![ The MDCT was proposed by Princen, Johnson, and
Bradley[1] in 1987, following earlier (1986) work by
Princen and Bradley[2] to develop the MDCT’s
underlying principle of time-domain aliasing
cancellation (TDAC), described below.
In MP3, the MDCT is not applied to the audio signal
directly, but rather to the output of a 32-band polyphase
quadrature filter (PQF) bank. The output of this MDCT
is post processed by an alias reduction formula to
reduce the typical aliasing of the PQF filter bank.](https://image.slidesharecdn.com/presentation1-150506050338-conversion-gate02/85/Audio-compression-33-320.jpg)
Audio compression
- 1. Prepared By:Avni Guna. Khushali Panasala. Yogesh Pandey. Priyanka Pandey. Hiteshri Patel. Guided By:Mr.Chandresh Parekh.
- 2. Introduction What Is Audio? Why Compression is needed? Audio Compression. Types Of Audio Compression. Standard codecs for audio compression. Categories of Audio Files MPEG Audio Encoding Steps MPEG Audio decoding Successor of MP3
- 3. High quality audio coding based on perceptual models has found its way to widespread application in broadcasting and Internet audio (e.g. mp3). Algorithms defined by the MPEG group (MPEG-1 Audio, e.g. MPEG Layer-3 (mp3), MPEG-2 Advanced Audio Coding, MPEG-4 Audio including its different functionalities) still define the state of the art.
- 4. Audio is an electrical or other representation of sound. An audio file format is a file format for storing digital audio data on a computer system.
- 5. • Compression is the reduction in size of data in order to save space or transmission time. • Compression can be used to: Reduce File Size Save disk space Reduce transmission time • Compression is performed by a program that uses an algorithm to determine how to compress or decompress data.
- 6. • Audio compression is a form of data compression designed to reduce the size of audio files. • There is a conditions on this definition : the audio file must still be playable after compression, without decompressing it to original size when you want to play it (for example with WinRAR). If the file is compressed 'too much' there will be loss of quality. The compression is done with a thing called a codec. This is an aggregation of the words: compressor and decompressor. This codec is a special algorithm to reduce the size.
- 8. There are mainly two types of audio compression show below: 1)Lossy Compression(MP3) 2)Lossless Compression(Winzip)
- 9. • A compression technique that does not decompress data back to 100% of the original. • Lossy methods provide high degrees of compression and result in smaller compressed files, but there is a certain amount of visual loss when restored. • Example:MP3
- 10. • A compression technique that decompresses data back to its original form without any loss. • The decompressed file and the original are identical. For example, the ZIP archiving technology (WinZip...) is the most widely used lossless method.
- 11. • Lossless audio files typically require more storage space than Lossy encoded ones. • However this type of format is often favored by users wanting to backup original audio CDs. • A perfect copy can be restored in the event of loss or damage to the CD. FLAC, Apple Lossless (ALAC) and WMA Lossless are examples of lossless compression formats.
- 12. For lossy compression: • Nero AAC Codec (Nero “advanced audio coding” codec): It was developed and distributed by Nero AG. • FAAC(Freeware Advanced Audio Coder):is an audio compression computer program that creates AAC sound files from other formats , it is the recommended format for the company's iPod music player.
- 13. For lossless compression • LPAC (Lossless predictive audio compression):is an improved lossless audio compression algorithm developed by Tilman Liebchen, Marcus Purat and Peter Noll. • ALAC (Apple Lossless Audio Codec):is an audio coding format, and its reference audio codec implementation, developed by Apple. • FLAC(Free Lossless Audio Codec): can typically reduce the original size of audio file to 50–60%, and decompressed it to an identical copy of the original audio data, developed by Josh Coalson. • WMA Lossless (Windows Media Audio Lossless): developed by Microsoft
- 14. Moving Picture Experts Group Aim to create standards relating to synchronized audio and video compression MPEG-1 MPEG-2
- 15. Quite good high bit-rate is available Digital audio tap Complex Digital Audio Broadcasting most complex audio transmission over ISDN lines
- 16. There are 3 categories in which certain Audio files belong to:- 1) Uncompressed: Ex) .Wav 2) Lossless: Ex) .WMA 3) Lossy: Ex) .Mp3
- 24. ◦ Hearing threshold level – a function of frequency ◦ Any frequency components below the threshold will not be perceived by human ear
- 25. A frequency component can be partly or fully masked by another component that is close to it in frequency A lower tone can effectively mask higher tone This shifts the hearing threshold
- 26. ◦ A quieter sound can be masked by a louder sound if they are temporally close Sounds that occur both (shortly) before and after volume increase can be masked
- 28. Power-law: larger values have less accuracy quantization: uniform or non-uniform quantization. coding: quantized spectral components are transmitted either directly, or as entropy coded words (Huffman coding) For better data compression, variable- length Huffman codes are used to encode the quantized samples. Resultant bitstream is now reduced, because of coarser quantisation, but can be further reduced by the use of Huffman coding.
- 29. formats encoded quantized samples into an encoded bit stream – final form in which the compressed signal is transmitted. Header (First 4 bytes of a frame) ◦ Contains: Frame Sync, MPEG Layer, Sampling Frequency, Number of Channels, CRC, etc. ◦ Variable bit rate mp3’s switch bitrate between frames
- 30. Decoder side relatively easier. The gain, scale factor, quantization steps recovered and used for reconstruct the filter bank responses Filter bank responses are combined to reconstruct the decoded audio signal
- 31. Advanced Audio Coding (AAC) – now part of MPEG- 4 Audio Inclusion of 48 full-bandwidth audio channels Default audio format for iPhone, iPad, Nintendo, PlayStation, Nokia, Android, BlackBerry Introduced 1997 as MPEG-2 Part 7 In 1999 – updated and included in MPEG-4 standard for lossy digital audio compression. Designed to be the successor of the MP3 format, AAC generally achieves better sound quality than MP3 at similar bit rates.
- 32. Opus is a lossy audio coding format developed by the Internet Engineering Task Force (IETF) that is particularly suitable for interactive real-time applications over the Internet. Opus incorporates technology from two other audio coding formats: the speech-oriented SILK and the low latency CELT.
- 33. The MDCT was proposed by Princen, Johnson, and Bradley[1] in 1987, following earlier (1986) work by Princen and Bradley[2] to develop the MDCT’s underlying principle of time-domain aliasing cancellation (TDAC), described below. In MP3, the MDCT is not applied to the audio signal directly, but rather to the output of a 32-band polyphase quadrature filter (PQF) bank. The output of this MDCT is post processed by an alias reduction formula to reduce the typical aliasing of the PQF filter bank.
- 34. Such a combination of a filter bank with an MDCT is called a hybrid filter bank or a sub band MDCT. AAC, on the other hand, normally uses a pure MDCT; only the (rarely used) MPEG-4 AAC-SSR variant (by Sony) uses a four band PQF bank followed by an MDCT. Similar to MP3, ATRAC uses stacked quadrature mirror filters (QMF) followed by an MDCT.
- 35. ISO/IEC • MPEG-1 Layer III (MP3) • MPEG-1 Layer II • MPEG-1 Layer I • AAC • MPEG-4 ALS • MPEG-4 SLS • MPEG-D USAC
- 36. ITU-T • G.711 • G.718 • G.719 • G.722 • G.723 • G.726 • G.728 • G.729
- 37. • audio compression is a key technology • many algorithms many applications • Better algorithms better quality, more compression
- 38. Thank You