How to select and use an audio codec and microcontroller for embedded audio feedback files
Developers also need to consider what they will be driving. Will the audio codec be directly driving headphones, one speaker or a pair of speakers, and what will the output rating be? If the system will be driving a 5-watt speaker, there are not many codecs for embedded systems that will do that. Instead, a developer may want to select the line out and use a separate Class-D amplifier to drive the speaker directly. This saves some cost while also providing design flexibility. Two final considerations are the internal routing and digital filtering capabilities. Here is where the real differentiation and cost differences are determined for an audio codec. For example, the TLV320AIC311IRHBR has de- pop and soft start capabilities to minimize speaker popping and allow for a smooth transition into audio playback. It also has an internal mixer for each output channel and digital volume control. It is up to the developer to carefully balance their needs from the audio codec with the BOM and the amount of board space that will be consumed by the circuitry. The audio playback system When working with an audio codec, it is important to realize that there are several different blocks outside the audio codec that are necessary to achieve successful audio playback. The exact blocks will vary slightly based on application
What are audio codecs? An audio codec is a hardware component that is capable of encoding or decoding a digital data stream containing audio information1. An audio codec is useful because it allows the audio processing to be offboarded from the microcontroller. This can significantly decrease the software complexity and also allow a less expensive and less capable microcontroller to be used for an application. A typical audio codec will contain several functional blocks: ■ An I2S interface to transmit or receive encoded digital audio data ■ An I2C interface to configure and read the audio codec’s control registers ■ A microphone input which is connected to an analog-to-digital converter (ADC)
Figure 1: The AK4637EN is an audio codec with a mono speaker output that has audio playback and recording capabilities. It also contains an internal audio block that can be used to filter incoming and outgoing audio to improve audio fidelity. Image source: AKM Semiconductor
features, in addition to a beep generator input that can be used to generate a beep using a pulse width modulation (PWM) signal at a desired frequency. Developers will find that the main differentiator for an audio codec is going to be whether it outputs mono or stereo audio, as well as the digital block capabilities. For example, the AK4637EN offers a high-pass filter, a low-pass filter, a four-band equalizer, an auto- leveling channel feature and a single-band equalizer. The latter can be used as a notch filter. How a developer sets up these digital filters can dramatically affect how a system sounds. The audio codec can sometimes intimidate a developer that is new to audio playback. For example, while the AK4637EN is a simple
audio codec, a quick examination of the datasheet shows that it has 64 configurable registers. That might seem like a lot at first, but most of those registers are used to set the filter coefficients for the various digital filters that are available. There are only a handful that need to be used to get the system outputting audio properly, making the driver development for an audio codec far simpler than a newbie might imagine.
Figure 2: The TI TLV320AIC3110IRHBR is an audio codec with stereo output and amplification in addition to a microphone input. The codec can drive 1.29 watts from internal amplifiers and has programmable digital audio blocks. Image source: Texas Instruments
■ At least one audio output channel such as a speaker
output, but most also include a line out and may include multiple speaker outputs for stereo playback ■ A digital block that contains high-pass, low-pass, notch, and equalizer filters to tune audio playbacks and recordings An example audio codec that is quite popular due to its low cost and audio capabilities is the AK4637EN 24-bit audio codec from AKM Semiconductor (Figure 1). The AK4637EN has all these
weigh the design requirements against the key solution parameters. The first consideration is the required output from the audio codec. There are several different choices. For example, the AK4637EN has a line output and a mono speaker output. There are other codecs like the Texas Instruments TLV320AIC3110IRHBR stereo audio codec that can drive two speakers at 1.29 watts (Figure 2).
Other audio codecs like the Maxim Integrated MAX9867 are designed to only drive a pair of headphones (Figure 3). The MAX9867 has the typical I2S and I2C digital interfaces, but it also contains stereo microphone inputs and two line ins that can be digitally selected. Deciding between these three solutions as to what the output type will be (or even the input) is a critical early decision.
How to select an audio codec
One of the key drivers to selecting anything in product development is cost, and audio codecs are no different. Still, it is important to keep in mind that developers get what they pay for, so when it comes to audio, a team must carefully
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