Noise, Hum and other … 1
LAB Audio Technology®
Noise, disturbance and other …1
In signal transmission, the result obtained is determined by the presence of two other elements: the noise and disturbances.
The noise is due to the physical nature of the devices involved in the generation and transport of the signal, and the only way to minimize it is to apply the so-called “filtering” techniques. These techniques must be used carefully in order to properly improve the Signal / Noise (S / R) ratio.
The disturbances can be eliminated with selective filtering techniques, with the shielding of the circuits, with the use of quality cables and connectors and with a careful design of signal paths and connections.
The disturbances in audio
The disturbances in the audio is always possible precisely because of the large amount of connections present in the studio and on stage.
Disturbances can be of various nature and therefore of different origin:
Conducted Noise: when we have two or more circuits with common conductors (e.g. power supplies and ground connections) mutual interference is possible, because current in one circuit can generate voltage in another. This type of disturbance is often found in “series” power supplies between digital and analog circuits. The problems increase with increasing frequencies used in the circuits.
The connection tracks and cables are part of the circuit and assume increasing inductance values according to their size and working frequency.
A typical connection that can cause conducted disturbances (as described above) are cheap multiple sockets that series power supplies to electronic products. The best solution is a “star” connection which is able to reduce the reciprocal influences of the circuits.
When we talk about signals and signal transmissions it is always necessary to have a reference “to ground”. An incorrect ground connection (or reference to ground) can create unwanted inductive couplings. Inductive disturbances are caused by inductive coupling. When a current flows in a circuit, a magnetic flux is produced which is chained in a mutual inductance on another circuit, so as to induce a disturbance in voltage.
A typical example of inductive disturbance are the magnetic fields generated by conductors crossed by alternating current. Their “mutual induction” is a function of the current intensity that passes through them, their mutual distance and their distance from the ground plane, which is the reference and carries the return current.
In the case of insulated conductors, the simplest solution to minimize inductive disturbances is to twist the conductors to protect the circuit. A typical example would be twisted pair or USB cable conductors. See fig.1 below
Disturbances due to electromagnetic coupling are very common.
In order to better clarify, it is necessary to explain that each conductor has its own parasitic capacity, that is, the capacity that distances the behavior of the components from their ideal values.
We often consider cables as simple conductors, but in their practical application they can be schematized as series resistors and parallel capacities. The presence of capacitive components between the conductors creates a circuit that is similar to RC filters.
In the case of a capacitive coupling, which occurs every time two cables are placed side by side, there is a coupling of the electric field whose variation induces a current.
The amount of the noise decreases as the capacitive coupling decreases, and therefore with the distance between the cables or with the presence of a capacitance between the ground and the cable, but this last solution can create bandwidth problems in high impedance circuits.
As an empirical reference, the order of magnitude of the capacities of a cable, both coaxial and braided, is about 100 pF, and it is necessary to take this value into account in order to make correct measurements or understand any limitations of the bandwidths.
A typical example of minimizing capacitive coupling is the calibration of the oscilloscope probes to allow a correct reading on the instrument. Parasitic capacitances in a circuit, in addition to inducing disturbances, could create instability in the circuit itself.
If the disturbances deriving from capacitive and inductive couplings are due to proximity (i.e. for distances shorter than the wavelength of the electrical signals in question), in the case of coupling by radiation and therefore disturbance by radiation, the source of disturbance is a signal of the order of Mhz or higher, which brings with it a power transfer. In this case the conductors behave like antennas, capturing the signals and transforming them into disturbances