Digital Audio, Networking, and Distributed Information

Key Insight

Digitized sound made a major consumer impact with the compact disc (CD) in 1983, storing 74 minutes of audio on a 12-centimeter disk. Sound, being vibration, is converted from analog electrical currents to digital values using Pulse Code Modulation (PCM). An analog-to-digital converter (ADC) samples the sound wave's voltage at a constant rate; for CDs, this is 44100 samples per second, which is more than double the maximum human hearing frequency of 20,000 Hz, adhering to Nyquist's theorem. Each sample on a CD utilizes 16 bits, allowing a dynamic range of 96 decibels, which spans from the threshold of hearing to the threshold of pain.

Stereo CD audio requires 176400 bytes per second (44100 samples/sec multiplied by 2 bytes/sample multiplied by 2 channels), accumulating to approximately 783.2 MB for a 74-minute disc. Digitized sound offers superior fidelity as digital data can be copied perfectly without degradation, unlike analog recordings. Beyond audio, CDs store data as CD-ROMs (typically 660 MB), a prevalent medium for software distribution. Multimedia, incorporating digitized sound and video, is now standard on home computers, facilitated by sound boards equipped with ADCs and digital-to-analog converters (DACs), supporting various sampling rates (e.g., 8000 Hz to 44100 Hz) and bit depths (8 or 16 bits). Voice synthesis converts text into waveform data, while voice recognition, the more complex task of converting waveform to ASCII, often requires user training.

The concept of interconnected computers for information sharing dates back to George Stibitz's remote computer operation in 1940. Early network communication over telephone lines used modems (modulator/demodulators) to convert bits to sound and vice versa, with early modems achieving speeds of 30 bytes per second. The Internet, a decentralized network, transmits data using protocols like TCP/IP, which divides larger blocks of data into smaller packets for efficient transmission and reassembly. The World Wide Web, built upon HTTP, uses HTML (Hypertext Markup Language) for web pages, supporting hypertext links, formatted text, and embedded images, fulfilling a goal similar to Bush's Memex. HTML files are text-based, enabling searchability and client-side scripting (JavaScript). Java, an object-oriented programming language, offers a platform-independent solution for running compiled programs (Java byte codes for the Java Virtual Machine) securely across various operating systems. Future high-speed data transmission will increasingly rely on optical fiber, capable of gigahertz rates by transmitting light (photons).