Audio Signal Chain Design for Non-Audio Experts

Audio Signal Chain Design for Non-Audio Experts

In this article, we will begin to decode audio product specifications to find the best product for the audio application - not the fancier, but the key is to meet the product requirements, just like a glove fits the hand.

Converters

Audio converters fall into three categories: analog-to-digital converters (ADCs), digital-to-analog converters (DACs), and ADCs and DACs integrated into the same device (CODECs). In a software context, a codec is similar to the software used to encode and decode MP3 formats. But in a hardware context, it is an interface to the analog domain.

Control Interfaces

There are many different control interfaces. Some simple converters generally use a hardware control interface. The control pins are usually connected to VDD, GND, or GPIO processor pins. If your system does not change configuration or you have a lot of GPIO space on your processor, this is the easiest way to start. Permanent hardware settings (configured when you design the printed circuit board) offload a redundant software driver that you may have to write to use software controlled converters. Some examples of hardware driven ADCs and DACs include the PCM1803/ PCM1789.

Software control interfaces are typically driven by I2C or SPI serial ports, which are found on some microprocessors and DSPs. Some devices driven in software mode often have more flexibility than their hardware controlled versions. Software controlled converters usually have some internal registers that can be written from an external source. From the perspective of the overall system solution, this adds some complexity to the mix.

However, there are some tricks to make it easier: you can write to your "driver" to change settings during operation; you can also dump all configurations or register code to flash memory. In this way, during startup, the entire configuration is sent out the power-up serial port.

Dynamic Range, SNR and THD+N

Audio standards for measuring product performance (not just converters, but entire signal chains) are defined by the Audio Engineering Society (AES): "AES17-1998 (r2004): AES Standard Methods for Digital Audio Engineering - Standards for Measurement of Digital Audio Equipment (Revised AES17-1991)".

These tests are based on the difference between full scale (maximum input/output) and the background noise level. For example, using a 1 kHz input -60dB below full scale and then measuring the background noise allows for a test of signal-to-noise ratio (SNR) or dynamic range (same in converters). For THD+N measurements, engineers run the device under test (DUT) at -1 dB below full scale and make similar measurements. I recommend downloading the AES document and reading it carefully.

Since most output signal chains require an audio converter and audio amplifier, and audio amplifiers generally introduce their own noise, specify a converter that exceeds your requirements. CD quality sound is often quoted as having a 96-dB dynamic range (the actual number may be slightly higher, but the basic calculation is the number of bits times 6, or 16 bits x 6 = 96 dB).

ADCs are similar. Ideally, the best quality input conversion is achieved by using an input amplifier that can convert the highest level input signal to just below the ADC full-scale input. This results in the best SNR performance of the converter.

For more information on how to specify the right audio converter for your system, visit the TI E2E community forum-style discussion audio community, or see "Why a better DAC?" and "Understanding excellent professional audio design: A block-by-block approach" (both articles on the Audio DesignLine).

About the author
Dafydd Roche is the marketing manager for home entertainment and professional audio products in TI's High Performance Analog Products Group. A graduate of the University of York (UK), Dafydd brings everything he has learned about audio and music production to his work, helping designers and consumers achieve clearer inputs and higher outputs.