Digital Recording Concepts - Appendix A
74 ADAT LX20 Reference Manual
A/D
Analog Signal Digital Signal
These numbers, which represent coded audio, are then converted into an audio
signal that can be recorded on tape. Fortunately, recording a number on tape doesn’t
degrade the signal.
During playback, a digital-to-analog (D/A) converter reads the numbers from tape
and outputs a corresponding level. This creates a “stairstep” reconstruction of the
original signal, which is close to (but not exactly) the same as the original signal.
To complete the process, a low pass filter smooths this stairstep signal. The result
is natural-sounding audio that sounds virtually unchanged from what was
originally recorded.
Digital audio requires lots of numbers to represent an analog version of the same
sound. Analog signals may require at least 20kHz frequency response to reproduce
audio faithfully. Digital signals for the same 20 kHz audio requires a frequency
response of several million Hertz. Due to the way individual numbers are
transmitted, digital audio requires a wide bandwidth recording medium
2
.
THE ADVANTAGES OF 20-BIT RECORDING
Just as the sample rate determines the frequency response of any digital system, the
word length (or quantization) determines its dynamic range (or signal-to-noise).
The four extra bits of ADAT Type II recording offer a potential of over 120 dB
dynamic range, while 16-bit linear systems’ maximum potential is 98 dB.
Another advantage is the lower distortion of 20-bit encoding. With over a million
different values (as opposed to 65,536 values in 16-bit), the A/D converters capture
a much more detailed signal.
So why not go to 24-bit? The increased potential of 24-bit encoding is overshadowed
by the technical limitations of the converters themselves, not to mention dynamic
range limitations of the analog microphones or other equipment being recorded. The
incremental improvement is, unfortunately, mostly lost in the real-world
limitations on electronic design such as thermal noise, interference etc.
If your application is extremely critical, it is possible to record at 24-bit resolution
on your ADAT-LX20 using external adapters (i.e., the Rane PAQRAT or the
Yamaha 02R). This method takes the “extra” bits and records them on higher
tracks, so you won’t get eight tracks without using more recorders and adapters.
2
.Bandwidth is a measure of the lowest to highest frequency a signal path can
handle.
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