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Storing Digital Data
Once the waveform has been transformed into digital bits, it must be stored.
When sampling in stereo at 48kHz using a 20-bit word size, the system has
to accommodate 1,920,000 bits per second. Though this is a lot of data, it is
well within the capabilities of personal computers.
Most computer-based digital recording systems record the data directly to
the computer’s hard disk. Today’s hard disks are capable of storing large
amounts of data, though the performance of hard drives can vary
substantially. The Echo Reporter program (included on the Mona CD-
ROM) will give you a general idea what the capabilities of your hard disk
are and how much information it can successfully store and retrieve. The
speed and size of your hard drive will be a major determining factor in how
many tracks of audio you will be able to simultaneously record and
playback.
Decibels
Audio signal levels are generally expressed in units called “decibels” which
are abbreviated as “dB”. This is a “logarithmic” scale where each doubling
of signal level is represented by an increase of 6 dB. Therefore a signal of 6
dB is twice as big as a 0 dB signal and a signal of 12 dB is four times as big
as a 0 dB signal.
Since digital audio signals are represented by binary data, each bit of audio
information represents 6 dB. A 16-bit number can represent a total range of
96 dB and a 24-bit number can represent a total range of 144 dB (6 times
the number of bits). It’s much easier to say that one signal is 72 dB less
than another instead of saying it is 1/4096 the size of the other one. It also
more accurately represents the way we hear sounds, since the smaller signal
in the above example will still be audible and not appear to be only 1/4096
as loud when we listen to it.
Just as there are different types of degrees used to represent temperature
(Fahrenheit, Celsius, etc), there are different types of decibels used to
represent the level of analog audio signals. The most common are dBu and
dBV decibels. Both of these represent voltage levels and still double for
every increase of 6 dB. It is only the reference point, or 0 dB level that is