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EDNE MARCH 2013

By Jerad Lewis • Analog Devices Inc Common inter-IC digital interfaces for audio data transfer Understandin g the pros and cons of diferent interfaces before selecting parts helps to streamline your component selection and ensure that you have the most eficient implementation A of the signal chain. s audio integrated circuits move to finer geometries, it becomes more difficult to design—and less cost-effective to integrate— high-performance analog circuits on the same piece of silicon with high-density digital circuits. Audio-system architects thus are pushing analog portions of the audio signal chain further toward the input and output transducers and are connecting everything in between digitally. A traditional audio signal chain may have analog signal connections between microphones, preamps, ADCs, DACs, output amplifiers, and speakers, as shown in Figure 1. As the analog circuits are pushed to the edges of the signal chain, however, digital interfaces between ICs in the chain become more prevalent. DSPs have always had digital connections, but now digital interfaces are being included on the transducers and amplifiers, which typically have had only analog interfaces. IC designers are integrating ADCs, DACs, and modulators in the transducers on opposite ends of the signal chain, thereby eliminating the need to route any analog audio signals on the PCB, as well as reducing the number of devices in the signal chain. Figure 2 shows an example of a completely digital audio signal chain. Many standards exist for transmitting digital audio data. Some formats, such as I2S (inter-IC sound), TDM (time-division multiplexed), and PDM (pulse-division multiplexed), are typically used to enable inter-IC communication on the same PCB. Other formats, such as S/PDIF and Ethernet AVB, primarily target data connections from one PCB to another through cabling. This article focuses on the differences, advantages, and disadvantages of the inter-IC digital audio formats. Choosing audio components with mismatched digital interfaces needlessly complicates the system design. Understanding the pros and cons of the different interfaces before selecting parts helps to streamline component selection and ensure the most efficient implementation of the signal chain. Inter-IC Sound (more commonly called “I squared S” or “I two S”) is the most common digital audio format used for audio data transfer between ICs. Philips Semiconductors— now NXP—introduced the I2S standard in 1986; the format was revised in 1996. The interface was first popularly used in CD-player designs and now can be found in almost any application that involves the transfer of digital audio data from one IC to another. Most audio ADCs, DACs, DSPs, and sample-rate converters, as well as some microcontrollers, include I2S interfaces. An I2S bus uses three signal lines for data transfer: a frame clock, a bit clock, and a data line. The receiving IC, the transmitting IC, or even a separate clock-master IC can generate the two clocks, depending on the system architecture (Figure 3). An IC with an I2S port often can be set to be in either master or slave mode. Unless the design uses a sample-rate converter in the signal chain, a system will usually have a single I2S master device so that there are no issues with data synchronization. The Philips standard for these signals uses the designations WS for word select, SCK for the clock, and SD for the data, although IC manufacturers seem to use those names only rarely in their IC data sheets. Word select is also commonly called LRCLK, for “left/right clock,” and SCK may ANALOG SIGNALS DIGITAL SIGNALS ANALOG SIGNALS DAC MICROPHONE PREAMP 10100101 10100101 ADC DSP SPEAKER/ HEADPHONE AMPLIFIER Figure 1 A traditional audio signal chain may have analog signal connections between microphones, preamps, ADCs, DACs, output amplifiers, and speakers. SPEAKER 34 EDN Europe | MARCH 2013 www.edn-europe.com


EDNE MARCH 2013
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