Basic Concepts of Audio, Video and Compression

1
Basic Concepts of Audio, Video and Compression

• Anupriya Sharma

2
Content

• Introduction on Multimedia
• Audio encoding
• Video encoding
• Compression

3
Video on Demand

• Video On Demand (a) ADSL vs. (b) cable

4
Multimedia Files

• A movie may consist of several files

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Multimedia Issues

• Analog to digital
• Problem need to be acceptable by ears or eyes
• Jitter
• Require high data rate
• Large storage
• Compression
• Require real-time playback
• Scheduling
• Quality of service
• Resource reservation

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Audio

• Sound is a continuous wave that travels through
  the air.
• The wave is made up of pressure differences.

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How do we hear sound?

• Sound is detected by measuring the pressure level
  at a point
• When an acoustic signal reaches the otter- ear
  (Pinna), the generated wave will be transformed
  into energy and filtered through the middle-ear.
  The inner-ear (Cochlea) transforms the energy
  into nerve activity.
• In similar way, when an acoustic wave strikes a
  microphone, the microphone generates an
  electrical signal, representing the sound
  amplitude as a function of time.

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Basic Sound Concepts

• Frequency represents the number of periods in a
  second (measured in hertz, cycles/second)
• Human hearing frequency range 20 Hz - 20 kHz
  (audio), voice is about 500 Hz to 2 kHz.
• Amplitude of a sound is the measure of
  displacement of the air pressure wave from its
  mean.

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Computer Representation of Audio

• Speech is analog in nature and it is converted to
  digital form by an analog-to-digital converter
  (ADC).
• A transducer converts pressure to voltage levels.
  
• Convert analog signal into a digital stream by
  discrete sampling
• Discretization both in time and amplitude
  (quantization)

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Audio Encoding (1)

• Audio Waves Converted to Digital
• electrical voltage input
• sample voltage levels at intervals to get a
  vector of values (0, 0.2, 0.5, 1.1, 1.5, 2.3,
  2.5, 3.1, 3.0, 2.4,...)
• A computer measures the amplitude of the waveform
  at regular time intervals to produce a series of
  numbers (samples).
• The ADC process is governed by four factors
  sampling rate, quantization, linearity, and
  conversion speed. binary number as output

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Audio Encoding (2)

• Sampling Rate rate at which a continuous wave is
  sampled (measured in Hertz)
• Examples CD standard - 44100 Hz, Telephone
  quality - 8000 Hz
• The audio industry uses 5.0125 kHz, 11.025 kHz,
  22.05 kHz, and 44.1 kHz as the standard sampling
  frequencies. These frequencies are supported by
  most sound cards.
• Question How often do you need to sample a
  signal to avoid losing information?

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Audio Encoding (3)

• Answer It depends on how fast the signal is
  changing,. Real Answer twice per cycle (this
  follows from Nyquist sampling theorem)
• Nyquist Sampling Theorem If a signal f(t) is
  sampled at regular intervals of time and at a
  rate higher than twice the highest significant
  signal frequency, then the samples contain all
  the information of the original signal.
• Example CD's actual sampling frequency is 22050
  Hz, but because of Nyquist's Theorem, we need to
  sample the signal twice, therefore the sampling
  frequency is 44100Hz.

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Audio Encoding (4)

• The best-known technique for voice digitization
  is Pulse-Code Modulation (PCM).
• PCM is based on the sampling theorem.
• If voice data are limited to 4000 Hz, then PCM
  samples 8000 samples/second which is sufficient
  for the input voice signal.
• PCM provides analog samples which must be
  converted to digital representation. Each of
  these analog samples must be assigned a binary
  code. Each sample is approximated by being
  quantized as explained above.

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Audio Encoding (5)

• Quantization (sample precision) the resolution
  of a sample value.
• Samples are typically stored as raw numbers
  (linear PCM format) or as logarithms (u-law or
  A-law)
• Quantization depends on the number of bits used
  measuring the height of the waveform
• Example 16-bit CD quality quantization results
  in over 65536 values
• Audio Formats are described by the sample rate
  and quantization
• Voice quality 8-bit quantization, 8000 Hz u-law
  mono (8kBytes/s)
• 22 kHz 8-bit linear mono (22 kBytes/second) and
  stereo (44 kBytes/s)
• CD quality 16-bit quantization, 44100 Hz linear
  stereo (176.4 kBytes/s 44100 samples x 16
  bits/sample x 2 (two channels)/8000)

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Audio Formats

• Audio formats are characterized by four
  parameters
• Sample Rate sampling frequency per second
• Encoding Audio data representation
• U-law CCITT G.711 standard for voice data in
  telephone companies (USA, Canada, Japan)
• A-law CCITT G.711 standard for voice data in
  telephony elsewhere (Europe,)
• A-law and u-law are sampled at 8000
  samples/second with precision of 12 bits and
  compressed to 8 bit samples.
• Linear PCM uncompressed audio where samples are
  proportional to audio signal voltage
• Precision number of bits used to store each
  audio sample.
• Channel Multiple channels of audio may be
  interleaved at sample boundaries

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Basic Concepts of Video

• Visual Representation Video Encoding
• Objective is to offer the viewer a sense of
  presence in the scene and of participation in the
  events portrayed
• Transmission
• Video signals are transmitted to a receiver
  through a single television channel
• Digitalization
• Analog to digital conversion, sampling of
  gray/color level - Quantization

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Visual Representation (1)

• Video Signals are generated at the output of a
  camera by scanning a two-dimensional moving scene
  and converting them into a onedimensional
  electric signal
• A moving scene is a collection of individual
  images, where each scanned picture generates a
  frame of the picture
• Scanning starts at the top-left corner of the
  picutre and ends at the bottom-right.
• Aspect Ratio ratio of picture width and height
• Pixel discrete picture element digitized
  light point in a frame
• Vertical Frame Resolution number of pixels in
  picture height
• Horizontal Frame Resolution number of pixels in
  picture width
• Spatial Resolution Vertical x Horizontal
  Resolution
• Temporal Resolution Rapid succession of
  different frames

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Visual Representation (2)

• Continuity of Motion
• Minimum 15 frames per second
• NTSC 29.97 Hz repetition rate, 30 frames/sec
• PAL 25 HZ, 25 frames/sec
• HDTV 59.94 Hz, 60 frames/sec
• Flicker Effect
• is a periodic fluctuation of brightness
  perception. To avoid this effect, we need at
  least 50 refresh cycles per second. This is done
  by display devices using display refresh buffer
• Picture Scanning
• Progressive scanning single scanning of a
  picture
• Interlaced scanning the frame is formed by
  scanning two pictures at different times, with
  the lines interleaved, such that two consecutive
  lines of a frame belong to alternative field
  (scan odd and even lines separately)
• NTSC TV uses interlaced scanning to trade-off
  vertical with temporal resolution
• HDTV and computer displays are high
  spatio-temporal videos and use progressive
  scanning.

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Video Color Encoding (3)

• During the scanning, a camera creates three
  signals RGB (red, greed and blue) signals.
• For compatibility with black-and-white video and
  because of the fact that the three color signals
  are highly correlated, a new set of signals of
  different space are generated.
• The new color systems correspond to the standards
  such as NTCS, PAL, SECAM.
• For transmission of the visual signal we use
  three signals 1 luminance (brightness- basic
  signal) and 2 chrominance (color signals).
• YUV Signal (Y 0.3R0.59G0.11B, U (B-Y) x
  0.493, V(R-Y) x 0.877)
• Coding ratio between components Y,U,V 422
• In NTSC signal the luminance and chrominance
  signals are interleaved
• The goal at the receiver is (1) separate
  luminance from chrominance components, and (2)
  avoid interference between them (cross-color,
  cross luminance)

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Basic Concepts of Image Formats

• Important Parameters for Captured Image Formats
• Spatial Resolution (pixels x pixels)
• Color encoding (quantization level of a pixel
  e.g., 8-bit, 24-bit)
• Examples SunVideo' Video Digitizer Board allows
  pictures of 320 by 240 pixels with 8-bit
  gray-scale or color resolution.
• For a precise demonstration of image basic
  concepts try the program xv which displays images
  and allows to show, edit and manipulate the image
  characteristics.
• Important Parameters for Stored Image Formats
• Images are stored as a 2D array of values where
  each value represents the data associated with a
  pixel in the image (bitmap or a color image).
• The stored images can use flexible formats such
  as the RIFF (Resource Interchange File Format).
  RIFF includes formats such as bitmats,
  vector-representations, animations, audio and
  video.
• Currently, most used image storage formats are
  GIF (Graphics Interchange Format), XBM (X11
  Bitmap), Postscript, JPEG (see compression
  chapter), TIFF (Tagged Image File Format), PBM
  (Portable Bitmap), BMP (Bitmap).

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Digital Video

• The process of digitizing analog video involves
• Filtering, sampling, quantization
• Filtering is employed to avoid the aliasing
  artifacts of the follow-up sampling process
• Filtered luminance and chrominance signals are
  sampled to generate a discrete time signal
• Digitization means sampling gray/color levels in
  the frame at MxN array of points
• The minimum rate at which each component (YUV)
  can be sampled is the Nyquist rate and
  corresponds to twice the signal bandwidth
• Once the points are sampled , they are quantized
  into pixels i.e., the sampled value is mapped
  into integer. The quantization level depends on
  how many bits do we allocate to present the
  resulting integer (e.g., 8 bits per pixel, or 24
  bits per pixel)

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Digital Transmission Bandwidth

• Bandwidth requirements for Images
• Raw Image Transmission Bandwidth size of the
  image spatial resolution x pixel resolution
• Compressed Image Transmission Bandwidth
  depends on the compression scheme(e.g., JPEG) and
  content of the image
• Symbolic Image Transmission bandwidth size of
  the instructions and variables carrying graphics
  primitives and attributes.
• Bandwidth Requirements for Video
• Uncompressed Video Bandwidth image size x frame
  rate
• Compressed Video Bandwidth depends on the
  compression scheme(e.g., Motion JPEG, MPEG) and
  content of the video (scene changes).
• Example Assume the following video
  characteristics - 720,000 pixels per
  image(frame), 8 bits per pixel quantization, and
  60 frames per second frame rate. The Video
  Bandwidth 720,000 pixels per frame x 8 bits per
  pixel x 60 fps
• which results in HDTV data rate of 43,200,000
  bytes per second 345.6 Mbps When we use MPEG
  compression, the bandwidth goes to 34 Mbps wit
  some loss in image/video quality.

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Compression Classification

• Compression is important due to limited bandwidth
  
• All compression systems require two algorithms
• Encoding at the source
• Decoding at the destination
• Entropy Coding
• Lossless encoding
• Used regardless of medias specific
  characteristics
• Data taken as a simple digital sequence
• Decompression process regenerates data completely
• Examples Run-length coding, Huffman coding,
  Arithmetic coding
• Source Coding
• Lossy encoding
• Takes into account the semantics of data
• Degree of compression depends on data content
• Examples DPCM, Delta Modulation
• Hybrid Coding
• Combined entropy coding with source coding
• Examples JPEG, MPEG, H.263,

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Compression (1)
Uncompressed Picture
Picture Preparation
Picture Processing
Adaptive Feedback
Quantization
Entropy Encoding
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Compression (2)

• Picture Preparation
• Analog-to-digital conversion
• Generation of appropriate digital representation
• Image division into 8x8 blocks
• Fix the number of bits per pixel
• Picture Processing (Compression Algorithm)
• Transformation from time to frequency domain
  (e.g., Discrete Cosine Transformation DCT)
• Motion vector computation for motion video
• Quantization
• Mapping real numbers to integers (reduction in
  precision)
• Entropy Coding
• Compress a sequential digital stream without loss

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Compression (3)(Entropy Encoding)

• Simple lossless compression algorithm is the
  Run-length Coding, where multiple occurring bytes
  are grouped together as Number-OccuranceSpecial-Ch
  aracterCompressed-Byte. For example,
  'AAAAAABBBBBDDDDDAAAAAAAA' can be encoded as
  6!A5!B5!D8!A', where !' is the special
  character. The compression ratio is 50
  (12/24100).
• Fixed-length Coding each symbol gets allocated
  the same number of bits independent of frequency
  (L log2(N)), N number of symbols
• Statistical Encoding each symbol has a
  probability of frequency (e.g., P(A) 0.16, P(B)
  0.51, P(C) 0.33)
• The theoretical minimum average number of bits
  per codeword is known as Entropy (H). According
  to Shannon
• H SPilog2Pi bits per codeword

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Huffman Coding
P(ACB) 1
0
1
P(AC) 0.49
P(B) 0.51
0
1
Symbol Code A 00 C
01 B 1
P(A) 0.16
P (C) 0.33
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JPEG Joint Photographic Experts Group

• 6 major steps to compress an image (1) block
  preparation, (2) DCT (Discrete Cosine Transform)
  transformation, (3) quantization, (4) further
  compression via differential compression, (5)
  zig-zag scanning and run-length coding
  compression, (6) Huffman coding compression
• Quantization step represents the lossy step where
  we loose data in a non-invertible fashion.
• Differential compression means that we consider
  similar blocks in the image and encode only the
  first block and for the rest of the similar
  blocks, we encode only differences between the
  previous block and current block. The hope is
  that the difference is a much smaller value,
  hence we need less bits to represent it. Also
  often the differences end up close to 0 and can
  be very well compressed by the next compression -
  run-length coding.
• Huffman compression is a lossless statistical
  encoding algorithm which takes into account
  frequency of occurrence (not each byte has the
  same weight)

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JPEG Block Preparation

• RGB input data and block preparation
• Eyes responds to luminance (Y) more than
  chrominance (I and Q)

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Image Processing

• After image preparation we have
• Uncompressed image samples grouped into data
  units of 8x8 pixels
• Precision 8bits/pixel
• Values are in the range of 0,255
• Steps in image processing
• Pixel values are shifted into the range
  -128,127 with center 0
• DCT maps values from time to frequency domain
• S(u,v) ¼ C(u)C(v)SSS(x,y)cos(2x1)up/16
  cos(2y1)vp/16
• S(0,0) lowest frequency in both directions DC
  coefficient determines the fundamental color of
  the block
• S(0,1), , S(7,7) AC coefficients

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Quantization

• Goal of quantization is to throw out bits
• Consider example 1011012 45 (6bits) we can
  truncate this string to 4 bits 10112 11 or to
  3 bits 1012 5 (original value 40) or 1102 6
  (original value 48)
• Uniform quantization is achieved by dividing DCT
  coefficient value S(u,v) by N and round the
  result.
• JPEG uses quantization tables

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Entropy Encoding

• After image processing we have quantized DC and
  AC coefficients
• Initial step of entropy encoding is to map 8x8
  plane into 64 element vector using Zig-Zag

• DC Coefficient Processing use
• Difference coding
• AC Coefficient Processing apply
• Run-length coding
• Apply Huffman Coding on DC and
• AC coefficients

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MPEG Motion Picture Experts Group

• MPEG-1 was designed for video recorder-quality
  output (320x240 for NTSC) using the bit rate of
  1.2 Mpbs.
• MPEG-2 is for broadcast quality video into
  4-6Mbps (it fits into the NTSC or PAL broadcast
  channel)
• MPEG takes advantage of temporal and spatial
  redundancy. Temporal redundancy means that two
  neighboring frames are similar, almost identical.
  
• MPEG-2 output consists of three different kinds
  of frames that have to be processed
• I (Intracoded) frames - self-contained
  JPEG-encoded still pictures
• P (Predictive) frames - Block-by-block difference
  with the last frame
• B (Bidirectional) frames - Differences with the
  last and next frames

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The MPEG Standard

• I frames - self-contained, hence they are used
  for fast forward and rewind operations in VOD
  applications
• P frames code interfame differences. The
  algorithm searches for similar macroblocks in the
  current and previous frame, and if they are only
  slightly different, it encodes only the
  difference and the motion vector in order to find
  the position of the macroblock for decoding.
• B frames - encoded if three frames are available
  at once the past one, the current one and the
  future one. Similar to P frame, the algorithm
  takes a macroblock in the current frame and looks
  for similar macroblocks in the past and future
  frames.
• MPEG is suitable for stored video because it is
  an asymmetric lossy compression. The encoding
  takes long time, but the decoding is very fast.
• The frames are delivered at the receiver in the
  dependency order rather than display order, hence
  we need buffering to reorder the frames.

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MPEG/Video I-Frames

• I frames (intra-coded images)
• MPEG uses JPEG compression algorithm for I-frame
  encoding
• I-frames use 8x8 blocks defined within a
  macro-block. On these blocks, DCT is performed.
  Quantization is done by a constant value for all
  DCT coefficients, it means no quantization tables
  exist as it is the case in JPEG

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MPEG/Video P-Frames

• P-frames (predictive coded frames) requires
  previous I-frame and/or previous P-frame for
  encoding and decoding
• Use motion estimation method at the encoder
• Define match window within a given search window.
  Match window corresponds to macro-block, search
  window corresponds to an arbitrary window size
  depending how far away we are willing to look.
• Matching methods
• SSD correlation uses SSD Si(xi-yi)2
• SDA correlation uses SAD Sixi-yi

37
MPEG/Video B-Frame

• B-frames (bi-directionally predictive-coded
  frames) require information of the previous and
  following I and/or P-frame

- ½ x (
)
DCT Quant. RLE
0001110
Motion Vectors (two)
Huffman Coding
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MPEG/Audio Encoding

• Precision is 16 bits
• Sampling frequency is 32 KHz, 44.1 KHz, 48 KHz
• 3 compression methods exist Layer 1, Layer 2,
  Layer 3 (MP3)

Decoder is accepting layer 2 and layer 1 32
kbps-320kpbs, target 64 kbps Decoder is
accepting layer 1 32kbps-384kbps, target 128
kbps 32kbps-448kbps, target 192 kbps
Layer 3
Layer 2
Layer 1
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MPEG/System Data Stream

• Video is interleaved with audio.
• Audio consists of three layers
• Video consists of 6 layers
• (1) sequence layer
• (2) group of pictures layer (Video Param,
  Bitstream Param, )
• (3) picture layer (Time code, GOP Param, )
• (4) slice layer (Type, Buffer Param, Encode
  Param, .)
• (5) macro-block layer (Qscale, )
• (6) block layer (Type, Motion Vector, Qscale,)