Digitizing audio

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Comparing analog and digital audio
Understanding sample rate
Understanding bit depth
Audio file contents and size
How Soundbooth digitizes audio

Comparing analog and digital audio

In analog and digital audio, sound is transmitted and stored in very different ways.

Analog audio: positive and negative voltage

A microphone converts the pressure waves of sound into voltage changes in a wire: high pressure becomes positive voltage, and low pressure becomes negative voltage. When these voltage changes travel down a microphone wire, they can be recorded onto tape as changes in magnetic strength or onto vinyl records as changes in groove size. A speaker works like a microphone in reverse, taking the voltage signals from an audio recording and vibrating to re‑create the pressure wave.

Digital audio: zeroes and ones

Unlike analog storage media such as magnetic tape or vinyl records, computers store audio information digitally as a series of zeroes and ones. In digital storage, the original waveform is broken up into individual snapshots called samples. This process is typically known as digitizing or sampling the audio, but it is sometimes called analog‑to‑digital conversion.

When you record from a microphone into a computer, for example, analog-to-digital converters transform the analog signal into digital samples that computers can store and process.

Understanding sample rate

Sample rate indicates the number of digital samples taken of an audio signal each second. This rate determines the frequency range of an audio file. The higher the sample rate, the closer the shape of the digital waveform is to that of the original analog waveform. Low sample rates limit the range of frequencies that can be recorded, which can result in a recording that poorly represents the original sound.

Two sample rates

A.: Low sample rate that distorts the original sound wave
B.: High sample rate that perfectly reproduces the original sound wave

To reproduce a given frequency, the sample rate must be at least twice that frequency. (See Nyquist frequency.) For example, CDs have a sample rate of 44,100 samples per second, so they can reproduce frequencies up to 22,050 Hz, which is just beyond the limit of human hearing, 20,000 Hz.

The following table lists the most common sample rates for digital audio:

Sample rate	Quality level	Frequency range
11,025 Hz	Poor AM radio (low‑end multimedia)	0–5,512 Hz
22,050 Hz	Near FM radio (high‑end multimedia)	0–11,025 Hz
32,000 Hz	Better than FM radio (standard broadcast rate)	0–16,000 Hz
44,100 Hz	CD	0–22,050 Hz
48,000 Hz	Standard DVD	0–24,000 Hz
96,000 Hz	High-end DVD	0–48,000 Hz

Understanding bit depth

Bit depth determines dynamic range. When a sound wave is sampled, each sample is assigned the amplitude value closest to the original wave’s amplitude. Higher bit depth provides more possible amplitude values, producing greater dynamic range, a lower noise floor, and higher fidelity. For the best audio quality, remain at 32‑bit resolution while transforming audio in Soundbooth, and then convert to a lower bit depth for output.

Bit depth	Quality level	Amplitude values	Dynamic range
8‑bit	Telephony	256	48 dB
16‑bit	CD	65,536	96 dB
24‑bit	DVD	16,777,216	144 dB
32‑bit	Best	4,294,967,296	192 dB

Higher bit depths provide greater dynamic range.

Audio file contents and size

An audio file on your hard drive, such as a WAV file, consists of a small header indicating sample rate and bit depth, and then a long series of numbers, one for each sample. These files can be very large. For example, at 44,100 samples per second and 16 bits per sample, a mono file requires 86 KB per second—about 5 MB per minute. That figure doubles to 10 MB per minute for a stereo file, which has two channels.

How Soundbooth digitizes audio

When you record audio in Soundbooth, the sound card starts the recording process and specifies what sample rate and bit depth to use. Through Line In or Microphone In ports, the sound card receives analog audio and digitally samples it at the specified rate. Soundbooth stores each sample in sequence until you stop recording.

When you play a file in Soundbooth, the process happens in reverse. Soundbooth sends a series of digital samples to the sound card. The card reconstructs the original waveform and sends it as an analog signal through Line Out ports to your speakers.

To sum up, the process of digitizing audio starts with a pressure wave in the air. A microphone converts this pressure wave into voltage changes. A sound card converts these voltage changes into digital samples. After analog sound becomes digital audio, Soundbooth can record, edit, and process it—the possibilities are limited only by your imagination.