1442 Chapter 38
38.2.3 Integration of Digital Products into Analog
Systems
38.2.3.1 Dynamic Range
The dynamic range of analog systems is characterized by
a noise floor, nominal operating level, and maximum out-
put level before a rated distortion is exceeded. The noise
floor is usually constant and does not change with the
audio signal. Distortion generally increases with increas-
ing level. The increase in distortion as the maximum out-
put level is approached may be gradual or sudden. Most
professional analog audio equipment today has a maxi-
mum output level in the range from +18 to +28 dB rela-
tive to 0.775 V (dBu), and a nominal operating level in
the 0 to +4 dBu range. While optimum operation requires
matching of the maximum output levels so that all
devices in the signal chain reach maximum output level
at the same time, in practice many engineers do not
bother to match maximum output levels, relying instead
on the easier matching of nominal levels. The best signal
levels to run through typical analog equipment are mod-
erate levels, far away from the noise floor, but not too
close to the maximum output level.
Digital equipment, on the other hand, has a different
set of characteristics. Distortion decreases with
increasing level, and reaches the minimum distortion
point just before the maximum output level. At that
maximum output level distortion rises very abruptly.
The noise floor of digital equipment is often not
constant. In some cases the noise is very signal depen-
dent, and sounds to the ear much more like distortion
than noise. These characteristics come together to
suggest that the optimal signal levels would be those
close to but just a little below the maximum level.
38.2.3.2 Level Matching
As we combine analog and digital equipment in the same
system, the different characteristics of the two technolo-
gies suggest that for maximum performance and widest
dynamic range we must align the maximum output
levels.
Each device has its own dynamic range, but those
ranges will have different characteristics. In all cases we
want the audio signal to stay as far as possible from the
noise floor.
In any system some device will have the smallest
dynamic range, and thus set the ultimate limitation on
the performance of the system.
The system as a whole may not perform as well as
the worst performing component, unless care has been
taken to assure that all devices reach their own
maximum output level at the same time.38.2.3.3 Level Matching ProcedureTo match the maximum output levels, apply a midfre-
quency tone to the input of the first device in the system.
Increase the applied level and/or the gain of the device
until the maximum output level is reached as determined
by the increase in distortion. This point may be deter-
mined by using a distortion meter, watching the wave-
form using an oscilloscope for the onset of clipping, or
listening with a piezo tweeter connected to the output of
the device.
This latter technique was developed by Pat Brown of
Syn-Aud-Con. If a frequency in the range of 400 Hz is
selected, the piezo tweeter can’t reproduce it, and will
remain silent. When the device under test exceeds its
maximum output level, the resulting distortion will
produce harmonics of the 400 Hz tone that fall in the
range the piezo tweeter can reproduce, and it will sound
off in a very noticeable way. The level is then reduced
until the tweeter just falls silent, and maximum level has
been determined. Rane has produced a commercial
tester based on this concept called the Level Buddy.
Once the first device is at its maximum output level,
the gain of the second device is adjusted to achieve its
maximum output level. In some cases the input of the
second device will be overloaded by the maximum
output level of the first device and no adjustment of the
gain of the second device will eliminate the distortion.
In such a case, an attenuator must be used between the
devices to drop off the level so the second device’s input
is not overloaded. One place where such an attenuator is
often needed is at the input of a power amplifier. Many
times professional power amplifiers have input overload
points far lower than the maximum output levels of any
of the common devices used to drive them.
Once the second device is at maximum output level,
the process is repeated in turn for each subsequent
device in the system.
Once all device interfaces have been optimized, the
system is capable of maximum possible performance.38.2.3.4 Minimization of ConversionsUp until now, digital components have been treated just
like the analog components they have replaced in the sys-