| 1. Digital Audio | 2. Rules of Audacity | 3. Setup, Audio Import and Playback |
| 4. Recording with Audacity | 5. Common Editing Tasks |
| What's digital audio made of? |
Recording digital audio is similar to taking a picture with a digital camera. You take snapshots.
Let's examine the audio material on an audio CD (also called CDDA-Compact Disc Digital Audio).
Here we find two channels (left and right) of 44100 snapshots per second. These snapshots are called samples and in this case are stored as Pulse Code Modulation data (PCM). The important thing to know about PCM is that it's the closest thing a digital device can come to the actual audio signal. Your computer has many other ways of storing audio, but PCM is the only one that encodes raw, uncompressed, audio. Two common audio file formats, WAV and AIFF, store PCM data. To contrast, MP3 files do not store PCM data - they store compressed audio.
The number of samples taken per second is called the sample rate. In general, the higher the sample rate, the higher the quality of the audio. The sample rate of a CD (44100 Hz, or 44100 samples per second) is good enough to reproduce most sounds on all but the best speaker systems. Some audiophiles prefer sounds recorded at 96000 Hz or higher.
Each sample on an audio CD is represented by a 16-bit number. 16 bits means that each sample can take on any of 2^16 different values. It is also possible to store audio using 8 bits or 24 bits. Just like with the sample rate, the audio quality goes up with the number of bits. Audacity currently only supports 16-bit audio.
| How does audio get digitized? |
This is where a part of your soundcard comes in. It is the Analog-to-Digital Converter (ADC).
It interprets an electrical signal and assigns an absolute value to it,
which it writes to memory, so that software can read and store it away
in further places, such as your RAM or hard disk. The chain of software
your audio data passes through may be
ADC -> RAM -> soundcard driver -> recording software
Playback of digital audio uses a Digital-to-Analog Converter (DAC). This takes the sample and sets a certain voltage on the analog outputs to recreate the signal, that the Analog-to-Digital Converter originally took to create the sample, as faithfully as possible. The first CD players did only that and it sounded horrible. Nowadays DACs use Oversampling to smooth out the audio signal. The quality of the filters in the DAC also contribute to the quality of the recreated analogue audio signal. The filter is part of a multitude of stages that make up a DAC.