Music 2734A/B Study Guide - Midterm Guide: Digital Audio Workstation, Electrical Audio, Harry Nyquist
19 Dec 2016
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Integrity within an Audio Chain (Microphones)
• we hear analog copies of sound sources
• colouring can occur at every stage from mic to loudspeaker
• maintain a neutral audio path
• copy duplicates the original as closely as technology allows
• audio signal passes through a number of devices as it travels along the chain
• small amounts of deterioration introduced by each device sum together to produce degradation
• a microphone makes a single-point electrical copy of the complex waveforms that strike its
diaphragm
• transduction turns those soundwaves into an analog signal equivalent to what our eardrums
would hear at the mics location
• computer-based recording emerges in early 1980s
• digital process now occupies a central position in the audio chain
• the way devices convert electrical current to and from digital information now critical to
preserving the integrity of the original signal
Overview of the Processes Involved
• mechanical energy created by soundwaves striking the diaphragm
• induces an electrical current of constantly varying voltage
• analogous to the continuously changing air pressure of the waveforms that set the diaphragm in motion
• weak current prevents use of this signal in the next stages of the audio chain
• an amplifier generates line-level output
• sent to an analog-to-digital converter (ADC) to change voltage into a digital form computers
can recognise
• numeric information is the basic material recordists manipulate in a digital audio workstation
(DAW)
• sound quality cannot deteriorate when it is stored, copied, or processed in a computer
• signal then enters a digital-to-analog converter (DAC)
• binary numbers decoded to recreate the original variations in voltage
• line-level output goes to an amplifier then loudspeakers
• speakers transduce the electrical current into mechanal vibrations that generate soundwaves
which travel to the listeners eardrums
The Process: Basic Concepts and Terminology
Analog Audio
• analog the representation of a signal by continuously variable and measurable physical
quantities, such as, pressure or voltage
• diaphragm measures constantly changing air pressure
• mechanical motion induces an electrical current
• voltage continuously varies up and down in the same way air pressure constantly increases and decreases
• conversion of soundwaves to electrical energy enables amplification, recording, editing,
mixing, etc.
Digital Audio
• convert electrical current to digital information using a series of discrete binary numbers (0, 1)
• these numbers represent the changing voltage in an analog signal
• fluctuating voltage equates directly to the amplitude variations of soundwaves
Pulse Code Modulation (PCM)
• invented by Alec Reeves in the late 1930s
• PCM the standard method for digitally encoding analog waveforms (it is the format used in
both WAV and AIFF)
• three components: sampling, quantising, and encoding
• a device samples (measures) the voltage of an analog signal at a regular interval
• it quantises (rounds) that measurement to the nearest value on a predetermined scale, each step of which has been
assigned a discrete numerical value (whole-number / integer)
• the device encodes this value in binary digits original signal can be used in digital systems
Bit
• the term abbreviates the expression binary digit
• binary means something based on or made up of two things
• in digital audio systems, these two things are the numbers 0 and 1
Bit Depth (word length)
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• computers store data in number sequences that are multiples of 8 digits
• each of the numbers is either 0 or 1
• a group of eight digits called a byte
• one or more bytes comprise a binary word
• 16-bit audio (the standard for CDs) every binary word contains 16 numbers
• 24-bit audio each word has 24 numbers
• bit depth stipulates how many 0s and 1s represent each sample of a waveform
• each bit in a binary word equates to roughly 6.02 dB of dynamic range
• dynamic range is the difference between the softest and loudest sound a system can produce
• in theory, the total dynamic range available on a CD is 96.3 dB (6.02 x 16)
• 24-bit audio allows for a range of 144.5 dB, which approximates what humans can hear
• todays technology not capable of reaching these theoretical limits
• for orchestral music, the system must be able to handle more than 120 dB so it can reproduce
the ensembles full dynamic range
Bit Rate
• bit rate indicates how many bits are transmitted per unit of time in digital audio
• the unit of time is often seconds, as in bps or bits per second
Analog to Digital Converter (ADC)
• converts analog signals to digital code using pulse code modulation
Sampling
• the process of measuring the voltage of an electrical audio signal at a regular interval
• measurements outputted as binary numbers
• CD norm measured 44,100 times per second
• theory established by Harry Nyquist between 1924 and 1928 helps us understand why
• an analog signal can be recreated accurately only if measurements are taken at a rate equal to or greater than twice the
highest frequency in the signal
• the maximum frequency a digital system can represent is about half the sampling rate
• 44.1K exceeds the minimum sampling rate required for the upper limit of human hearing (20
kHz)
• therefore, waveforms can be reconstructed adequately
• lower sampling rates prohibit the faithful restoration of signals
• aliasing occurs when too few samples cause a device to interpret the voltage data as a
waveform different from the one originally sampled
• measure voltage at a rate higher than 44.1K, even subtler details of signals can be represented numerically
• finer variations in voltage can be facilitated and frequencies beyond 20 kHz can be encoded
• Nyquist theory determines the bandwidth digital systems can represent
• the greater the sampling rate, the higher the frequency that can be encoded
• many engineers believe ultrasonic frequencies lying above the upper limit of normal human
hearing interact with lower frequencies to create audible content that has desirable
psychoacoustic effects
Quantisation
• sampling imposes a succession of discrete measurement points on a signal at a regular interval
• the non-continuous nature of these points means that digital systems cannot precisely copy the smoothly varying
voltages of electrical current
• basic explanation of the principles converters use
• converters store each sample temporarily, holding it until the next sample is taken
• thus, the system ignores the continuously varying voltage between the sampling points
• a loss of information occurs
• converters compare the measured voltage to a scale that consists of a series of specific steps
instead of an infinite continuum of values
• a large range of precise measurements must be mapped onto a smaller set of whole numbers / integers
• when the voltage at the sample falls between two integers, the measurement has to be rounded (quantised) to the
closest step or number
• hence, quantisation changes the signal to match the points on the scale, a practice that
introduces errors into the system (heard as white noise or hiss)
• size of error depends on the number of steps in the scale
• 4-bit scale has 16 possible steps
• 8-bit scale 256 steps
• a 16-bit scale 65,536 steps
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• 24-bit scale 16,777,216 steps
• scales with higher numbers of bits have more finely graded steps
• reduces the size of the rounding error and thus noise
• as the difference between the actual voltage at the sampling point and its numeric representation becomes smaller,
many more subtle features of a waveform can be encoded
• dither reduces audibility of the quantisation defect
• adds specially constructed noise to the signal to randomise the negative effects
• people most sensitive to frequencies between 2 and 4 kHz
• devices can move the noise to a frequency range humans do not hear well
• noise shaping noise moved to a less audible range
• but note: the noise produced by a 16-bit scale is for all intents and purposes inaudible
• any distortion 80 dB or more below the main signal will not be heard anyway, regardless of its
makeup
Digital to Analog Converter (DAC)
• converts digital code to an analog signal (voltage), so non-digital systems can use the
information
• changes the numeric encoding back to the voltage points the binary words represent
• results in an electrical current comprised of single-point voltage levels
• this discrete waveform sent through a reconstruction filter to interpolate (fill in) the missing
data
• restores the signal to smoothly varying voltages
Resolution
• sound quality of digital audio depends primarily on sample rate and bit depth
• sample rate how frequently the waveform is measured
• bit depth the number of binary digits that store the data
• higher sample rates increase the bandwidth devices can encode
• larger bit depths reduce the noise in the system
• these determine the resolution of the audio
• todays high-resolution audio bit depth of at least 24 and a sample rate at or greater than 96 kHz
• the greater the resolution of the system, the more accurately it can represent the amplitudes of waveforms.
Microphone
• measures variations in air pressure
• transduces soundwaves from acoustic energy to mechanical energy to electrical energy
Condenser Microphones
• operate electrostatically
• capsule a movable diaphragm and a fixed backplate
• form the two electrodes of a capacitor (previously called a condenser)
• given a constant charge of DC voltage by an external power source (often supplied from a pre-
amp and called phantom power)
• soundwaves cause the distance between the two surfaces to change
• charge-carrying ability (capacitance) of the structure to fluctuate around its fixed value
• variation in voltage creates an electrical current that corresponds to the acoustic sound wave
• vacuum tube (valve) or transistor then boosts the current to ready the signal for post-
microphone amplification
• condensers employ either a pressure or a pressure-gradient principle of operation
Pressure Transducer (omnidirectional polar pattern)
• a single circular diaphragm clamped inside a completely enclosed casing
• only the front face exposed to the sound field
• sounds arriving from all directions exert equal force on the diaphragm
• diaphragm responds identically to every pressure fluctuation on its surface
• non-directional, that is, an omnidirectional (360o), response pattern
• diaphragm made of polyethylene (trade name Mylar)
• thinly coated on one side with a metal, such as, gold
• small holes on the backplate dampen the diaphragms motion by capturing air as the diaphragm moves back and forth
• a small tube connects the interior chamber to the exterior so that the internal and external air
pressure remain equal
Pressure-Gradient Transducer
• directional polar pattern
• figure 8 or bi-directional
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Document Summary
Digital audio: convert electrical current to digital information using a series of discrete binary numbers (0, 1, these numbers represent the changing voltage in an analog signal, fluctuating voltage equates directly to the amplitude variations of soundwaves. Bit: the term abbreviates the expression (cid:1688)binary digit(cid:1689, binary means something based on or made up of two things, in digital audio systems, these two things are the numbers 0 and 1. Bit rate: (cid:1688)bit rate(cid:1689) indicates how many bits are transmitted per unit of time in digital audio, the unit of time is often seconds, as in (cid:1688)bps(cid:1689) or (cid:1688)bits per second(cid:1689) Analog to digital converter (adc: converts analog signals to digital code using pulse code modulation. Microphone: measures variations in air pressure, transduces soundwaves from acoustic energy to mechanical energy to electrical energy. Dynamic and ribbon mics: operate on an electromagnetic principle, some form of electrically conductive metal moves within a magnetic field to generate electrical current.
