PDM data connections are becoming more common in portable audio applications, such as cell phones and tablet computers. This is an advantage in these size-constrained applications because PDM audio signals can be routed around noisy circuitry, such as LCD screens, without having to deal with interference issues analog audio signals might have.
With PDM, up to two audio channels can be transmitted with only two signal lines. Figure 4 shows a system diagram with two PDM sources driving a common data line into a receiver. A clock generated by the system master can be used by two slave devices, which use alternate edges of the clock to output their data on a common signal line.
The data is modulated at a 64x rate, resulting in a clock that is typically between 1.0 and 3.2 MHz. The bandwidth of the audio signal increases as the clock rate increases, so lower frequency clocks are used in systems where a reduced bandwidth can be traded off for lower power consumption.
Figure 4: Dual-source PDM connection diagram
A PDM-based architecture differs from I2S and TDM in that the decimation filter is in the receiving IC, rather than the transmitting IC. The output of the source is the raw high-sample-rate modulated data, such as the output of a Sigma-Delta modulator, rather than a decimated data, as it is in I2S. A PDM-based architecture reduces the complexity in the source device, and often makes use of decimation filters that are already present in a codec’s ADCs.
This allows system designers to not only use audio codecs that they may already be using, but also take advantage of a digital data connection’s reduced sensitivity to interference. Also, these decimation filters may be more efficiently implemented in the finer silicon geometries used for fabricating a codec or processor, rather than what is used on the microphone ICs.
Codecs, DSPs, and amplifiers have had I2S ports for years, but until now a system’s input devices, such as microphones, have had either analog or PDM outputs. As the digital interfaces are pushed further towards the ends of the signal chain, new ICs will be needed to support these new system architectures.
Microphones that have an integrated I2S interface, such as the Analog Devices ADMP441 MEMS microphone, make it easier for designers to build this component into systems where PDM microphones are not easily used or where analog interfaces are not desired. Only a subset of audio codecs accepts a PDM input and very few audio processors outside of those specifically designed for mobile phones and tablets natively accept this type of data stream.
In some designs, an I2S output microphone could completely eliminate the need for any analog front-end circuits, since many designs may only have an ADC and PGA in order to support a microphone input to the processor. An example of a system like this is a wireless microphone with a digital transmitter. The wireless transmitter SOC may not have a built-in ADC, so using an I2S output microphone enables the connections between the transducer and transmitter to be completely-digital.
I2S, TDM, and PDM audio interfaces each have their advantages and applications for which they are best-suited. As more audio ICs are transitioning from analog to digital interfaces, system designers and architects will need to understand which of these interfaces will be most appropriate for their particular design. With a digital signal chain from microphone to DSP to amplifier, analog signals can be pushed completely off of the PCB and exist only in the acoustic domain.
About the author
Jerad Lewis is an applications engineer for MEMS microphones at Analog Devices. He joined the company in 2001 after getting his BSEE from Penn State University. Since then, he’s supported different audio ICs, such as converters, SigmaDSPs, and MEMS microphones.
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