Audio Interfacing Review Sheet PDF

MUS 21800 – Audio Interfacing Review Sheet Sound Transmission Formats Analog audio uses continuous electrical flow to represent audio signals. Digital audio uses discrete/discontinuous values (1s & Os) to represent audio signals. An example of an Analog to Digital convertor are...

MUS 21800 – Audio Interfacing Review Sheet Sound Transmission Formats Analog audio uses continuous electrical flow to represent audio signals. Digital audio uses discrete/discontinuous values (1s & Os) to represent audio signals. An example of an Analog to Digital convertor are the inputs on an audio interface. An example of a Digital to Analog convertor are the outputs from an audio interface A microphone, speaker, and headphones are transducers because they change one form of energy into another. A microphone changes mechanical energy into electrical voltage. Speakers and headphones change electrical voltage into mechanical energy. Magnetic tape recorders convert electrical audio signals into analogous patterns of magnetic fluctuation. Analog Audio Level Standards The lowest level is microphone level. It’s measured in single millivolts. The next highest level is instrument level. It’s measured in tens of millivolts. The next highest level is line level. It’s measured in single volts The highest level is speaker level. It’s measured in tens of volts. Analog Audio Level Transformations Preamplification is required when changing from microphone or instrument level to line level (level goes up). Amplification is required when changing from line level to speaker level (level goes up). An active speaker accepts line level and then changes it to speaker level with the on-board amplifier in the speaker. A passive speaker accepts speaker level from a power amplifier. It does not have an on-board amplifier. Basic Electricity Valence Electrons (the outer most electron shell) are unstable and can move from the valence shell of one atom to another atom’s valence shell. Valence electrons that have broken free from the atom are called Free Electrons. Electrical flow involves the movement of valence electrons from one atom to another. When free electrons are connected to neutral or positively charged atoms, current will flow. Atoms with few valence electrons are good conductors (copper, aluminum) Atmos with a lot of valence electrons are good insulators (rubber, plastic, glass). 1  Current = Rate of flow of free electrons through a conductor. Measured in amperes/amps. Voltage = Electromotive force that “pushes” free electrons. Measured in volts. Resistance = Impedes to the flow of current. Measured in Ohms Ω. Electromagnetic Induction and AC Electromagnetic induction requires a moving magnetic field and a fixed conductor or a moving conductor and a fixed magnetic field. The three components used in a moving coil microphone to create output are a magnet, coil of wire, and a diaphragm. The three components used in a ribbon microphone to create output are a magnet, corrugated aluminum ribbon, and a step-up transformer. The three components used in a passive guitar pickup to create output are a magnet, coil of wire, and a steel string. Direct Current (DC) only moves in one direction as opposed to AC which alternates between positive and negative. United States AC Standards The AC frequency in the United State is 60 Hertz or cycles per second. The standard household voltage in the United States is 120 volts. The standard household amperage used in circuits in the United States is 10, 15, or 20 amps. Impedance The amount of resistance in an AC circuit is the same for all frequencies. It’s NOT frequency dependent. Impedance (Z) in AC circuits is a frequency dependent type of resistance. Resistance changes with frequency. Impedance is measured in Ohms Ω In Bridging Impedance systems, the input/load impedance must be at least 10x the output/source impedance In Matching Impedance systems, the input/load impedance and the output/source impedance should be the same. Some microphone preamps, and devices like the Cloudlifter Z, can provide different impedance loads to moving coil and ribbon microphones as well as instrument level inputs. Hight Z and Low Z High Z interconnections are susceptible to electromagnetic noise, handling noise, and stored energy on the cable which creates a low pass filter reducing high frequencies. To reduce these issues, instrument cables should be no longer than 15 ft. and have low capacitance. The only High Z system used in a professional audio studio is for instrument level found in electric guitar and bases. 2  All microphone level and line level equipment used in a professional audio studio are always Low Z. Unbalanced Connections The unbalanced system rejects noise with the braided shield that is connected to ground. Know the typical connectors for the unbalanced system (slide 59). The Unbalanced System has two connection points: the inner conductor and shield/ground. Unbalanced cabling needs to be kept as short as possible whether it’s Hi or Low Z. Conductors that carry AC current (power cables) create fluxing magnetic fields that are at right angles to the conductor. The magnitude and rate of the magnetic field is analogous to the AC current. Balanced Connections The Balanced System uses twisted pair cable The balanced system has three connection points: Hot/+, Cold/-, and Shield/Ground. The same signal is on both the conductors of the twisted pair The Cold/- connection is polarity reversed from the Hot/+ connection While the conductors are polarity reversed, the noise accumulating on these conductors is not. They are the same polarity. In balanced connections any signals that are different are accepted at the input. The Cold/- signal is then polarity reversed so that it’s the same polarity as the Hot/+ signal. This means their volumes are summed (constructive interference slide 63). Signals that are common (the noise collecting on the twisted pair) are rejected at the input. Know the typical connectors for the balanced system (slide 65) Star Quad cable uses four conductors and the shield/ground. Two conductors for the Hot/+ and two for the Cold/-. Star Quad cable provides superior rejection of EMI compared to twisted pair. Audio Interfacing Examples Electric Guitars and Basses are Unbalanced, Hi-Z, and instrument level. Professional microphones are Balanced, Low-Z, microphone level. Microphone preamps accept Balanced, Low-Z, Microphone level and many preamps accept Unbalanced, Hi-Z, Instrument Level at their inputs. Microphone preamps produce Balanced, Low-Z, Line Level at their outputs. The Lynx Aurora accepts Balanced, Low-Z, Line Level at its analog inputs and produces Balanced, Low-Z, Line level at its analog outputs. DIs accept Unbalanced, Hi-Z, Instrument Level at their inputs and produce Balanced, Low-Z, Microphone Level at their outputs. 3  Passive DIs employ a step-down transformer which has more turns at the primary winding than at the secondary. Passive DIs use a 10:1 winding ratio. Transformers are Bi-Directional. They can be used as a step-up transformer when the secondary winding (output) is used as the input. Active DIs require power and employ a tube or FET (field effect transistor). They are unidirectional, unlike passive DIs. A Reamp Box accepts Balanced, Low-Z, Line Level at its inputs and produces Unbalanced, Hi-Z, Instrument Level at its output. Two DIs can be used for a remote amp setup. The “sending” DI can be passive or active. It steps down the level and Z and balances the guitar signal. The “receiving” DI must be passive. The “sending” signal is connected to its output (secondary winding) so that the signal can be stepped up in level and Z and the signal can be made unbalanced. 4