Alesis ADAT-XT20 User Manual download pdf (Page 114)

Digital Recording Concepts: Appendix A

ADAT-XT20 Reference Manual 113

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, this stairstep signal is smoothed by a

low pass filter. 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

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-XT20 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.

WHY S-VHS?

S-VHS recording technology offers more than enough bandwidth to record eight

tracks of digital audio. S-VHS tapes are built to higher standards than

standard VHS tapes, and can take the tape shuttling required by professional

audio applications. S-VHS tape cassettes are also inexpensive compared to

reel-to-reel tape, readily available, compact, and easy to transport and store.

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