UNIT V

Linear Predictive coding With the advent of inexpensive digital signal processing circuits, the source simply analyzing the audio waveform to determine a selection of the perceptual features it contains. These are then quantized and sent and the destination uses them, together with a sound synthesizer, to regenerate a sound i.e., perceptually comparable with the source audio signal. This is the basis of LPC technique.

Code excited LPC Code Excited Linear Prediction (CELP) is a speech coding algorithm originally proposed by M.R. Schroeder and B.S. Atal in At the time, it provided significantly better quality than existing low bit-rate algorithms, such as RELP and LPC vocoders (e.g. FS-1015). Along with its variants, such as ACELP, RCELP, LD-CELP and VSELP, it is currently the most widely used speech coding algorithm. CELP is now used as a generic term for a class of algorithms and not for a particular codec. Code Excited Linear Prediction (CELP) is a speech coding algorithm originally proposed by M.R. Schroeder and B.S. Atal in At the time, it provided significantly better quality than existing low bit-rate algorithms, such as RELP and LPC vocoders (e.g. FS-1015). Along with its variants, such as ACELP, RCELP, LD-CELP and VSELP, it is currently the most widely used speech coding algorithm. CELP is now used as a generic term for a class of algorithms and not for a particular codec. The principles are The principles are CELP uses more sophisticated model of vocal tract. CELP uses more sophisticated model of vocal tract. Standard audio segments are stored as waveform templates. Encoder and decoder both have same set of templates. It is called codebook. Standard audio segments are stored as waveform templates. Encoder and decoder both have same set of templates. It is called codebook. Every digitized segment is compared with waveform templates in codebook. Every digitized segment is compared with waveform templates in codebook. The matching template is differentially encoded codeword selects the matching template from codebook. The matching template is differentially encoded codeword selects the matching template from codebook.

Perceptual Coding: Frequency Masking: When multiple signals are present, a strong signal may reduce the level of sensitivity of the ear to other signals, which are near to it in frequency, which is known as frequency masking. Frequency Masking: When multiple signals are present, a strong signal may reduce the level of sensitivity of the ear to other signals, which are near to it in frequency, which is known as frequency masking. Temporal Masking: When the ear hears a loud sound, it takes a short but finite time before it can hear a quieter sound, which is known as temporal masking. Temporal Masking: When the ear hears a loud sound, it takes a short but finite time before it can hear a quieter sound, which is known as temporal masking.

MPEG audio coders The coders associated with the audio compression part of the MPEG standards are known as MPEG audio coders and a number of these use perceptual coding.

Dolby audio coders Dolby AC – 1 is used for audio recording. It is MPEG audio coding standard. It is used psychoacoustic model at the encoder has fixed bit allocations to each subband.

Video compression-Principles The types of frames in video compression are Frames that are encoded independently, they are called as Intracoded Frames or I-Frames. Frames that are encoded independently, they are called as Intracoded Frames or I-Frames. Frames that are predicted are intercoded frames. Frames that are predicted are intercoded frames.

Video compression-Principles GOP and prediction span GOP and prediction span GOP is a group of pictures i.e., the number of frames / pictures between uccessive I-frames. Typical value of GOP varies from 3 to 12. Prediction span: The number of frames between a P-frame and immediately preceding I or P frame is called prediction span. The Typical value of the prediction span lies from 1 tp 3. D-Frame D-Frame D-frames are inserted at regular intervals in the encoded sequence of frames. D-Frame is mainly used in movie and video on demand application. These are highly compressed frames and are ignored during the decoding of P and B frames. It consists of only DC coefficients and hence they generate low-resolution picture. The low-resolution pictures generated by D-frames are useful in fast forward and rewind applications. Motion estimation and motion compensation Motion estimation and motion compensation The accuracy of prediction is determined by how well any movement is estimated. This operation is called motion estimation and additional information must also be sent to indicate the difference is called motion compensation.

H.261 H.261 is video coding standard published by the ITU (International Telecom Union) in It was designed for datarates which are multiples of 64Kbit/s, and is sometimes called p x 64Kbit/s (p is in the range 1-30). These datarates suit ISDN lines, for which this video codec was designed for. H.261 is video coding standard published by the ITU (International Telecom Union) in It was designed for datarates which are multiples of 64Kbit/s, and is sometimes called p x 64Kbit/s (p is in the range 1-30). These datarates suit ISDN lines, for which this video codec was designed for. The coding algorithm is a hybrid of inter-picture prediction, transform coding, and motion compensation. The datarate of the coding algorithm was designed to be able to be set to between 40 Kbits/s and 2 Mbits/s. The inter-picture prediction removes temporal redundancy. The transform coding removes the spatial redundancy. Motion vectors are used to help the codec compensate for motion. To remove any further redundancy in the transmitted bitstream, variable length coding is used. The coding algorithm is a hybrid of inter-picture prediction, transform coding, and motion compensation. The datarate of the coding algorithm was designed to be able to be set to between 40 Kbits/s and 2 Mbits/s. The inter-picture prediction removes temporal redundancy. The transform coding removes the spatial redundancy. Motion vectors are used to help the codec compensate for motion. To remove any further redundancy in the transmitted bitstream, variable length coding is used.

MPEG Video standards The 3 MPEG standards are MPEG-1 MPEG-2 MPEG-4

MPEG-1 MPEG-1 defines a group of Audio and Video (AV) coding and compression standards agreed upon by MPEG (Moving Picture Experts Group). MPEG-1 video is used by the Video CD (VCD) format and less commonly by the DVD-Video format. The quality at standard VCD resolution and bitrate is near the quality and performance of a VHS tape. MPEG-1, Audio Layer 3 is the popular audio format known as MP3. As cheaper and more powerful consumer decoding hardware became available, more advanced formats such as MPEG-2 and MPEG-4 were developed. These newer formats are more complex and require more powerful hardware, but the formats also achieve greater coding efficiency, i.e., quality per bitrate.[citation needed]

MPEG-2 MPEG-2 is a standard for "the generic coding of moving pictures and associated audio information".[1] It describes a combination of lossy video compression and lossy audio compression (audio data compression) methods which permit storage and transmission of movies using currently available storage media and transmission bandwidth.

MPEG-4 MPEG-4 is an ISO/IEC standard developed by MPEG (Moving Picture Experts Group), the committee that also developed the Emmy Award winning standards known as MPEG-1 and MPEG-2. These standards made interactive video on CD-ROM, DVD and Digital Television possible. MPEG-4 is the result of another international effort involving hundreds of researchers and engineers from all over the world. MPEG-4, with formal as its ISO/IEC designation 'ISO/IEC 14496', was finalized in October 1998 and became an International Standard in the first months of The fully backward compatible extensions under the title of MPEG-4 Version 2 were frozen at the end of 1999, to acquire the formal International Standard Status early in Several extensions were added since and work on some specific work-items work is still in progress. MPEG-4 builds on the proven success of three fields: MPEG-4 is an ISO/IEC standard developed by MPEG (Moving Picture Experts Group), the committee that also developed the Emmy Award winning standards known as MPEG-1 and MPEG-2. These standards made interactive video on CD-ROM, DVD and Digital Television possible. MPEG-4 is the result of another international effort involving hundreds of researchers and engineers from all over the world. MPEG-4, with formal as its ISO/IEC designation 'ISO/IEC 14496', was finalized in October 1998 and became an International Standard in the first months of The fully backward compatible extensions under the title of MPEG-4 Version 2 were frozen at the end of 1999, to acquire the formal International Standard Status early in Several extensions were added since and work on some specific work-items work is still in progress. MPEG-4 builds on the proven success of three fields: Digital television; Digital television; Interactive graphics applications (synthetic content); Interactive graphics applications (synthetic content); Interactive multimedia (World Wide Web, distribution of and access to content) Interactive multimedia (World Wide Web, distribution of and access to content)