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Lecture 11 – MP3 Audio & Introduction to MPEG-4 (Part 6)

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Lecture 11 – MP3 Audio & Introduction to MPEG-4 (Part 6) 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 CS 414 - Spring 2011 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 Characteristics Precision 16 bits Sampling frequency: 32KHz, 44.1 KHz, 48 KHz 3 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 7 7 (2i +1)(k −16)π St[i] = ∑3∑cos( *(C[k + 64 j]* x[k + 64 j] k =0 j=0 64 i ∈[0,31], St[i]- 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 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 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 CS 414 - Spring 2011 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 Source: http://wiki.hydrogenaudio.org/images/e/ee/Mp3filestructure.jpg CS 414 - Spring 2011 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, 4:4:4 Progressive and interlaced frame coding Four 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 manipulation e.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 compression beyond statistical- and DCT-based techniques committee 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 content defer 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 CS 414 - Spring 2011 ISO N3536 MPEG4 MPEG-4 Example CS 414 - Spring 2011 ISO N3536 MPEG4 MPEG-4 Example Daras, P. MPEG-4 Authoring Tool, J. Applied Signal Processing, 9, 1-18, 2003 CS 414 - Spring 2011 Interactive Drama CS 414 - Spring 2011 http://www.interactivestory.net MPEG-4 Characteristics and Applications CS 414 - Spring 2011 Media Objects An object is called a media object real 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 character furniture sprite voice desk globe CS 414 - Spring 2011 ISO N3536 MPEG4 Video Syntax Structure New MPEG-4 Aspect: Object-based layered syntactic structure CS 414 - Spring 2011 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 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. CS 414 - Spring 2011 Temporal Scalability CS 414 - Spring 2011 Composition (cont.) Encode objects in separate channels encode 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 CS 414 - Spring 2011 ISO N3536 MPEG4 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 tracks each track composed of media elements (one reserved for BIFS information) each media element is an object each 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 relation e.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 7 Used by high-definition video media like Blu- ray Disc CS 414 - Spring 2011.
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