This note was written in, if memory serves, 1986. Hence the references to mixing consoles you may never have heard of. The technology of analogue mixing has not changed much in the intervening years, so the data here is still representative.
NOMINAL SIGNAL LEVELS.
Signals vary continuously in their actual amplitude, so it is
necessary to specify a nominal or "normal" signal level.
Explicitly or otherwise, this assumes a continuous sine wave
of a specified RMS voltage. The level may be quoted in Volts RMS but is more commonly
expressed in decibels, eg dBu or dBv. Decibels alone are
merely a ratio, so they must be based on a reference if they
are to refer to an absolute rather than a relative level.
As an example, professional mixing consoles use a standard
nominal level of +4 dBu, equivalent to 1.228 V. The "u" means
that the reference level is 775 mV. This rather odd-looking
value was chosen because it gives 1 mW of power in a 600 Ohm
load. (The 600 Ohm load is a historical hangover from line
transmission systems, as it is the characteristic impedance of
spaced copper wires on a standard-sized telegraph pole; you
don't need to worry about this) This could also be expressed as +1.8 dBv, the "v" meaning a
reference level of 1 Volt.
The nominal level is sometimes the maximum that can be recorded on
a given medium, but this is not true for electronic systems
such as mixing consoles, where the maximum possible level is
often 20 dB or more above the nominal level.
NOISE.
Noise is the hiss in the background. All electronic equipment adds a (hopefully) small amount of noise to the signals being handled. This is unavoidable, and the measure of a quiet piece of equipment is that it adds as little noise as possible. The word "noise" is sometimes used to include any hums, buzzes or other extraneous sounds the system may be subject to; this is not very helpful as it confuses inevitable white noise with interference that is avoidable with correct design.
The noise level is often called the "Noise Floor" as it sets
the lower limit to the dynamic range. This does not mean that
anything lower in level than the noise floor is totally
inaudible; for example a hard-edged buzz would probably
audible (given enough amplification) even if it was 20 dB
below the noise floor.
Noise is generated in every conductor or component that has
resistance. In addition to this, all semiconductors or other
active components add extra noise of their own. From this it
may sound as though it is very hard to make a quiet piece of
equipment; but in practice most of these noise sources make a
negligible contribution, and only a few need close attention.
This is because the various source of noise are all
uncorrelated- ie they are all random, but differently random,
and so when the are added together some cancellation as well
as summation occurs. If two correlated signals of equal
amplitude are added together, the total amplitude is doubled.
(6 dB increase) In contrast, if two different sources of white
noise are added, the level only goes up by 3 dB. This is
mathematically known as RMS summing.
If one of the noise sources is reduced in amplitude, its
contribution to the total drops away very quickly:
Source 1Source 2Summed total
0 dB 0 dB +3.01 dB
0 dB -1 dB +2.54 dB
0 dB -2 dB +2.12 dB
0 dB -3 dB +1.76 dB
0 dB -6 dB +0.97 dB
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Roughly speaking, one dB is the smallest perceptible change in
sound level. Therefore adding any noise source that is less
than half the noise level existing already makes very little
difference.
HEADROOM.
Every electronic channel, be it digital or analogue, has a
maximum signal level it can pass. Attempts to exceed this
typically result in hard clipping, where the waveform is
levelled off flat when it reaches the limits.
The maximum level is much harder to define for analogue tape
machines. Here a higher recording level can be obtained by
accepting some distortion on signal peaks. This is possible
because it is low-order distortion, generating mostly third
harmonics. Hard clipping such as occurs in electronic
circuitry or digital storage, generates lots of high-order
harmonics that are far more obtrusive. Hard clipping is not
normally detectable if it is very brief (say 1 millisecond)
but if sustained sounds very unpleasant.
Both in recording and live music, the level of signals
arriving is not entirely predictable. It is clearly desirable
to have some safety margin between the "usual" operating level
and the clipping point.
DYNAMIC RANGE.
The difference between the maximum level and the noise floor
is known as the dynamic range. In electronic circuitry this is
usually 100 dB or more, the exception being mic amplifiers
working at high gain.
. | | Noise floor | Headroom | Dynamic range
Analogue tape. | 0 dB
Digital-83 dB+10 dB93 dB
Mixing console-84 dBu+26 dBu110 dB
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THE ASSESSMENT OF NOISE IN MIXING CONSOLES.
The noise performance of a complex system such as a mixing
console is not simply described. An almost infinite number of
variations in console set-up, and measurement technique and
interpretation exist. The final noise level at an output
depends on the operational configuration in use as much as on
the technological decisions made at the design stage, and so
this document has been produced to clarify some of the
measurement philosophies and techniques used in the design,
testing, and specification of consoles.
Most of the actual data quoted are taken from the
Soundcraft Series 6000, as one of the quietest consoles ever
built, but the general principles are applicable to all mixing
consoles. It is assumed that the reader is familiar with audio
basics such the difference between dBu and dBv.
In recording work the most critical situation is mixdown,
as the maximum number of potential noise sources are being
added together. The situation is simplified because all inputs
are line inputs, and these will be set at or near unity gain,
assuming the levels of console and tape machine are properly
aligned. Where inputs are being mixed in directly from
sequenced synthesisers, these are likely to be noisier than
any line input amplifier, and the discreet use of noise gating
is not unusual.
Consoles for PA work are normally used in a way that
approximates to mixdown in a recording console, as a large
number of inputs are mixed down to stereo. However in this
case many of the inputs will be used for microphones, probably
with widely varying input gains, and the situation becomes
more complex. In general it seems impossible to specify one
particular configuration that would provide a realistic
assessment of noise performance in all live situations.
NOISE AND THE LAYING OF TRACKS.
Soundcraft noise specifications are set using true-RMS
readings. It is not our practice to quote noise measurements
made with weighting filters (such as A-weighting) because it
is our feeling that this would tend to mislead people. Once
again, it would also limit the choice of test gear that can be
used to check that a console is performing to specification.
The last two points apply to all our noise measurements,
and not just those involving input preamps.
We shall now examine the noise structure of a Series 6000
console in recording mode. Typically the mic input gain might
be set to +50dB, and at this setting the preamp EIN is still
as good as -127 dBu. From this we can quickly find that that
the preamp noise output is -77 dBu, and while this may seem
high at first sight, even an absolutely noiseless circuit
could only improve this by 2 dB. The EQ section that follows
the preamp generates less than -95 dBu, and therefore its
contribution is completely negligible.
Similarly the noise contribution from the fader post-amp
is approximately -104 dBu, and this makes no measurable
difference.
Assuming that one channel is routed to a group in order to
send it to the multitrack, the extra noise from the group
summing amplifier will be about -101.5 dBu, which is once more
a negligible increase.
It can therefore be clearly seen that in recording mode,
the performance of the Series 6000 is determined to a very
large extent by noise from outside the console, in the shape
of the inevitable microphone Johnson noise.
NOISE AT MIXDOWN AND IN PA WORK.
At mixdown there are typically many inputs, all feeding
the stereo mix bus in varying amounts. The most obvious of
these are the input channels, as considered above, but it is
important not to forget the presence of the monitor sections,
which may well have a different noise characteristics. The
same applies to any effect returns routed to the stereo bus.
Each input will have varying frequency characteristics when
the EQ is in use, and if input are sub-grouped this adds an
extra layer of complication.
The number of variables is large, and the interactions
between them complex, and so we have attempted to standardise
a measurement configuration that would, as closely as
possible, represent an 'average' mixdown insofar as such an
animal exists. The basis of this configuration is the
recognition that if 16 or 24 input channels are all mixed into
the stereo bus with their faders at 0dB, the result is likely
to be an inharmonious crunching as the mix summing amplifier
clips. For most of our tests we have chosen to use 16 inputs
as this is possible on more types of console, though the
principle is of course the same for 24 or more.
Assuming that the inputs are uncorrelated, their voltages
will sum in an RMS fashion- in other words the result is not
the arithmetic sum but the square-root of the sum of the
squares. Therefore 16 inputs will combine to give a signal
roughly 4 times greater (the square root of 16) and somewhere
in the system attenuation must be introduced to reduce the
signal to the nominal level; in other words, the inputs are
summing to unity. We felt that it was more likely that this
should be introduced by pulling down the channel faders to -12
dB, (rather than pulling down the master fader) as this
protects the mix summing amps from overload.
In fact, mix summing overload is very unlikely on the
Series 6000, as the discrete/integrated summing-amp technology
used allows the amplifiers to be run at a low gain without
sacrificing noise performance. This gain structure also
conveniently allows the mix inserts to run at a nominal -10
dBv. (equal to -7.8 dBu, sometimes called Tascam level)
Other decisions had to be taken to define the test
configuration. Line gain is set to unity, ie +4 dBu in for
nominal internal level, and the line inputs are terminated in
a short circuit as the output impedance of professional
equipment feeding it is unlikely to be higher than 100 Ohms.
This is negligible compared with the line amps internal
impedances. (It is not inappropriate here to underline the
fact that the output impedance of an amplifier does not
determine its current-drive capability- it is possible to have
a stage with a 10 Ohm source impedance that cannot fully drive
a load below 10 kOhm )
Since the relative amounts of cut and boost applied are
impossible to predict, it was decided that EQ should be
switched in but set flat. The control centre-detents help here
as the configuration must be relatively quick to set up, and
repeatable.
Our normal procedure is to begin measurements at the mix
outputs, with the mix fader down. This parameter is important
because it determines the ultimate quietness reached when
doing an overall fade-out. Then the audio path is successively
extended to the 16 inputs, first by fully advancing the mix
fader to 0 dB, then routing the channels to mix with channel
faders down, and then advancing these faders to the previously
described setting of -12 dB. Finally the EQ section is
switched in. This stepwise process gives a very clear picture
of how noise builds up through a console. Note that the
monitor sections are switched on, and panned hard left so that
their effect can be determined by recording the results for
both left and right mix outputs. Likewise the channels are
panned hard left to discriminate between mixing noise and
noise from the fader post-amp. The figures below are actual
measurements from a Series 6000 prototype.
. | LEFT RIGHT
Mix fader down. -102 dBu -102 dBu
Mix fader 0 dB. -85.7 dBu -87.5 dBu
16 channels routed. -83.5 dBu -84.5 dBu
Chan faders to -12 dB. -81.5 dBu -84.5 dBu
EQ section in. -81.0 dBu -84.5 dBu
All channels and monitor sections are panned fully Left.
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DIGITAL EQUIPMENT AND NOISE.
When digital recording systems were first introduced they
were nominally 16-bit. This gives a theoretical noise floor 96
dB below maximum level. In practice 90 dB was more likely, and
if maximum level corresponds to +4 dBu, this indicates a noise
level at each multitrack output of -86 dBu.
It is necessary now to consider the amount of headroom
left when the nominal operating levels are set up. Unlike
analogue tape, digital clips suddenly and with painful
clarity, and so it is essential to leave enough headroom to
prevent this. If 10 dB of headroom is left on all tracks that
are not completely predictable in level, then this implies
that the noise level actually present at the console line
inputs is -76 dBu. After this level has been passed through
channel faders set to -12 dB as above, and then summed to
unity, it remains at -76 dBu, well above any of the noise
readings quoted in the above table. Thus the console will be
at least 5 dB quieter than the digital equipment it is
connected to, even in the worst case. It is of course more
likely that the panpots are on average central, which yields a
larger margin of about 8 dB.
Many digital systems now have 20-bit or more resolution;
though in general only 20 bits is achieved in practice. This
reduces the theoretical noise floor by 4 bits, or 24 dB.
Therefore the noise level at the recorder output should be -
110 dB. In practice the analogue input/output circuitry of the
recorder is unlikely to be able to achieve this, and
assessment of the relative noise performances of recorder and
console must be done on a case-by-case basis.
Douglas Self