102
Dynamic Range
Dynamic range represents the difference between the maximum signal that can be
recorded and the “noise floor”, or level of noise with no signal present. A system
with a high dynamic range will be quieter than one with a lower dynamic range.
Dynamic range is a very important specification, and your echo card uses
converters that have very high dynamic range.
Theoretically, a 24-bit system has a dynamic range of 144dB and a 16-bit system
has a dynamic range of 96dB. Two questions immediately come to mind:
1) Why does my Echo card only have a dynamic range of 106 dB?
2) For mastering 16-bit CDs with a dynamic range of 96dB, isn’t anything
more than 96dB just overkill?
First, today’s analog-to-digital converters typically produce a full-scale input
voltage with an input of +7dBu. If they were to have 144dB of dynamic range,
they would have to be capable of resolving signals as small as –137 dBu (7dBu –
144dBu) or approximately 10 nano-volts. That’s 10
one-billionths
of a volt!
Transistors and resistors produce noise in this range just by having electrons
moving around due to heat. Even if the converters could be perfectly designed to
read these levels, the low noise requirements of the surrounding circuitry such as
power supplies and amplifiers would be so stringent that they would either be
impossible or too expensive to build.
In answering the second question, consider the fact that music is often compressed
or amplified after it is recorded, and that some headroom is necessary when
recording to avoid clipping. The only way that 96dB would be adequate is if all
music were recorded so that the peaks were just under full-scale and no
compressing or amplification was going to be applied after recording. Any time
recorded music is amplified, so is the noise at the low end. Your Echo card has
enough dynamic range to allow sufficient headroom and post-processing to be
applied while still keeping the noise either off the CD completely or down as far as
possible.
