Converters
Audio converters come in three flavors: analog-to-digital converters (ADC), digital-to-analog converters (DAC) and ADCs and DACs combined on the same device (CODEC). In the software world, a codec is similar to software used to encode and decode MP3 formats. In the hardware world, it is an interface to and from the analog domain.

Control Interface
There are a variety of control interfaces. Simple converters typically come with hardware control interfaces. Control pins are usually tied to V_DD, GND or GPIO processor pins. If your system isn't going to change configurations or you have plenty of GPIO on your processor space, this is the simplest way to get started. A permanent hardware setup (configured when you design the printed circuit board) removes one extra software driver that you might have to write with a software controlled converter. Examples of hardware-driven ADCs and DACs include the PCM1803 / PCM1789.

Software-controlled interfaces are typically driven by either I²C or SPI serial ports found on microprocessors and DSPs. Devices driven in software mode usually offer more flexibility than their hardware-controlled cousins. Software-controlled converters usually have registers inside that can be written from an external source. From a total system solution perspective, some complexity is added to the mix.

However, there are a few tricks to make it simpler: you can write your own "driver" to alter settings during run-time. Or, you can dump all the configuration or register codes into flash memory; then, during startup, send the whole configuration out the serial port on power-up.

Dynamic Range, SNR and THD+N
The audio standard adopted to measure a product's performance (not just the converter, but the whole signal chain) was defined by the Audio Engineering Society (AES): "AES17-1998 (r2004): AES standard method for digital audio engineering--Measurement of digital audio equipment (Revision of AES17-1991)".

These tests are based on the difference between full-scale (maximum input /output) versus background-noise levels. For instance, testing signal-to-noise-ratio (SNR) or dynamic range (the same in converters) is done by using an input that is -60dB below full scale at 1 kHz, and measuring the background noise. For THD+N measurements, engineers run the device under test (DUT) at -1 dB below full scale and do a similar measurement. I recommend downloading the AES document and going through it in detail.

As most outputs signal chains require an audio converter and audio amplifier, and audio amplifiers typically add their own noise, specify a converter that's a touch better than your requirements. CD-quality audio is usually referred to as 96-dB dynamic range (the actual number is a little higher, however, a basic calculation is 6 times number of bits, or 16 bits × 6 = 96 dB).

ADCs are similar. Ideally, the best-quality input conversion can be obtained by utilizing an input amplifier that will bring the highest level of input signal to just below the full scale input of the ADC. This results in the best SNR from the converter.

Sources of information on specifying suitable audio converters for your system include Texas Instruments' audio community for a forum-type discussion at TI E2E Community, or see "Why use a better DAC?" and "Understanding superior professional audio design: A block-by-block approach" (both at Audio DesignLine).

About the Author
Dafydd Roche is the Home Entertainment and Professional Audio Marketing Manager for Texas Instruments' High Performance Analog group. A graduate from the University of York (UK), Dafydd pours his passion and knowledge of audio and music making into his work, helping designers and consumers get cleaner inputs and louder outputs!

Previous installments of this series:

• "SIGNAL CHAIN BASICS (Part 24): Basic networking using the IEEE 802.15.4 PHY/MAC protocol", click here
• "SIGNAL CHAIN BASICS (Part 23): EIA-485: Receiver equalization boosts networking performance", click here
• "SIGNAL CHAIN BASICS (Part 22): Phantom microphone power--the ghost in the machine", click here
• "SIGNAL CHAIN BASICS (Part 21): Understand and configure analog and digital grounds", click here
• "SIGNAL CHAIN BASICS (Part 20): Understand the basics of op amps and speed", click here
• "SIGNAL CHAIN BASICS (Part 19): Exploring and understanding linear voltage regulators", click here
• "SIGNAL CHAIN BASICS (Part 18): The op amp as integrator", click here
• "SIGNAL CHAIN BASICS (Part 17): Hysteresis--Understanding more about the analog voltage comparator", click here
• "SIGNAL CHAIN BASICS (Part 16): Understanding the analog voltage comparator", click here
• "SIGNAL CHAIN BASICS (Part 15): Analog/digital converter--dynamic parameters", click here
• "SIGNAL CHAIN BASICS (Part 14): Analog/digital converter--static parameters", click here
• "SIGNAL CHAIN BASICS (Part 13): Putting the Bode plot to use", click here
• "SIGNAL CHAIN BASICS (Part 12): The Bode plot, an essential ac-parameter display tool", click here
• "SIGNAL CHAIN BASICS (Part 11): Introducing voltage- and power-conditioning circuits", click here
• "SIGNAL CHAIN BASICS (Part 10): Exploring the Delta-Sigma Converter", click here
• "SIGNAL CHAIN BASICS (Part 9): SAR Converter Operation Explored", click here
• "SIGNAL CHAIN BASICS (Part 8): Flash- and Pipeline-Converter Operation Explored", click here
• "SIGNAL CHAIN BASICS (Part 7): Op Amp Performance Specification--Bias Current", click here
• "SIGNAL CHAIN BASICS (Part 6): Op Amp Input Voltage Offset", click here
• "SIGNAL CHAIN BASICS (Part 5): Introduction to the Instrumentation Amplifier", click here
• "SIGNAL CHAIN BASICS (Part 4): Introduction to analog/digital converter (ADC) types", click here
• "SIGNAL CHAIN BASICS (Part 3): Analog and the digital world", click here
• "SIGNAL CHAIN BASICS (Part 2): Op Amp--Basic operations", click here
• "SIGNAL CHAIN BASICS: Operational Amplifier--The Basic Building Block", click here