DIGITAL MEDIA BASICS

Digital media technology is constantly evolving. To implement it, you need a good grasp of the terminology used in the field. There are several useful publications and web resources for broadening your knowledge of digital media technology. David Katz and Rick Gentile’s Embedded Media Processing is one of my favorite resources. In this section, I’ll briefly describe some basic terms.

Sound is a wave form of energy that propagates through air or some other medium. Amplitude and frequency are the two attributes that define sound. Music is an art form, which involves structured and audible sound. Although the definition is seemingly subjective, it’s the most common. What’s music to one person can be noise to another.

The audio signal can come from a transducer (e.g., a microphone) that converts sound waves into electrical signals, or it can be created (synthesized) electronically. On the other end, an analog audio signal coming to the speaker or an earphone is converted into a pressure change (sound wave).

To process sound, you need to convert both ways between analog and digital signals. An ADC and DAC do this. Devices combining one or more of each are called codecs. Note that the word codec is used for both the hardware devices and software algorithms.

To correctly represent the digital signal, refer to the Nyquist theorem. (The sampling frequency must be at least twice the highest frequency of the signal.) You can hear sounds between 20 and 20,000 Hz, so the sampling rate has to be at least 40 kHz. The commonly used sampling rate for CD quality audio is 44.1 kHz. The amplitude of sound is measured in decibels. The range for a human ear is from 0 (the threshold of hearing) to 120 dB SPL (the threshold of pain).

Dynamic range is a ratio of the maximum signal level to the minimum signal level (or the noise floor). For digital audio systems, the dynamic range depends on precision (number of bits representing the signal). For 16-bit codecs, the dynamic range is 96 dB.

There are standards for connecting physical devices in digital audio systems. The most common are I²S and AC97. The former, which I used for my project, is a three-wire serial interface for the digital transmission of audio signals. It has a bit clock, data, and left/right synchronization lines. Intel created the popular AC97 standard for PC audio.

The popular audio file formats are wave and MP3. The latter is one of the most popular lossy audio compression codecs that can achieve a compression ratio of up to 12:1. Lossy codecs use a technique called perceptual encoding, which takes advantage of your ears’ physiology. Although the data is lost, the decoded sound is close to the original.