Owner`s manual

®
4
The Meaning of Signal-to-Noise Ratio
Signal-to-noise ratio is a common term used on spec sheets. As common as it is, many people (both
learned and not) misunderstand or misuse it. Signal-to-noise ratio is the relationship between a
device’s normal operating level, the noise floor, and peak clipping.
The noise floor, or the Ultimate noise
All electronic equipment operating above absolute zero generates noise. Fortunately, modern
electronic design techniques can identify each noise source, all the way from the tape head in a
cassette machine, to each stage in the amplifiers following it on the way to the speakers. Careful
design minimizes each potential source, resulting in optimum performance. Of course, cost is a
limiting factor in the overall design process at least if most of us are to be able to purchase the
device. The bottom line is the thermal noise generated by the electrical resistance of the source. In
the case of a cassette machine, the source is the tape head itself. Even if the amplifiers following it
were noiseless, they would still amplify the thermal noise of the head itself. This then is the Ulti-
mate noise.
The normal, residual noise output of any electronic device is known as its noise floor. Again, it is
the product of the thermal noise of the source, plus the noise contributions of each succeeding
amplifier stage.
Peak clipping, or the peak ceiling
At the other end of the scale is peak clipping. Peak clipping occurs when an amplifier’s output
signal can no longer rise as directed by the input signal. Typically, it occurs when the peak value of
the output signal attempts to exceed the value of the power supply voltages supplied to the ampli-
fier stage.
Figure 2 shows this graphically. The amplifier supply voltages are + and - 12 volts. The signal can
vary around ground (zero volts), up to 12 volts positive (above ground) or 12 volts negative (below
ground). If the signal were to try to exceed this limit, clipping occurs once the instantaneous signal
level reaches 12 volts. At this time, the output can go no further, and the signal peak is truncated or
flattened until the signal level falls below 12 volts. The signal level at which clipping occurs is
known as the peak ceiling.
Figure 2. Clipping.
Operating level, headroom, and signal-to-noise ratio
Somewhere between the noise floor and peak clipping is the normal operating level of a device.
Looking at this relationship graphically (Figure 3), the distance (difference) between operating
level and the noise floor is known as the working signal-to-noise ratio. Finally, the distance be-
tween peak clipping and the noise floor is known as the dynamic range.