Echo Audio MiaMIDI Owner's Manual download pdf (Page 99)

will result in reduced high frequency reproduction. Your Echo card allows you to

sample sound at up to 96,000 times per second.

Once the waveform has been transformed into digital bits, it must be stored. When

sampling in stereo at 96kHz using a 24-bit word size, the system has to

accommodate 4,608,000 bits per second. In the past, storing this vast amount of

data was problematic. Today, 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 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 6dB. Therefore a signal of 6dB is twice as

big as a 0dB signal and a signal of 12dB is four times as big as a 0dB signal.

Since digital audio signals are represented by binary data, each bit of audio

information represents 6dB. A 16-bit number can represent a total range of 96dB

and a 24-bit number can represent a total range of 144dB (6 times the number of

bits). It’s much easier to say that one signal is 72dB 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 6dB. It is

only the reference point, or 0dB level that is different. A 0dBV signal has a

voltage level of 1.0 volts. A 0dBu signal has a voltage level of .775 volts. Since

.775 is approximately 2dB less than 1.0, converting dBV levels into dBu levels is

as simple as subtracting 2dB (2.21 to be exact).

Signals are also occasionally represented with units of dBm. This is an older unit

that measures power instead of voltage levels with 0dBm representing 1 milliwatt.

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Echo Audio MiaMIDI Owner's Manual Page 99