Difference between revisions of "Understanding Digital Audio"
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Revision as of 01:24, 9 March 2015
Digital Audio is the representation of sound waves digitally.
Contents
Basic Principles
The most basic way to think of digital audio is like animation or video. By playing a number of individual "frames" quickly enough, it gives the appearance of motion. In the case of audio, the frames are called samples. Unlike video, however, digital audio is not an illusion - the digital representation of the sound is capable of storing all the information contained in and can be converted back into identical sound waves, at least theoretically.
The amount of sound information a digital audio signal can represent is determined by the bit depth and the sample rate. Bit depth is the number of bits per sample. Each sample represents the amplitude of the sound wave at that point in time. The number of samples per unit of time is called the sample rate, typically represented in kHz, which is the number of thousand samples per second. Typical values for these are 16-bit/44.1 kHZ (used on typical consumer CD's) and 24-bit/96 kHZ (for commercial recording). The higher the bit-depth, the more distinct values are possible for each sample, allowing more and more accuracy in recording. The sample rate determines the highest frequency that can be accurately represented. The Nyquist Frequency is 1/2 the sample rate and is the highest-pitched frequency that can be accurately represented. A sample rate of 44.1 kHZ means the highest-representable frequency is 22.05 kHZ, which is roughly the maximum frequency that humans are capable of hearing.
A device called an analog-to-digital converter (ADC) turns analog electrical waves into digital signals. Conversely, a digital-to-analog converter (DAC) turns digital audio into analog electrical signals. ADC's and DAC's typically feature a low-pass filter to remove high-frequencies that cannot be accurately represented by their maximum sample rate. This is at the Nyquist frequency. Without filtering out such frequencies during the analog-to-digital conversion, certain frequencies may be incorrectly represented as lower frequencies, which is called aliasing. Similarly, the digital-to-analog conversion will produce frequencies above the Nyquist frequency which are aliases of lower frequencies. They should be filtered out for accuracy (however, it shouldn't matter if the Nyquist frequency exceeds the range of human hearing).
While digital audio is theoretically capable of perfectly representing analog sound, in practice, devices can inaccurately measure amplitude or mis-time a sample. The quality of such devices is paramount to accurate recording and reproduction of sound. Commercial audio production uses bit depths and sample rates higher than the final mastered product to ensure any error is minimized. While there is no need for frequencies between 22.05 kHZ and 48 kHZ (as no human can hear them), the higher sample rate prevents signal degradation as digital audio streams are manipulated and mixed together.