![]() It embraces SBR technology to encode and store high frequency information as part of the standard, and is able to deliver near-CD quality sound at 64 kbit s − 1. It is specifically optimised for very-low-bit-rate applications such as audio streaming and podcasting, and is now the standard technology used in digital radio broadcasting. ![]() The most recent AAC extension is High-Efficiency AAC (HE-AAC) also known as AACplus. Therefore AAC has integrated different applications into a single framework covering music synthesis, low-bit-rate speech coding, text-to-speech synthesis and general perceptual audio compression across a host of different bit rates. Since AAC involves many varied processes in analysing different types of audio signal, no single algorithm is able to meet the diverse set of requirements it must fulfil. Figure 3.8 MPEG-AAC audio encoder familyĪAC-LC (low complexity) is the most widely used coding profile in this standard, and the default format for Apple’s iTunes. The connections are shown by three dashed vertical lines. The three squares are connected to the three arrows between the four rectangles. Between each pair of rectangles is a short horizontal arrow pointing to the right, so three arrows in total. From left to right, these are labelled MPEG-2 AAC-LC, MPEG-4 AAC-LC, MPEG-4 HE-AAC and MPEG-4 HE-AAC v2. From left to right, these are perceptual noise substitution (PNS), spectral band replication (SBR) and parametric stereo (PS).Ībove the three squares is a row of four rectangles. The figure shows three squares in a row, representing the key tools that have been instrumental in the advancement of the MPEG-4 AAC family of standards. While each tool to some extent adds complexity to the encoder, it also provides notable improvements in coding efficiency and corresponding audio quality. ![]() Figure 3.8 identifies three key tools that have been instrumental in the advancement of this standard:įurther information on each of these is readily available on the Web. MPEG-4 AAC and its variants excel at low bit rates by virtue of a series of extensions and tools that have evolved and subsequently become embedded into the standard. This is a consequence of AAC supporting scalable representations in terms of sample amplitudes (or S/ N ratio) and sampling rates. Instead the file is coded once, then streamed at a variable bit rate depending on the connection speed and network traffic conditions. In terms of coding, it uses either 2048 or 256 sub-bands compared to 32 for MP3, thus providing better frequency resolution for the psychoacoustic modelling and perceptual masking steps.Īnother noteworthy feature of AAC encoders is that audio files do not have to be encoded at a specific streaming speed. In comparison with MP3, AAC offers a range of sampling rates up to 96 kHz, and also supports up to 48 channels (mono, stereo and multichannel surround sound). The broad consensus is that, subjectively, the AAC encoder (.mp4 files) provides better audio quality for the same bit rate as MP3, with greater flexibility and functionality. So-called ‘5.1 surround sound’ includes five full bandwidth channels (left, right, centre, left surround and right surround), with the ‘point 1’ referring to a dedicated low frequency effect (LFE) channel carrying bass information in the 3 to 120 Hz band.ĪAC has now been formally embedded in both the MPEG-2 and MPEG-4 audio standards it is the default format for various multimedia applications and services, from YouTube to Apple’s iTunes. Its development was also motivated by the quest for efficient coding of multichannel surround-sound signals. MPEG-4 AAC (advanced audio coding) was designed as the successor to MP3 for low-bit-rate perceptual audio compression, with efficient internet multimedia streaming applications in mind.
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