Multimedia IS411P Lecture 02 - Digital Audio
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Questions and Answers

Which process converts continuous time into discrete values?

  • Sampling (correct)
  • Quantization
  • Filtering
  • Coding

What is the primary function of a Digital to Analog Converter (DAC)?

  • To convert an analog signal into a digital signal
  • To reconstruct the original analog signal from digital data (correct)
  • To quantize an analog signal into discrete values
  • To sample a continuous signal

What does quantization involve during the conversion process?

  • Assigning discrete values to continuous sample values (correct)
  • Dividing the signal into discrete time intervals
  • Enhancing the amplitude of the signal
  • Storing quantized values temporarily

Which term describes the fixed size of each quantization interval?

<p>Quantization step (D)</p> Signup and view all the answers

In the context of audio signals, what does a higher sampling rate typically achieve?

<p>Better representation of the original signal (A)</p> Signup and view all the answers

What is typically used to hold the sampled value constant until the next sampling interval?

<p>Sampling and hold circuit (A)</p> Signup and view all the answers

Which of the following best describes the coding stage in the analog-to-digital conversion process?

<p>Representing quantized values digitally (B)</p> Signup and view all the answers

What is the primary goal of using a low-pass filter in the DAC process?

<p>To eliminate high-frequency noise from the reconstructed signal (B)</p> Signup and view all the answers

What does the sampling rate depend on in the ADC process?

<p>The maximum frequency of the analog signal (A)</p> Signup and view all the answers

What does increasing the number of bits per sample do to the signal-to-noise ratio (SNR)?

<p>Increases the SNR by approximately 6 dB per bit (A)</p> Signup and view all the answers

According to Nyquist theorem, how many samples are required per cycle to accurately represent a sound wave?

<p>Two samples (D)</p> Signup and view all the answers

What is quantization error also referred to as?

<p>Quantization noise (D)</p> Signup and view all the answers

What is the relationship between the number of quantization levels (Q) and the bits (b) used in representation?

<p>b = log2(Q) (D)</p> Signup and view all the answers

What phenomenon occurs when a signal to be sampled has frequency components higher than half the sampling rate?

<p>Aliasing (D)</p> Signup and view all the answers

Which formula expresses the signal-to-noise ratio (SNR) in decibels?

<p>SNR = 20 log10(S/N) (B)</p> Signup and view all the answers

How does the number of quantization levels affect the amplitude fidelity of the digital signal?

<p>More quantization levels increase fidelity (A)</p> Signup and view all the answers

What determines the volume of sound?

<p>Amplitude (A)</p> Signup and view all the answers

Which unit measures the frequency of sound?

<p>Hertz (Hz) (A)</p> Signup and view all the answers

What is the range of frequencies that the human ear can typically perceive?

<p>20 Hz - 20 kHz (A)</p> Signup and view all the answers

What effect occurs when a sound source moves toward an observer?

<p>Doppler Effect (B)</p> Signup and view all the answers

What are harmonics in relation to sound waves?

<p>Whole number multiples of a fundamental frequency (C)</p> Signup and view all the answers

In audio signals, what do we call the difference between the upper and lower limit of the sound range that a person can hear?

<p>Dynamic Range (D)</p> Signup and view all the answers

What does the velocity of sound depend on?

<p>Medium through which it travels (B)</p> Signup and view all the answers

Which characteristic of sound represents the distance from one crest of a wave to the next?

<p>Wavelength (A)</p> Signup and view all the answers

Flashcards

Sampling Rate

The rate at which analog signals are sampled to create digital representations.

Quantization Levels

The discrete steps used to represent the amplitude of a signal in a digital format.

Quantization Error

The difference between the actual analog signal value and the quantized digital value.

Bits per Sample

The number of bits used to represent each sampled value.

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Fidelity

The closeness of the recorded version to the original signal or sound.

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Nyquist Theorem

States that the sampling rate must be at least twice the highest frequency component in the signal to avoid aliasing.

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Aliasing

A distortion introduced when the sampling rate is too low, causing higher-frequency components to appear as lower-frequency components.

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Signal-to-Noise Ratio (SNR)

A measure of the quality of a signal, expressed in decibels, comparing the signal strength to the noise level.

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Quantization Step (q)

The difference between adjacent quantization levels.

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Relationship between bits and quantization

The number of bits (b) determines the number of quantization levels (Q): Q = 2b

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Amplitude

The volume of a sound, measured in decibels.

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Period

The time taken for two consecutive peaks (crests) of a wave.

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Frequency

The number of wave peaks (or cycles) per second, measured in Hertz (Hz).

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Bandwidth

The range of frequencies within a signal (highest minus lowest).

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Wavelength

The distance between two adjacent wave peaks.

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Human Hearing Range

Audible frequencies range from 20Hz to 20kHz.

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Doppler Effect

Change in frequency of sound waves due to the motion of the sound source relative to the listener.

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Harmonics

Frequencies that are whole-number multiples of a fundamental frequency.

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Sound Wave

Continuous disturbances in air pressure.

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Velocity of Sound

Speed of sound waves; varies depending on the medium.

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Threshold of Audibility

The lowest sound intensity we can hear.

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Threshold of Pain

The highest sound intensity that causes pain.

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Analog Signal

A continuous electrical signal that represents an audio wave.

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Digital audio waveform representation

A representation of audio created by converting a continuous audio waveform into a digital series of numbers.

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Sampling

Converting a continuous signal into discrete values.

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Quantization

Making discrete samples by assigning a number within a specific range.

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Coding

Representing quantized values digitally.

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ADC

Analog to Digital converter

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Sampling Rate

The frequency at which audio is sampled.

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Quantization Step

The difference between adjacent levels.

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DAC

Digital to Analog Converter

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Audio Waveform

Describes the shape of an analog audio signal over time.

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Study Notes

Multimedia (IS411P) Lecture Notes

  • Course: Multimedia (IS411P)
  • Faculty: Computers & Information Science
  • Department: Information Systems
  • Academic Year: 2024-2025
  • Lecture: 02
  • Date: 15/10/2024

Digital Audio

  • Amplitude: Determines sound volume, measured in decibels (dB).
  • Period: Time between two crests (peaks) in a sound wave, measured in seconds.
  • Frequency (pitch): Number of peaks per second, measured in Hertz (Hz).
  • Bandwidth (BW): Difference between highest and lowest frequencies in a signal.
  • Wavelength (λ): Distance from midpoint of one crest to the midpoint of the next.
  • Human Hearing Range: 20 Hz to 20 kHz.
  • Human Maximum Sensitivity Range: 2 kHz - 4 kHz.
  • Velocity of Sound: Varies with medium, but can be determined by measuring the time required for sound waves to travel a given distance. Values for various mediums are shown in the table on page 5.

Doppler Effect

  • Sound waves are compressions and rarefactions of air.
  • When a sound source moves towards a listener, the frequency increases.
  • When a sound source moves away from a listener, the frequency decreases.

Harmonics

  • Most vibrating objects produce complex sounds.
  • Harmonic series: frequencies are whole number multiples of a fundamental frequency.
  • A complex sound wave is comprised of various frequencies.

Basic Characteristics of Audio Signal

  • Audio is caused by disturbances in air pressure.
  • Frequency range of audible sound: 20-20,000 Hz.
  • Amplitude: dynamic range is large, from audibility threshold to pain threshold; measured in decibels (dB).

Digital Representation of Audio

  • Continuous audio waveforms converted to an electrical signal by a microphone.
  • Analog signals converted to digital signals for processing/communication (ADC, Analog to Digital Converter). Three stages of ADC: sampling, quantization, coding.

Sampling

  • Continuous time converted to discrete values.
  • Time axis divided into fixed intervals.
  • Value of analog signal taken at the start of each interval.
  • Determined by sampling rate (sampling frequency)

Quantization

  • Process of converting continuous sample values to discrete values.
  • Signal's range divided into fixed intervals, each interval has a number.
  • The size of each interval is referred to as the quantization step.

Coding

  • The process of representing quantized values digitally (e.g. using binary values).
  • Eight quantized levels can be represented via 3 bits.

Digital-to-Analog Converter (DAC)

  • Reconstructs original analog signal from digital data.
  • Each quantized value is held for the sampling interval, resulting in step signals.
  • Step signals passed through a low-pass filter to approximate the original signal.

Signal-to-Noise Ratio (SNR)

  • Measures signal quality in decibels (dB).
  • Defined as: SNR = 20 log₁₀(S/N), where S is the maximum signal amplitude and N is the quantization noise.

Nyquist Theorem

  • Two samples per cycle (per wave) are necessary to represent a given wave.
  • To represent a 440 Hz sound, sampling rate must be 880 samples per second at minimum.
  • Sampling rate= 2 x Highest Frequency.

Aliasing

  • Distortion that happens when a signal has frequencies greater than half the sampling rate.
  • A serious problem in sampling systems, cannot be removed by post-processing after recording.
  • High frequencies filtered out prior to sampling for prevention.
    • falias = fsampling - ftrue for ftrue < fsampling < 2 x ftrue

Quality of Sound

  • Telephone conversation bandwidth = 3300 Hz.
  • CD-ROM sampling rate = 44 kHz per channel.
    • CD ROMs are important media for multimedia.

Sound Formats

  • Stereo recordings (2 channels) are more realistic, but take twice the storage space of mono recordings.

  • Formulas to calculate storage space for mono/stereo recordings:

    • Mono: File size = Sampling rate * Duration * (bits per sample/8) * 1
    • Stereo: File size = Sampling rate * Duration * (bits per sample/8) * 2

Use of Audio in Multimedia

  • Audio can serve as either content (dialogues, instructions) or ambient (background music, sound effects).

Video

  • Delivery of information per second is greater than with any other multimedia element.
  • DVDs make large video distribution easier, similar to the ease of analog-to-digital conversion of audio using CDs.

Analog Video

  • Only analog video is used in broadcasting for now.
  • Some movies/video may be digitally processed prior to broadcast.
  • Three analog video standards globally: NTSC, PAL, SECAM.

Analog Video Standards

  • NTSC (National Television System Committee): 30 frames/sec, 525 lines, 16 million colors.
  • PAL (Phase Alternation by Line): 25 frames/sec., 625 lines
  • SECAM (Sequential Couleur Avec Mémoire): 25 frames/sec., 625 lines

Digital Video

  • Increasing trend towards digital video, even in consumer electronics.
  • Digital video is easy to access and edit.
  • Video editing involves various operations: removing/inserting frames, mixing audio, etc.

Timecode

  • Unit for measuring video clip duration.
  • Can serve as a frame address.
  • SMPTE standard for timecode: hours:minutes:seconds:frames

Digitizing Analog Video

  • Video capture cards accept video input from devices, audio is sampled separately, software synchronizes both.
  • Cards must support appropriate frame rates (e.g., 30 fps) to avoid issues.

Keyframes

  • A complete image frame, unlike frames that only show differences.
  • Serve as reference points in video; rest of the frames rely on them to reconstruct the full picture.
  • Keyframe interval indicates how often a keyframe appears in a video stream.

Compression

  • Restructuring data to reduce file size.
  • Video files are compressed during capture, decompressed during playback.
  • Various codecs (compression/decompression algorithms) are available for digital videos.
  • Important codec characteristics: whether they are symmetric or asymmetric.

Factors Affecting Compression

  • Frames per second (fps)
  • Number of keyframes
  • Data rate for playback

File Formats for Video

  • AVI (PC) and QuickTime (Macintosh) are common formats for saving edited digital videos.
  • Both utilize similar video compression/decompression strategies.
  • Conversion programs are available.

Video on the Internet

  • Streaming video enables real-time video transmission via the Internet.
  • Quality depends on factors like bandwidth, sound intensity/frequency, and difference in information between successive frames.

Surround Video

  • Provides photorealistic visuals in Web pages, enabling real-time navigation around a 360-degree image.

Quiz (Example Questions)

  • Total frames in a clip with a given timecode and frames per second are given.
  • Uncompressed video file size calculation given video dimensions, color depth, and frames per second.
  • Given fps and keyframe interval, calculating number of keyframes in a second.

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Description

This quiz covers key concepts from Lecture 02 of the Multimedia (IS411P) course, focusing on digital audio fundamentals. Topics include amplitude, frequency, bandwidth, and the Doppler Effect. Test your knowledge on the properties of sound and human hearing ranges.

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