3.12.1 Digital Audio

Learn the concept of digital audio.

Digital audio involves taking standard audio signals and converting it into binary data that the computer can use, process, and store. The natural sounds that we hear every day is referred to as analog audio.

The audio card installed in PCs is responsible for making the switch between digital and analog audio signals so that we can hear sounds as they are and so that computers can store and edit sounds.

Analog Audio

Analog signals are transmitted through the air in the form of sound waves. The vibration in the air creates alternating regions of high and low pressure, moving outward from the surface of the source of the sound. This is why speakers have big, round surfaces called drivers that vibrate, creating the pressure waves. When the pressure waves propagate out, they carry the sound to our ears.

When measuring analog signals, two different values are used to describe what the sound wave looks like. Those two things are called amplitude and frequency.

• Amplitude measures the strength of the sound signal, or basically how loud the sound is. A signals amplitude is shown on the graph by the distance from the peak to the trough.
• Frequency measures how many times the signal changes amplitude in one second. On the graph, its how many pairs of peaks and troughs there are in one second.

When analog devices record audio signals, they just make a copy of the wave using some medium. However, computers aren't analog devices. They use digital signals, which requires converting the signals type through a process called sampling.

Sampling

When a computer digitizes an analog signal, it splits the wave up into chunks called samples. Samples are specific values that can identify a waves amplitude and frequency at a given point in time. Sampling covers the entire wave, creating samples at arbitrary moments throughout the sound signal.

Using the group of samples, the computer can create an approximation of what the analog wave looks like, but in digital form. The accuracy of the approximation is referred to as fidelity. There are several parameters that define a digital audio signal's fidelity. The first is the bits per sample (bps), and it refers to the amount of digital bits used to represent the value of each sample.

A sample's bps specifies how many possible values one given sample can have to measure an analog wave at that point. Low bps returns low fidelity since the amount of different values being used to approximate the value of the samples is low, and high bps returns a higher quality digital audio signal, since we get a more accurate approximation of the sample's value.

Another thing that affects the quality of digital audio signals is the sampling rate. The sampling rate specifies the number of samples that are taken for every one second of analog audio, measured in hertz (Hz). e.g. 8000 Hz => 8,000 samples for every 1sec of analog audio A higher sampling rate typically results in increased digital audio quality.

All of this conversion is the responsibility of the installed video card. The standard for sound cards is to be able to output 16-bit, 44 kHz audio, the equivalent of CD quality. High-end sound cards are supposed to output 24-bit 192 kHz audio. However, increased quality also results in increased file size.

Compression

One 24-bit, 96 kHz sample would use 34 MB of disk space. For a six-minute song, that would result in a file size of over 200 MB. To get around these, audio file compression techniques were developed to save storage size. The goal of audio compression is to reduce the file size of the digital audio signals all while maintaining a level of fidelity.

The most common compression scheme is called MPEG-2 Audio Layer III compression (or MP3 THE GOAT!!!!!!). MP3 is able to compress audio files to just about 9% of their original size. Still, MP3 is considered a lossy compression algorithm, which means it trades a loss in fidelity for smaller file sizes, which is good. Other compression schemes include:

• Windows Media Audio (WMA)
• Advanced Audio Coding (AAC)
• Dolby Digital

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