Digital Sound & Music: Concepts, Applications, & Science, Chapter 5, last updated 6/25/2013
19
Figure 5.20 Signal path in digital audio recording
Now let's get back to how a recording session works. During the recording session, the
microphone, an analog device, receives the audio signal and sends it to the audio interface in
analog form. The audio interface could be an external device or an internal sound card. The
audio interface performs analog-to-digital conversion and passes the digital signal to the
computer. The audio signal is received by the computer in a digital stream that is interpreted by
a driver, a piece of software that allows two hardware devices to communicate. When an audio
interface is connected to a computer, an appropriate driver must be installed in the computer so
that the digital audio stream generated by the audio interface can be understood and located by
the computer.
The driver interprets the audio stream and sends it to an audio input buffer in RAM.
Saying that the audio buffer is in RAM implies that this is a software buffer. (The audio interface
has a small hardware buffer as well, but we don't need to go to this level of detail.) The audio
input buffer provides a place where the audio data can be held until the CPU is ready to process
it. Buffering of audio input is necessary because the CPU may not be able to process the audio
stream as soon as it comes into the computer. The CPU has to handle other processes at the same
time (monitor displays, operating system tasks, etc.). It also has to make a copy of the audio data
on the hard disk because the digitizing and recording process generates too much data to fit in
RAM. Moving back and forth to the hard drive is time consuming. The input buffer provides a
holding place for the data until the CPU is ready for it.
Let's follow the audio signal now to its output. In Figure 5.20, we're assuming that you're
working on the audio in a program like Sonar or Logic. These audio processing programs serve
as the user interface for the recording process. Here you can specify effects to be applied to
tracks, start and stop the recording, and decide when and how to save the recording in permanent
storage. The CPU performs any digital signal processing (DSP) called for by the audio
processing software. For example, you might want reverb added to the singer’s voice. The CPU
applies the reverb and then sends the processed data to the software output buffer. From here
the audio data go back to the audio interface where it is converted back to analog format and sent
to the singer’s headphones or a set of connected loudspeakers. Possibly, the track of the singer’s
voice could be mixed with a previously recorded instrumental track before it is sent to the audio
interface and then to the headphones.
For this recording process to run smoothly -- without delays between when you speak
into the microphone and without breaks in the audio you must choose and configure your
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