Signals and Transmission Flashcards
Balanced line
signal in, differential amp inverts signal so there is one positive and one negative; has 2 conductors going down the line, plus a shield to ground; another differential amp at the other end inverts the signal again so noise is cancelled but signal is added together and gives +6dB of gain
Unbalanced line
Has two paths, one inner insulated conductor and shield; shield must act as both return path for signal and as shield tied to the ground, making it easier for noise to get into the audio signal
Common mode rejection
In a balanced line, there are two inner conductors, which carry audio, and a shield, which does not carry audio; any noise current will be equal and in phase in both conductors (common to both of them), and the signal will be out of phase (different); a differential amplifier is used at the receiving end of the cable so that it rejects the common mode of the signal
Impedance
Measured in ohms, it is a device’s opposition to the flow of AC current. Reactive devices such as loudspeakers, capacitors and inductors exhibit impedances which vary with frequency.
High impedance
Has a high resistance to current; requires higher voltage, but has low current
Low impedance
Has low resistance to current; requires lower voltage, and has higher current (used in professional audio)
Line level
the average audio voltage level of a signal at a particular point in an audio system
Line level (consumer)
-10 dBV (.316 V)
Line level (professional)
+4 dBu (1.23 V or higher)
Microphone level
-60 dBV (.001 V) to -40 dBV (.010 V)
Instrument level
0 dBu (.775 V)
Gain
The amount of increase in power of a signal by the amplifier
dBu /dBv
compares voltage; 0dBu=.775 V (across 600 ohms yields 1mW)
dBm
dB milliwatt; compares electrical power; 0 dBm=1mW
Unity gain
No amplification or attenuation applied to a signal
Gain structure
the settings of all the sequential amplification (gain) stages in a device or system; good gain structure is a result of the analysis and setting of all gain stages to ensure the system has good dynamic range, no clipping, and is high enough above the noise floor for a good signal-to-noise ratio
Peak hold
indicates the peak level of the signal and holds that level until it is either exceeded by a higher peak or the indicator is reset by a time delay or manual reset button
Peak level
the instantaneous level of a signal
Peak meter
shows the instantaneous level of a signal; changes faster than a VU meter (10-12ms versus 300ms)
Transient
the beginning of an audio signal (nonperiodic); often much louder than the rest of the signal
Decibel (dB)
used to describe gains and losses in signal power within an audio system; represents a power ratio, and is not a quantity of anything
+4 dBu
the industry standard SOL (standard operating level) for professional audio equipment
-10 dBu
the SOL (standard operating level) for consumer audio equipment
dBFS
decibels full-scale; measure of decibels closer to the true peak analog voltage equivalent
of the digital sample’s bit value
VU
volume unit; averages the absolute value of amplitude, is good for measuring perceived loudness
VU meter
shows the volume unit of a signal; good for measuring perceived loudness
SPL
sound pressure level; the variation in air pressure that a waveform causes, measured in a decibel scale and referred to a pressure of 20 micropascals
Crest factor
the difference between the average and peak level of a signal
Reference level
a signal level near the maximum possible that the device can handle but low enough to ensure low distortion
Noise floor
the noise power generated by the device in the absence of any input signal
Clipping
what typically happens when an audio amplifier output is overloaded or its input stage is overdriven; the waveform appears to be flat on its peak, containing lots of harmonic distortion
Dynamic range
the number of decibels between the peak level and the noise floor, indicating the ‘maximum-to-minimum’ range of signal levels that may be handled by the system
Signal to noise ratio
the number of decibels between the ‘reference level’ and the noise floor of the system
Bandwidth
The range of frequencies over which a device will operate; formally, the measurement is taken from the points at which the response is 3 dB down at the frequency extremes
compared with mid-frequencies
Transient response
also called slew rate; measures an amplifier’s or a microphone’s ability to quickly respond to the onset of a sound (its transient or attack)
Frequency response
a measure of a device’s ability to reproduce an input signal accurately across the frequency spectrum, as compared to a reference input signal, generally at 1 kHz
Crosstalk
a signal flowing along one path may induce a small signal in another
Distortion
any change in the original waveform of the signal in question
Harmonic distortion
non-linear distortion (the output signal is no longer proportional to the input signal), caused by clipping of a waveform
3rd harmonic distortion
a measurement of the amplitude of the third harmonic of the input frequency
THD
total harmonic distortion; the percentage of the output signal made up of all distortion products
IMD
intermodulation distortion; low and high frequencies interact and cause one or the other to modulate, generating new frequencies (sum and difference tones) that were not part of the original signal
Signal path
the route an audio signal follows
Power amplifier
job is to boost the line-level signal output to a signal strong enough to drive the speakers or monitors; use wall line voltage and large internal transformers to boost the input audio signal from around a volt to tens of volts
Instrument amplifier
electronic device that converts the often barely audible or purely electronic signal of a musical instrument into a larger electronic signal to feed to a loudspeaker
Line amplifier
any amplifier with a line level output and an output impedance of 600 ohms or so; used in the broadcast industry for sending signals from place to place and in recording studios to send signals between audio devices such as reverberators
Op amp
operational amplifier; theoretically has a gain of negative infinity, but practical designs
only approximate this ideal; if there is room above 0 on a fader or pot, pushing the fader or pot beyond this point typically activates the op amp
Mic preamp
boosts a low-level microphone signal to a usable line level
Active DI
direct box with an electronic circuit which presents a constant very high impedance to the instrument and provides a constant low-impedance output; requires a power source, usually batteries or phantom power
Passive DI
direct box containing just a transformer; does not require power supply
Phase
two waves of the same frequency are ‘in phase’ when their positive and negative half-cycles coincide exactly in time; if they are out of phase, cancellation of sound results
Phase correlation
the relationship between the left and right channels of a stereo signal
Phase correlation meter
a meter that shows the relationship between the left and right signals of a stereo waveform
Phase cancellation
when two waveforms’ positive and negative half-cycles match up when played together, the waveforms cancel out to zero, causing silence
Polarity
the alignment of a waveform relative to another waveform
Polarity inverter
a switch found on inputs of many consoles that flips the waveform upside-down, changing the polarity; also called phase reverse
Phantom power
preamp power is carried by the same two wires that carry the signal; the key to its operation is the fact that the signal is alternating current and the power is direct current, and they can be separated by the action of a transformer (as in condenser microphones)
usually 48 volts (P48)
Pad
built-in fixed resistive network that reduces output level
