Yamaha PM4000, Руководство по эксплуатации

Страница: 81 / 132

5.2 Dynamic Range and Headroom
5.2.1 What Is Dynamic Range?
Every sound system has an inherent noise floor,
which is the residual electronic noise in the system
equipment (and/or the acoustic noise in the local envi-
ronment). The dynamic range of a system is equal to the
difference between the peak output level of the system
and the noise floor.
5.2.2 The Relationship Between Sound
Levels and Signal Levels
A concert with sound levels ranging from 30 dB SPL
(near silence) to 120 dB SPL (threshold of pain) has a
90 dB dynamic range. The electrical signal level in the
sound system (given in dBu) is proportional to the
original sound pressure level (in dB SPL) at the micro-
phone. Thus, when the program sound levels reach
120 dB SPL, the maximum line levels (at the console’s
output) may reach +24 dBu (12.3 volts), and maximum
power output levels from a given amplifier may peak at
250 watts. Similarly, when the sound level falls to 30 dB
SPL, the minimum line level falls to -66 dBu
(0.388 millivolts) and power amplifier output level falls
to 250 nanowatts (250 billionths of a watt).
The program, now converted to electrical rather than
acoustic signals, still has a dynamic range of 90 dB:
+24 dBu - (-66 dBu) = 90 dB. This dB SPL to dBu or
dBm correspondence is maintained throughout the
sound system, from the original source at the micro-
phone, through the electrical portion of the sound
system, to the speaker system output. A similar rela-
tionship exists for any type of sound reinforcement,
recording studio, or broadcast system.
Note: Refer to Figure 5-1 (next page) while reading the
following disucssions of headroom and dynamic range.
5.2.3 A Discussion Of Headroom
The average line level in the typical commercial
sound system just described is +4 dBu (1.23 volts),
corresponding to an average sound level of 100 dB SPL.
This average level is usually called the “nominal”
program level. The difference between the nominal and
the highest (peak) levels in a program is the headroom.
In the above example, the headroom is 20 dB. Why is
this so? Subtract the nominal from the maximum and
see: 120 dB SPL - 100 dB SPL = 20 dB. The headroom is
always expressed in just plain “dB” since it merely
describes a ratio, not an absolute level; “20 dB” is the
headroom, not “20 dB SPL”. Similarly, the console
output’s electrical headroom is 20 dB, as calculated
here: +24 dBu - (+4 dBu) = 20 dB. Again, “20 dB” is the
headroom, not “20 dBu”. Provided the 250-watt rated
power amplifier is operated just below its clipping level
at maximum peaks of 250 watts, and at nominal levels
of 2.5 watts, then it also operates with 20 dB of head-
room (20 dB above nominal = 100 times the power).
5.2.4 What Happens When The Program
Source Has Wider Dynamics Than
The Sound Equipment?
If another mixing console were equipped with a
noisier input circuit and a less capable output amplifier
than the previous example, it might have an electronic
noise floor of -56 dBu (1.23 millivolts), and a peak
output level of +18 dBu (6.16 volts). The dynamic range
of this system would only be 74 dB. Assuming the
original program still has an acoustic dynamic range of
90 dB, it is apparent that 16 dB of the program will be
“lost” in the sound system. How is it lost? There may be
extreme clipping of program peaks, where the output
does not rise higher in response to higher input levels.
Quiet passages, corresponding to the lowest signal
levels, may be buried in the noise. Typically, portions of
that 16 dB difference in dynamic range between the
sound system capability and the sound field at the
microphone will be lost in both ways. A system with
+24 dBu output capability and a -66 dBu or better noise
floor, or +18 dBu output capability and -82 dBu noise
floor, would be able to handle the full 90 dB dynamic
range. Thus, for high quality sound reinforcement or
music reproduction, it is necessary that the sound
system be capable of low noise levels and high output
capability.
In the special case of an analog audio tape recorder,
where the dynamic range often is limited by the noise
floor and distortion levels of the tape oxide rather than
the electronics, there is a common method used to avoid
program losses due to clipping and noise. Many profes-
sional and consumer tape machines are equipped with a
noise reduction system, also known as a compander (as
designed by firms like Dolby Laboratories, Inc. and dbx,
Inc.). A compander noise reduction system allows the
original program dynamics to be maintained through-
out the recording and playback process by compressing
the program dynamic range before it goes onto the tape,
and complementarily expanding the dynamic range as
the program is retrieved from the tape. Compact (laser)
discs, and digital audio tape recording, and the FM or
vertical recording used in modern stereo VCR
soundtracks are all additional methods of recording
wide dynamic range programs which, in turn, demand
playback systems with wide dynamic range.
Page 5-2