Signal Levels,Dynamic Range And Headroom - Yamaha P2075 Operating Manual

Noise and losses in low impedance and
high impedance lines
The length and type of cable can affect system frequency
response and susceptibility to noise. The impedance of the
line has a major influence here, too.
Signal cables from high impedance sources (actual output
impedance of 5000 ohms and up), should not be any longer
than 25 feet, even if low capacitance cable is used. The
higher the source impedance, the shorter the maximum
recommended cable length.
For low impedance sources (output impedances of 600 ohms
or less), cable lengths of 100 feet or more are acceptable. For
very low impedance sources of 50-ohms or less, cable lengths
of up to 1000 feet are possible with minimal loss.
In ай! cases, the frequency response of the source, the desired
frequency response of the system, and the amount of capaci-
tance and resistance in the cable together affect actual high
frequency losses. Thus, these suggested cable lengths should
not be considered "absolute" rules.
Susceptibility to noise is another factor which affects cable
length. All other factors being equal (which they seldom are),
if a given noise voltage is induced in both a high impedance
and a low impedance cable, the noise will have a greater
. impact on the high impedance circuit. Consider that the
noise energy getting into the cable is more-or-less constant in
both instances. The low impedance input is being driven
primarily by power, whereas the high impedance input is
being driven primarily by voltage. The induced noise energy
must do MORE WORK when it drives a lower impedance —
it does not have much power — so less noise is amplified by
the input circuit. The induced noise energy is not loaded by a
high impedance input, and so it is amplified.
Signal levels,dynamic range and headroom
STANDARD OPERATING LEVELS
There are a number of different "standard" operating levels
in audio circuitry. It is often awkward to refer to a specific
level (i.e., +4 dBu) when one merely wishes to describe a
general sensitivity range. For this reason, most audio engi-
neers think of operating levels in three general categories:
1. MIC LEVEL OR LOW LEVEL р
This range extends from no signal up to about —20 dBu
(77.5 mV), or —20 dBm (77.5 mV across 600 ohms = 10
millionths of a watt). It includes the outputs of micro-
phones, guitar pickups, phone cartridges, and tape heads,
prior to any form of amplification (i.e., before any mic,
phono, or tape preamps). While some mics can put out
more level in the presence of very loud sounds, and a
hard-picked guitar can go 20 dB above this level (to 0 dBu
or higher), this remains the normal, average range.
2. LINE LEVEL OR MEDIUM LEVEL
This range extends from —20 dBu or —20 dBm to +30
dBu (24.5 V) or +30 dBm (24.5V across 600 ohms = 1
watt). It includes preamp and console outputs, and most
of the inputs and outputs of typical signal processing
equipment such as limiters, compressors, time delays, `
reverbs, tape decks, and equalizers. In other words, it
covers the output levels of nearly all equipment except
power amplifiers.
3. SPEAKER LEVEL AND HIGH LEVEL
This covers all levels at or above *30 dBu (24.5V) *30
dBm (24.5 V across 600 ohms = 1 watt). These levels
include power amplifier speaker outputs, AC power lines,
and DC control cables carrying more than 24 volts.
Let's discuss these levels in the context of a sound system.
The lowest power levels in a typical sound system are present
at the output of microphones or phono cartridges. Normal
speech at about one meter from the "average" dynamic
microphone produces a power output from the microphone
of about one trillionth of a watt, Phono cartridges playing an
average program selection produce as much as a thousand
times this output — averaging a few billionths of a watt.
These signals are very weak, and engineers know that they
cannot be "run around" a chassis or down a long cable
without extreme susceptibility to noise and frequency
response errors. This is why microphone and phono preamps
are used to boost these very low signal levels to an interme-
diate range called "line level". Line levels are between 10
millionths of a watt and 250 thousandths of a watt (1/4
watt). These levels are related to the "dBm" unit of measure-
ment as follows:
—20 dBm = 10 microwatts (0.00001 watts)
OdBm= 1 milliwatt (0.001 watts)
+4 аВт = 2.5 milliwatts (0.0025 watts)
+24 dBm = 250 milliwatts (0.025 watts)
While some console and preamp outputs can drive lower
impedances, primarily for driving headphones, typical line
levels (measured in milliwatts) cannot drive speakers to
useable levels. Not only is the power insufficient for more
than "whisper" levels, the console circuits are designed to
operate into loads of 600 ohms to 50,000 ohms; they cannot
deliver even their few milliwatts of rated power to a typical
8-ohm speaker without being overloaded. A power amplifier
must be used to boost the power output of the console so it
is capable of driving low impedance speaker loads and
delivering the required tens or hundreds of watts of power.