CS 414 - Spring 2011 CS 414 – Multimedia Systems Design Lecture 11 – MP3 Audio & Introduction to...
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Transcript of CS 414 - Spring 2011 CS 414 – Multimedia Systems Design Lecture 11 – MP3 Audio & Introduction to...
CS 414 - Spring 2011
CS 414 – Multimedia Systems Design Lecture 11 – MP3 Audio &Introduction to MPEG-4 (Part 6)
Klara Nahrstedt
Spring 2011
CS 414 - Spring 2011
Administrative MP1 – deadline February 18
Outline MP3 Audio Encoding MPEG-2 and Intro to MPEG-4 Reading:
Media Coding book, Section 7.7.2 – 7.7.5 Recommended Paper on MP3: Davis Pan, “A Tutorial on MPEG/Audio
Compression”, IEEE Multimedia, pp. 6-74, 1995 Recommended books on JPEG/ MPEG Audio/Video Fundamentals:
Haskell, Puri, Netravali, “Digital Video: An Introduction to MPEG-2”, Chapman and Hall, 1996
CS 414 - Spring 2011
MPEG-1 Audio Encoding
CharacteristicsPrecision 16 bitsSampling frequency: 32KHz, 44.1 KHz, 48 KHz3 compression layers: Layer 1, Layer 2, Layer 3
(MP3) Layer 3: 32-320 kbps, target 64 kbps Layer 2: 32-384 kbps, target 128 kbps Layer 1: 32-448 kbps, target 192 kbps
CS 414 - Spring 2011
MPEG Audio Encoding Steps
CS 414 - Spring 2011
MPEG Audio Filter Bank Filter bank divides input into multiple sub-bands
(32 equal frequency sub-bands) Sub-band i defined
- filter output sample for sub-band i at time t, C[n] – one of 512 coefficients, x[n] – audio input sample from 512 sample buffer
CS 414 - Spring 2011
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MPEG Audio Psycho-acoustic Model MPEG audio compresses by removing
acoustically irrelevant parts of audio signals Takes advantage of human auditory systems
inability to hear quantization noise under auditory masking
Auditory masking: occurs when ever the presence of a strong audio signal makes a temporal or spectral neighborhood of weaker audio signals imperceptible.
CS 414 - Spring 2011
CS 414 - Spring 2011
MPEG/audio divides audio signal into frequency sub-bands that approximate critical bands. Then we quantize each sub-band according to the audibility of quantization noise within the band
MPEG Audio Bit Allocation This process determines number of code bits allocated to each sub-
band based on information from the psycho-acoustic model Algorithm:
1. Compute mask-to-noise ratio: MNR=SNR-SMR Standard provides tables that give estimates for SNR resulting from
quantizing to a given number of quantizer levels
2. Get MNR for each sub-band
3. Search for sub-band with the lowest MNR
4. Allocate code bits to this sub-band. If sub-band gets allocated more code bits than appropriate, look up new
estimate of SNR and repeat step 1
CS 414 - Spring 2011
MP3 Audio Format
CS 414 - Spring 2011
Source: http://wiki.hydrogenaudio.org/images/e/ee/Mp3filestructure.jpg
MPEG Audio Comments Precision of 16 bits per sample is needed to get good SNR
ratio Noise we are getting is quantization noise from the
digitization process For each added bit, we get 6dB better SNR ratio Masking effect means that we can raise the noise floor
around a strong sound because the noise will be masked away
Raising noise floor is the same as using less bits and using less bits is the same as compression
CS 414 - Spring 2011
MPEG-2 Standard Extension MPEG-1 was optimized for CD-ROM and
apps for 1.5 Mbps (video strictly non-interleaved)
MPEG-2 adds to MPEG-1: More aspect ratios: 4:2:2, 4:4:4Progressive and interlaced frame codingFour scalable modes
spatial scalability, data partitioning, SNR scalability, temporal scalability
CS 414 - Spring 2011
MPEG-1/MPEG-2
Pixel-based representations of content takes place at the encoder
Lack support for content manipulatione.g., remove a date stamp from a video turn off “current score” visual in a live game
Need support manipulation and interaction if the video is aware of its own content
CS 414 - Spring 2011
MPEG-4 Compression
CS 414 - Spring 2011
Interact With Visual Content
CS 414 - Spring 2011
Original MPEG-4
Conceived in 1992 to address very low bit rate audio and video (64 Kbps)
Required quantum leaps in compressionbeyond statistical- and DCT-based techniquescommittee felt it was possible within 5 years
Quantum leap did not happen
CS 414 - Spring 2011
The “New” MPEG-4 Support object-based features for content
Enable dynamic rendering of contentdefer composition until decoding
Support convergence among digital video, synthetic environments, and the Internet
CS 414 - Spring 2011
MPEG-4 Components Systems – defines architecture, multiplexing structure, syntax Video – defines video coding algorithms for animation of synthetic and
natural hybrid video (Synthetic/Natural Hybrid Coding) Audio – defines audio/speech coding, Synthetic/Natural Hybrid Coding
such as MIDI and text-to-speech synthesis integration Conformance Testing – defines compliance requirements for MPEG-4
bitstream and decoders Technical report DSM-CC Multimedia Integration Framework – defines Digital Storage
Media – Command&Control Multimedia Integration Format; specifies merging of broadcast, interactive and conversational multimedia for set-top-boxes and mobile stations
CS 414 - Spring 2011
MPEG-4 Example
ISO N3536 MPEG4 CS 414 - Spring 2011
MPEG-4 Example
ISO N3536 MPEG4 CS 414 - Spring 2011
MPEG-4 Example
Daras, P. MPEG-4 Authoring Tool, J. Applied Signal Processing, 9, 1-18, 2003
CS 414 - Spring 2011
Interactive Drama
http://www.interactivestory.netCS 414 - Spring 2011
MPEG-4 Characteristics and Applications
CS 414 - Spring 2011
Media Objects
An object is called a media objectreal and synthetic images; analog and
synthetic audio; animated faces; interaction
Compose media objects into a hierarchical representation form compound, dynamic scenes
CS 414 - Spring 2011
Composition
Scene
furniture
desk globe
character
voicesprite
ISO N3536 MPEG4 CS 414 - Spring 2011
Video Syntax Structure
CS 414 - Spring 2011
New MPEG-4 Aspect: Object-based layered syntactic structure
Content-based Interactivity Achieves different qualities for different objects
with a fine granularity in spatial resolution, temporal resolution and decoding complexity
Needs coding of video objects with arbitrary shapes
Scalability Spatial and temporal scalability Need more than one layer of information (base and
enhancement layers)
CS 414 - Spring 2011
Examples of Base and Enhancement Layers
CS 414 - Spring 2011
Coding of Objects Each VOP corresponds to an entity that after
being coded is added to the bit stream Encoder sends together with VOP
Composition information where and when each VOP is to be displayed
Users are allowed to change the composition of the entire scene displayed by interacting with the composition information
CS 414 - Spring 2011
Spatial Scalability
CS 414 - Spring 2011
VOP which is temporally coincident with I-VOP in the base layer, is encoded as P-VOP in the enhancement layer. VOP which is temporally coincident with P-VOP in the base layer is encoded as B-VOP in the enhancement layer.
Temporal Scalability
CS 414 - Spring 2011
Composition (cont.)
Encode objects in separate channelsencode using most efficient mechanism transmit each object in a separate stream
Composition takes place at the decoder,rather than at the encoder requires a binary scene description (BIFS)
BIFS is low-level language for describing:hierarchical, spatial, and temporal relations
CS 414 - Spring 2011
MPEG-4 Rendering
ISO N3536 MPEG4 CS 414 - Spring 2011
Interaction as Objects
Change colors of objects Toggle visibility of objects Navigate to different content sections Select from multiple camera views
change current camera angle Standardizes content and interaction
e.g., broadcast HDTV and stored DVD
CS 414 - Spring 2011
Hierarchical Model
Each MPEG-4 movie composed of trackseach track composed of media elements (one
reserved for BIFS information)each media element is an objecteach object is a audio, video, sprite, etc.
Each object specifies its:spatial information relative to a parent temporal information relative to global timeline
CS 414 - Spring 2011
Synchronization
Global timeline (high-resolution units)e.g., 600 units/sec
Each continuous track specifies relatione.g., if a video is 30 fps, then a frame should
be displayed every 33 ms.
Others specify start/end time
CS 414 - Spring 2011
MPEG-4 parts MPEG-4 part 2
Includes Advanced Simple Profile, used by codecs such as Quicktime 6
MPEG-4 part 10 MPEG-4 AVC/H.264 also called Advanced
Video Coding Used by coders such as Quicktime 7Used by high-definition video media like Blu-
ray DiscCS 414 - Spring 2011