What Is the Digital Output in PCM Format?by David Lipscomb
Pulse code modulation, or PCM, is a digital representation of an analog waveform. PCM is the standard audio format for CDs, DVDs, Blu-ray discs and all other consumer digital audio products. PCM is compatible with any digital audio source and processor using fiber optic, digital coaxial and HDMI cables to carry the signals.
Quantization is the term that describes taking a large amount of data and converting it into a smaller set. Analog audio, for example, if recorded uncompressed to a CD, would take up far more space than the CD could hold. As a result, the data is quantized to file types that fit neatly onto a digital medium like a CD. Quantization is inherently "lossy" in that it represents less information than the original. However, tricks used in digital compression discard data people have trouble hearing, especially when masked by louder sounds occurring simultaneously.
PCM works by doubling the maximum sample rate of the highest analog frequency. PCM on a CD, for example, samples at 44.1 kilohertz, slightly greater than the 20 kilohertz high frequency maximum humans can hear. The need to double the maximum rate helps compensate for any compression that must occur, where data is thrown away to accommodate physical disc space requirements. Without doubling this maximum frequency, quantization errors occur, associated with harshness and poor overall PCM audio quality.
Tied to sample rate is bit depth, the amount of data, measured in bits, that comprise each sample. Bits are colloquially referred to as "ones and zeros," the binary components of digital information. Higher depth, combined with more samples, equals higher quality PCM audio. This is accomplished by increasing the signal-to-noise ratio that governs the amount of signal versus background or digital noise that otherwise interferes with a clean PCM sample recording.
Sampling rate defines the amount of digital samples taken of the original analog waveform in a given amount of time, usually per second. Loosely translated, higher sampling rates are generally associated with higher levels of sound quality. CDs use a PCM sample rate of 44.1 kilohertz, while higher quality formats such as DVD-Audio and Blu-ray use sample rates of 192 kilohertz. Higher sample rates more accurately reflect the original analog information, making the digital PCM version of the audio more faithful to the original master recording.
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