interfaces.  Please see below for an overview:

• Blue: Digital interfaces like PDM, I2S, TDM, Slimbus, Soundwire, and USB Audio

• Orange: Optical/Electrical interfaces like SPDIF and ADAT

• Red: Analog interfaces like Line in/out, XLR

Background

Inter-IC Audio Interfaces (Within Equipment)

Digital Audio Communication Standard/Protocol

2-Channel /Stereo Audio Interface

1. PDM (Pulse Density Modulation)

   • Supports up to 2 digital microphones

   • Bus Type: 2-wire interface

   • Pins: Data, Clock

2. I2S (Inter-IC Sound)

• Supports up to 2 channels

• Bus Type: 3/4-wire interface

• Configuration: Master/Slave, Half/Full Duplex

• Pins: BLCLK (Bit Clock), LRCLK (Word Clock), SDOUT (Data Out), SDIN (Data In), MCLK (Master Clock)

  • Note: Typically, BLCK, LRCLK, and SDOUT/SDIN are used.

Multi-Channel Audio Interface

• TDM (Time Domain Multiplexing)

  1. Supports up to 16 channels

  2. Bus Type: Master/Slave, Half/Full Duplex

  3. Pins: SCLK, FSYNC, SDATA-Din, SDATA-Dout

Advanced audio interface

Introductory Note: Some advanced audio interfaces simplify digital transmission by reducing the number of lanes to just two wires. This makes the hardware design more straightforward but necessitates a more complex audio hardware controller to manage the simplified lanes. 

1. Slimbus

• Combines audio and control streams

• Commonly found in mobile devices

• Wiring: CLK, Data

2. Soundwire

• Similar to Slimbus but supports PCM and PDM

• Wiring: CLK, Data

Analog Interface

• Line out/Line in

Inter-Board Audio Interfaces (Between Equipment)

Digital Audio 

1. SPDIF (Sony/Philips Digital Interface)

   • Audio Format: Uncompressed 2CH PCM, Compressed 5.1/7.1 (DD, DTS)

   • Physical Interface: Optical (Toslink), Electrical (Coaxial, RCA), ARC (HDMI)

2. ADAT (Alesis Digital Audio Tape)

   • Audio Format: Uncompressed 24Bits/48kHz/8CH PCM, 24Bits/96kHz/4CH PCM

   • Physical Interface: Optical (Toslink)

3. USB Audio

   • Physical Interface: USB transmission line and connectors

Analog Interface

1. Unbalanced Voltage

3. • Physical Interface: 3.5mm audio jack

2. Balanced Voltage

3. • Physical Interface: XLR connector

Wireless Interface

1. Bluetooth Classic (EDR, BDR)

3. • Profile: A2DP

   • Audio Format: Compressed (SBC, AAC, LDAC)

2. Bluetooth Low Energy (BLE)

• • Profile: GATT

  • Audio Format: Compressed (ADPCM, OPUS)

Design analysis

• The most common Inter-IC digital audio interfaces are I2S and TDM.

• The most common Inter-board digital audio interface is SPDIF, typically with an optical Toslink connector.

Q&A

• What are the key differences between Inter-IC and Inter-board audio interfaces?

  • Answer: Inter-IC interfaces are used for audio signal transmission within a single piece of electronic equipment, such as inside a smartphone or a computer. Inter-board interfaces, on the other hand, are used for audio signal transmission between different pieces of electronic equipment, like between a computer and external speakers.

• What types of audio formats are generally supported by digital audio interfaces like SPDIF and ADAT?

  • Answer: SPDIF generally supports uncompressed 2-channel PCM audio and compressed multi-channel audio formats like Dolby Digital (DD) and DTS. ADAT, on the other hand, supports uncompressed multi-channel PCM audio, commonly at 24-bits and 48kHz for up to 8 channels or 24-bits and 96kHz for up to 4 channels.

• What are the advantages of using a balanced analog audio interface over an unbalanced interface?

  • Answer: Balanced audio interfaces are less susceptible to external noise and interference, making them ideal for longer cable runs and professional audio applications. Unbalanced interfaces are simpler and less expensive but are more prone to noise interference, especially over longer distances

Summary & Conclusion

• Inter-IC connections are interfaces within electronic equipment.

• Inter-board connections are interfaces between different pieces of electronic equipment.

• Audio transmission mediums can be either optical or analog.

• Audio formats can be either raw (like PCM) or compressed (like AC3).

Further Reading

• Stereo Audio Digital Interface

• Multi-channel Audio Digital Interface