Audio Toyes #11 - What is Digital Audio
March 01, 2011
What is digital audio? What is meant by sample rate and bit rate? How does sound go from the air into my computer?
These are all good questions. A starting point towards finding these answers involve defining sound. Sound is a standing wave (energy) with areas of positive pressure and areas of negative pressure. Each wave has an amplitude and a wavelength. We need a device to measure this energy pattern in order to translate it to the digital realm. The capsule of a microphone is the front line of this process. A microphone capsule (a transducer) captures the pulsing alternating pressures of a sound wave and translates them to small electrical signals. These electrical signals are then analyzed by an analog to digital converter at a rate of 44,100 times a second. Each of these “samples” of the original sound represent a snapshot of the overall sound wave. When all the samples (snap shots) are placed side by side it is possible to visualize the complete sound wave. This is the process of digitizing the sound wave, breaking it into pieces, or digits. So the sample rate refers to the number of pictures taken per second of a particular sound wave. The standard CD quality audio has a sample rate of 44.1khz, 44,100 samples per second of audio.
There's one more thing to consider. What information is taken in these snap shots? Its the amplitude of the sound wave at that particular moment. It could be zero (silence) or it could be greater than zero (sound). In digital audio we can choose to differentiate between the loudness of two different snap shots by using a ladder of 16 steps (CD quality) or 24 steps (higher definition). This is referred to as the bit rate. A bit rate of 16 gives us 16 different volume levels at which to represent a snap shot of a sound wave. So in one second of audio at a sample rate of 44.1khz and a bit rate of 16 there are 44,100 pieces of information holding loudness(amplitude) data between 1 and 16. When played back through speakers we do not hear the individual snap shots but instead we perceive a smooth reproduction of the original sound we recorded.
We now have technology with the ability to convert analog sound into digital information at a sample rate of 192Khz at 24bit. Most audio engineers employ a sample rate of 48khz and 24 bit. If you have any questions about sample rates, bit rate, digital audio or sound waves please don't hesitate to email me I will be happy to elaborate.