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Conversion
Before you can store or transmit PCM data, you must first convert an analog signal to a set of numbers. An electronic circuit called an Analog-to-Digital converter, or A/D, performs this task. Thousands of times per second, the A/D converter measures the voltage of an incoming analog signal and produces an equivalent number; the higher the voltage, the larger the number. Other circuits connected to the A/D converter package the number as standard bytes of computer data. The data is easy to store and transmit with standard digital circuits, but you cannot hear it directly. Another circuit, called a Digital-to-Analog converter, takes the computer data and changes it back into an analog signal. From there, an amplifier and speaker turn the signal into sound.
Bit Resolution
Different A/D converters work at different bit resolutions. The more bits of resolution, the better the conversion quality. An 8-bit converter represents a signal with 256 levels of precision; this is the smallest number of bits in common use. This means, for a signal that reaches a maximum of 10 volts, an 8-bit converter is accurate to .039 volts. CD players use 16 bits, or 65,536 levels, accurate to .000154 volts. Professional audio equipment uses 24-bit A/D converters.
Sampling Frequency
In addition to a converter̵7;s bit resolution, the frequency at which it takes samples also plays an important role. The higher the frequency, the better the accuracy of the data. Telephone systems, for example, take samples at 8 kHz, compact discs use a 44.1 kHz rate and professional-quality equipment works at 48 or 96 kHz rates. At each sample, the A/D converter produces a number, so at the faster rates, it generates more numbers. This is a trade-off, as higher data rates consume memory resources faster.
Nyquist Limit
A parameter called the Nyquist Limit affects the highest frequency a PCM system can encode. It puts a theoretical ceiling on the highest audio frequency as one-half the sampling frequency. An A/D circuit produces one number for every sample of an incoming analog signal. For example, if it takes 1 sample per second of a 1 Hz signal, the positive and negative peaks of the wave cancel each other out, and the sample ends up being zero. You therefore need to double the sampling frequency to 2 Hz or reduce the signal frequency to one-half Hz. The circuit then captures the positive and negative peaks. The standard figure for the highest audible frequency is 20 kHz, so a CD̵7;s 44.1 kHz rate more than doubles it for good fidelity.