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DIGITAL MEDIA PROCESSING DIGITAL AUDIO PROCESSING

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Page 1: Chapter 3 part 3.pptx

DIGITAL MEDIA PROCESSING

DIGITAL AUDIO PROCESSING

Page 2: Chapter 3 part 3.pptx

What is dynamic range?

Dynamic range is the difference between two extremes. 

In audio, dynamic range is defined as the range between the highest and lowest amplitude moments of the sound over any given interval.

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Dynamic range

This is best illustrated by music > Classical symphonic music generally has a wide dynamic range. For example, “Beethoven’s Fifth Symphony” begins

with a high amplitude “Bump bump bump baaaaaaaa” and continues with a low-amplitude string section.

The difference between the loud and quiet parts is intentionally dramatic and is what gives the piece a wide dynamic range.

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Dynamic range The dynamic range of this clip is the difference

between the magnitude of the largest sample value and the magnitude of the smallest. measure the range from the highest-magnitude

sample either above or below the axis to the lowest-magnitude sample on the same side of the axis.

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The common parameters that can be set in dynamics processing are the threshold, attack time, and release time.

The threshold is an amplitude limit on the input signal that triggers compression or expansion. (The same threshold triggers the deactivation of compression or expansion when it is passed in the other direction.)

The attack time is the amount of time allotted for the total amplitude increase or reduction to be achieved after compression or expansion is triggered.

The release time is the amount of time allotted for the dynamics processing to be "turned off," reaching a level where a boost or attenuation is no longer being applied to the input signal.

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What is a dynamics processor? In its simplest form, a dynamics

processor is like an automatic volume control,  turning the volume up when it's too soft or

down when it's too loud.  In the audio world, dynamics processors

adjust the dynamic range of an audio signal by measuring a signal's amplitude over time, and setting up rules to react to any changes to that amplitude. 

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Dynamics processor Dynamics processors allow us to

manipulate a signal's amplitude and dynamic range in a variety of different ways sometimes reducing it, sometimes increasing it, and often doing

both over a period of time. 

Before adjusting the amplitude, we must first specify the amplitude level at which the processor begins to react >> threshold. 

Then we need to create a rule for what happens when that threshold is breached. 

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Basic types of Dynamic Processors 1 Compressors

The most common – the louder the signal is coming in, the less level it provides going out.

In a compressor, a target level is set — called the “threshold” — and any signal coming in that exceeds that level will be reduced.

The higher the level is above that threshold, the more reduction will occur.

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Basic types of Dynamic Processors 2 Expanders

The quieter the signal is coming in, the less level it provides going out.

In other words — it makes quiet signals even quieter. Much like a compressor, the threshold is set at a

certain level. Any signal that does NOT exceed that threshold is

reduced, and the quieter the signal, the more reduction is done.

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Basic types of Dynamic Processors 3 Limiters

Limiters are like super compressors. The idea is to ensure that the level does not exceed

the threshold. Because this amount of compression is extreme, a

limiter relies on certain functions and design that regular compressors do not have.

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Basic types of Dynamic Processors 4 Gates

Gates are like super expanders. Anything that does not exceed the threshold is

reduced to inaudible. Again, because gates are extreme, they often

require a slightly different design than a regular expander.

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Dynamics processing Dynamics processing refers to any kind of

processing that alters the dynamic range of an audio signal, whether by compressing or expanding it.

A dynamic processor is something that outputs a signal, where the level of the outgoing signal is based on the level of the incoming signal.

In other words, a loud signal coming in will come out differently than a quiet signal coming in.

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Use of dynamics processing The dynamics processing is useful if one needs to:

equalize the loudness of the sound, enlarge the dynamic range of the sound, attenuate or enhance selected frequency ranges, remove signal parts with level below the given

threshold, limit the maximum signal level value .

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Page 14: Chapter 3 part 3.pptx

Dynamics processing In dynamics processing, the two general possibilities are

compression and expansion,

each of which can be done in the upwards or downwards direction.

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Dynamics processing Compression : attenuates the higher amplitudes and boosts

the lower ones the result of which is less difference in level between the loud and

quiet parts, reducing the dynamic range. Expansion : boosts the high amplitudes and attenuates the

lower ones, resulting in an increase in dynamic range.

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Dynamics processing Downward compression attenuates signals that are above a

given threshold, not changing signals below the threshold. This reduces the dynamic range.

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Dynamics processing Upward compression boosts signals that are below a given

threshold, not changing signals above the threshold. This reduces the dynamic range.

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Dynamics processing Downward expansion attenuates signals that are below a

given threshold, not changing signals above the threshold. This increases the dynamic range.

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Dynamics processing Upward expansion boosts signals that are above a given

threshold, not changing signals below the threshold. This increases the dynamic range.

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Effect Of Compression & Expansion*

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Which of the Following is not a Filter?

• Graphic Equalizer on a stereo system• Tone control on a stereo system• Microphone• Mixing Board• Loudspeaker• Vocal Tract• Ear

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Definition

• A filter is any operation on a signal (From Rabiner et al, “Terminology in Digital Signal Processing.”

• Commonly, we limit the term filter to devices (hardware or software) that were designed specifically to boost or attenuate regions of a sound spectrum.

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What is a filter?** Musicians have been using filters for thousands of years to

shape the sounds of their art in various ways.  Any medium through which the music signal passes, whatever

its form, can be regarded as a filter. However, we do not usually think of something as a filter

unless it can modify the sound in some way. For example, speaker wire is not considered a filter, but the

speaker is (unfortunately). 

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What is a filter? Used by musicians

to shape the sounds of their art in various ways.  Filter >> any medium through which the music signal passes,

whatever its form. Therefore, an object is a filter if it can modify the sound in

some way. Speaker wire, the speaker?? 

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Filters

• Filters shape the frequency spectrum of a sound signal.

– Filters generally do not add frequency components to a signal that are not there to begin with.

– Boost or attenuate selected frequency regions

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What is a digital filter?

A digital filter is a filter that operates on digital signals.

It is a computation which takes one sequence of numbers (the input signal) and produces a new sequence of numbers (the filtered output signal).

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Digital filters

Digital filters are used for two general purposes: separation of signals

when a signal has been contaminated with interference, noise, or other signals.

signal restoration

when a signal has been distorted in some way.

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Digital filters

An audio filter is a frequency dependent amplifier circuit, working in the audio frequency range, 0 Hz to beyond 20 kHz.

Many types of filters exist for applications including graphic equalizers, synthesizers, 

sound effects, CD players and virtual reality systems.

Used to amplify, pass or attenuate (turn down) a range of sound frequencies.

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Digital filters

Digital filters can be implemented in two ways: by convolution (also called finite impulse

response or FIR) and by recursion (also called infinite impulse

response or IIR). Filters carried out by convolution can have far

better performance than filters using recursion, but execute much more slowly.

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Digital filters

Filters are defined by their slope, which determines the attenuation of signals outside the 'pass' band. 

Filters are used for 'corrective' equalization, as opposed to creative equalization. They are used to clean up a signal, rather than to

shape the sound creatively.

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**

These problems can be attacked with either analog or digital filters. Which is better?

Analog filters are cheap, fast, and have a large dynamic range in both amplitude and frequency.

Digital filters, in comparison, are vastly superior in the level of performance that can be achieved

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**

Digital filters can achieve thousands of times better performance than analog filters. This makes a dramatic difference in how filtering problems are approached.

With analog filters, the emphasis is on handling limitations of the electronics, such as the accuracy and stability of the resistors and capacitors.

In comparison, digital filters are so good that the performance of the filter is frequently ignored. The emphasis shifts to the limitations of the signals, and the theoretical issues regarding their processing.

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Digital filters

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Digital Filters

The purpose of these filters is to allow some frequencies to pass unaltered, while completely blocking other frequencies. The passband refers to those frequencies that are

passed, while the stopband contains those frequencies that are

blocked. The transition band is between.

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Digital Filters

High-pass, band-pass and band-reject filters are designed by starting with a low-pass filter, and then converting it into the desired response.

For this reason, most discussions on filter design only give examples of low-pass filters.

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Digital Filters

The goal of these filters is to separate one band of frequencies from another.

Custom filters are used when a special action is required by the filter, something more elaborate than the four basic responses (high-pass, low-pass, band-pass and band-reject).

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Filter types

A low-pass filter: pass through frequencies below their cutoff frequencies, and progressively attenuates frequencies above the cutoff frequency.

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Filter types

A high-pass filter does the opposite, passing high frequencies above the cutoff frequency, and progressively attenuating frequencies below the cutoff frequency.

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Filter types

A band-pass filter passes frequencies between its two cutoff frequencies, while attenuating those outside the range.

A band-reject filter, attenuates frequencies between its two cutoff frequencies, while passing those outside the 'reject' range.

An all-pass filter, passes all frequencies, but affects the phase of any given sinusoidal component according to its frequency.

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Revisit Audio Digital audio processing tools Audio coding techniques/technology Audio compression Dynamic processing Audio filtering Audio restoration

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Audio restoration

Audio restoration is the process of removing imperfections or degradations from sound recordings

Degradation of an audio source will be considered as any undesirable modification to the audio signal which occurs as a result of the recording process.

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Audio restoration

An ideal restoration would then reconstruct the original sound source exactly as received by the transducing equipment.

Of course, the ideal can never be achieved, and methods can only be devised which come close according to some suitable error criterion.

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Audio restoration

Analog methods are not sophisticated enough to perform a significant degree of noise reduction without interfering with the underlying signal quality

Digital methods allow for a much greater degree of flexibility in processing and hence greater potential for noise removal

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Audio restoration

Several distinct types of degradation in audio sources.

localized degradations and

global degradations.

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Audio restoration

Localized degradations discontinuities in the waveform which affect only

certain samples e.g. clicks, scratches, breakages and clipping.

Global degradations affect all samples of the waveform and e.g. background noise, wow and flutter and certain

types of non linear distortion.

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Click removal

Clicks - these are short bursts of interference random in time and amplitude

The term 'clicks' refer to a generic localized type of degradation which is common to many audio media.

All finite duration defects which occur at random positions in the waveform will be classified as clicks.

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Click removal

Clicks are perceived in a number of ways by the listener, ranging from tiny 'tick' noises which can occur in any recording medium through the characteristic 'scratch' and 'crackle' noise associated with most analogue disc recording methods.

Clicks are perceived as a variety of defects ranging from isolated ‘tick’ noises to the characteristic ‘crackle’ associated with 78rpm disc recordings.

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Page 49: Chapter 3 part 3.pptx

Click removal

There are many mechanisms by which clicks can occur.

Typical examples: specks of dirt and dust adhering to the grooves of a

gramophone disc or granularity in the material used for pressing records. damage to the disc in the form of small scratches on

the surface

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Click removal

Ticks can occur in digital recordings as a result of poorly concealed digital errors and timing problems.

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Click removal**

Two tasks can be identified for removing click: First, a detection procedure in which the values of

the noise switching process it is estimated (decide which samples are degraded).

Second, an estimation procedure in which we attempt to reconstruct the underlying audio data when corruption is present

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Low frequency noise pulses** A further problem which is common to several

recording media including gramophone discs and optical film sound tracks is that of low frequency noise pulses.

This form of degradation is typically associated with large scratches or even breakages in the surface of gramophone disc.

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Low frequency noise pulses usually a larger scale defect than clicks. caused by very large scratches or breakages in

the playback medium. These large discontinuities excite a low frequency

resonance in the pickup apparatus which is perceived as a low frequency ‘pop’or ‘thump’ noise.

This type of degradation is common in gramophone disc recordings and optical film sound tracks.

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Low frequency noise pulses* Low frequency noise pulses appear to be the

response of the playback system to extreme step like or impulsive stimuli caused bay breakages in the groove walls of gramophone discs or large scratches on an optical film sound track.

The audible effect of this response is a percussive "pop" noise or "thump" in the recording.

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Low frequency noise pulses This type of degradation is often the most

disturbing artifact present in a given extract. It is thus highly desirable to eliminate noise pulses

as a first stage in the restoration process.

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Background noise/hiss removal Random additive background noise is a form of

degradation common to all analogue measurement, storage and recording systems.

In the case of audio signals the noise, which is generally perceived as “hiss” by the listener, will be composed of electrical circuit noise, irregularities in the storage medium and ambient noise from the recording environment.

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Background noise removal

The combined effect of these sources will generally be treated as one single noise process,

Random noise generally has significant components at all audio frequencies, and thus simple filtering and equalization procedures are inadequate for restoration purposes.

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Background noise removal

In recording media which are also affected by local disturbances, such as clicks, standard practice is to restore these defects prior to any background noise treatment.

The most popular methods for noise reduction in audio signals are based upon short-time Fourier processing.

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Pitch variation defects

A form of degradation commonly encountered in disc magnetic tape and film sound recordings is an overall pitch variation not present in the original performance.

'wow‘ : smooth pitch variation over long time-scales

‘flutter’ : describes a pitch variation which varies more rapidly with time.

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Pitch variation defects

Several mechanisms by which wow can occur A variation of rotational speed of the recording

medium during either recording or playback. (tape or gramophone recordings)

Eccentricity in the recording or playback process for disc and cylinder recordings, (a hole which is not punched perfectly at the centre of a gramophone disc.)

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Pitch variation defects

It is possible for magnetic tape and optical film to become unevenly stretched during playback or storage; this too leads to pitch variation in playback.

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