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Comprehensive Exams > Signals and Transmission > Flashcards

Signals and Transmission Flashcards

1
Q

Balanced line

A

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

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2
Q

Unbalanced line

A

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

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3
Q

Common mode rejection

A

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

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4
Q

Impedance

A

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.

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5
Q

High impedance

A

Has a high resistance to current; requires higher voltage, but has low current

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6
Q

Low impedance

A

Has low resistance to current; requires lower voltage, and has higher current (used in professional audio)

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7
Q

Line level

A

the average audio voltage level of a signal at a particular point in an audio system

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8
Q

Line level (consumer)

A

-10 dBV (.316 V)

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9
Q

Line level (professional)

A

+4 dBu (1.23 V or higher)

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10
Q

Microphone level

A

-60 dBV (.001 V) to -40 dBV (.010 V)

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11
Q

Instrument level

A

0 dBu (.775 V)

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12
Q

Gain

A

The amount of increase in power of a signal by the amplifier

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13
Q

dBu /dBv

A

compares voltage; 0dBu=.775 V (across 600 ohms yields 1mW)

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14
Q

dBm

A

dB milliwatt; compares electrical power; 0 dBm=1mW

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15
Q

Unity gain

A

No amplification or attenuation applied to a signal

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16
Q

Gain structure

A

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

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17
Q

Peak hold

A

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

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18
Q

Peak level

A

the instantaneous level of a signal

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19
Q

Peak meter

A

shows the instantaneous level of a signal; changes faster than a VU meter (10-12ms versus 300ms)

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20
Q

Transient

A

the beginning of an audio signal (nonperiodic); often much louder than the rest of the signal

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21
Q

Decibel (dB)

A

used to describe gains and losses in signal power within an audio system; represents a power ratio, and is not a quantity of anything

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22
Q

+4 dBu

A

the industry standard SOL (standard operating level) for professional audio equipment

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23
Q

-10 dBu

A

the SOL (standard operating level) for consumer audio equipment

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24
Q

dBFS

A

decibels full-scale; measure of decibels closer to the true peak analog voltage equivalent
of the digital sample’s bit value

25
Q

VU

A

volume unit; averages the absolute value of amplitude, is good for measuring perceived loudness

26
Q

VU meter

A

shows the volume unit of a signal; good for measuring perceived loudness

27
Q

SPL

A

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

28
Q

Crest factor

A

the difference between the average and peak level of a signal

29
Q

Reference level

A

a signal level near the maximum possible that the device can handle but low enough to ensure low distortion

30
Q

Noise floor

A

the noise power generated by the device in the absence of any input signal

31
Q

Clipping

A

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

32
Q

Dynamic range

A

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

33
Q

Signal to noise ratio

A

the number of decibels between the ‘reference level’ and the noise floor of the system

34
Q

Bandwidth

A

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

35
Q

Transient response

A

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)

36
Q

Frequency response

A

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

37
Q

Crosstalk

A

a signal flowing along one path may induce a small signal in another

38
Q

Distortion

A

any change in the original waveform of the signal in question

39
Q

Harmonic distortion

A

non-linear distortion (the output signal is no longer proportional to the input signal), caused by clipping of a waveform

40
Q

3rd harmonic distortion

A

a measurement of the amplitude of the third harmonic of the input frequency

41
Q

THD

A

total harmonic distortion; the percentage of the output signal made up of all distortion products

42
Q

IMD

A

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

43
Q

Signal path

A

the route an audio signal follows

44
Q

Power amplifier

A

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

45
Q

Instrument amplifier

A

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

46
Q

Line amplifier

A

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

47
Q

Op amp

A

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

48
Q

Mic preamp

A

boosts a low-level microphone signal to a usable line level

49
Q

Active DI

A

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

50
Q

Passive DI

A

direct box containing just a transformer; does not require power supply

51
Q

Phase

A

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

52
Q

Phase correlation

A

the relationship between the left and right channels of a stereo signal

53
Q

Phase correlation meter

A

a meter that shows the relationship between the left and right signals of a stereo waveform

54
Q

Phase cancellation

A

when two waveforms’ positive and negative half-cycles match up when played together, the waveforms cancel out to zero, causing silence

55
Q

Polarity

A

the alignment of a waveform relative to another waveform

56
Q

Polarity inverter

A

a switch found on inputs of many consoles that flips the waveform upside-down, changing the polarity; also called phase reverse

57
Q

Phantom power

A

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)

58
Q

Pad

A

built-in fixed resistive network that reduces output level

Comprehensive Exams (25 decks)

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