AAC is a wideband audio coding algorithm that exploits two primary coding strategies to dramatically reduce the amount of data needed to represent high-quality digital audio. The MPEG-4 audio standard does not define a single or small set of highly efficient compression schemes but rather a complex toolbox to perform a wide range of operations from low bitrate speech coding to high-quality audio coding and music synthesis.
AAC encoders can switch dynamically between a single MDCT block of length points or 8 blocks of points or between points and points, respectively. AAC takes a modular approach to encoding. Depending on the complexity of the bitstream to be encoded, the desired performance and the acceptable output, implementers may create profiles to define which of a specific set of tools they want to use for a particular application.
Audio Object Types and their usage in brand new profiles. Applying error protection enables error correction up to a certain extent. Error correcting codes are usually applied equally to the whole payload. However, since different parts of an AAC payload show different sensitivity to transmission errors, this would not be a very efficient approach. Error Resilience ER techniques can be used to make the coding scheme itself more robust against errors. No licenses or payments are required to be able to stream or distribute content in AAC format.
However, a patent license is required for all manufacturers or developers of AAC codecs. However, it is restricted to only a single non-multiplexed AAC stream. In later years, Apple began offering music videos and movies, which also use AAC for audio encoding. On May 29, , Apple began selling songs and music videos free of DRM from participating record labels. These files mostly adhere to the AAC standard and are playable on many non-Apple products but they do include custom iTunes information such as album artwork and a purchase receipt, so as to identify the customer in case the file is leaked out onto peer-to-peer networks.
It is possible, however, to remove these custom tags to restore interoperability with players that conform strictly to the AAC specification.
The first such phone was the Nokia released in which also plays MP3s. However, this phone was a commercial failure and such phones with integrated music players did not gain mainstream popularity until when the trend of having AAC as well as MP3 support continued.
Most new smartphones and music-themed phones support playback of these formats. The list of systems which now or will soon use MPEG-2 is extensive and continuously growing: digital TV cable, satellite and terrestrial broadcast , Video on Demand, Digital Versatile Disc DVD , personal computing, card payment, test and measurement, etc. This is known as interaction, and is seen by many as the key discriminator between traditional video and MPEG At the moment, nobody is quite sure whether, when everything falls into place, the bulk of users will interact through their TV s or their Personal Computers - and exactly how much interaction the users want or will pay for.
The Moving Picture Coding Experts Group was established in January with the mandate to develop standards for coded representation of moving pictures, audio and their combination. The MPEG-2 standard has been extended by various groups including:. The MPEG-2 video compression algorithm achieves very high rates of compression by exploiting the redundancy in video information. MPEG-2 removes both the temporal redundancy and spatial redundancy which are present in motion video.
Temporal redundancy arises when successive frames of video display images of the same scene. Activity status:. Further, it is possible to switch into a 16x8 prediction mode, where separate motion vectors can be defined for the top and bottom halves of a macroblock.
The variable-length coding VLC tables were extended for better compression performance in higher data rates and resolutions. Methods of scalable coding are defined, which provide SNR scalability and spatial scalability over a limited number of layers each.
In this context, the bitstream is sub-divided into two or three parts, which means that by retaining or receiving only core parts of the stream, it is e.
To encode DCT coefficients related to the different resolution levels, differently optimized variable length codes are provided in the spatial scalable mode. A method of temporal scalability is defined, which allows prediction of additional inserted frames either from a base-layer sequence or from another frame of the enhancement layer sequence.
This method can also be used for encoding of stereoscopic sequences with an LRLRLR… interleaving of left and right pictures, as e. Methods of data partitioning for DCT coefficients are defined, which can improve the error resilience of video streams. Usage of B frames is not allowed. The standard defines 21 sets of capabilities, which are referred to as profiles , targeting specific classes of applications. For camcorders, editing, and professional applications, the standard contains four additional Intra-frame -only profiles, which are defined as simple subsets of other corresponding profiles.
These are mostly for professional e. As a result of the Scalable Video Coding SVC extension, the standard contains five additional scalable profiles , which are defined as a combination of a H. As the term is used in the standard, a " level " is a specified set of constraints that indicate a degree of required decoder performance for a profile. For example, a level of support within a profile specifies the maximum picture resolution, frame rate, and bit rate that a decoder may use.
A decoder that conforms to a given level must be able to decode all bitstreams encoded for that level and all lower levels. The maximum bit rate for High Profile is 1. Previously encoded pictures are used by H. This allows the encoder to make efficient decisions on the best way to encode a given picture.
At the decoder, such pictures are stored in a virtual decoded picture buffer DPB. The maximum capacity of the DPB is in units of frames or pairs of fields , as shown in parentheses in the right column of the table above, can be computed as follows:.
Where MaxDpbMbs is a constant value provided in the table below as a function of level number, and PicWidthInMbs and FrameHeightInMbs are the picture width and frame height for the coded video data, expressed in units of macroblocks rounded up to integer values and accounting for cropping and macroblock pairing when applicable. This formula is specified in sections A. It is important to note that the current picture being decoded is not included in the computation of DPB fullness unless the encoder has indicated for it to be stored for use as a reference for decoding other pictures or for delayed output timing.
Thus, a decoder needs to actually have sufficient memory to handle at least one frame more than the maximum capacity of the DPB as calculated above. Versions of the H. Each version represents changes relative to the next lower version that is integrated into the text. Bold faced versions are versions that include relatively major technical enhancements.
Because H. Compression efficiency depends on video algorithmic implementations, not on whether hardware or software implementation is used.
Therefore, the difference between hardware and software based implementation is more on power-efficiency, flexibility and cost. To improve the power efficiency and reduce hardware form-factor, special-purpose hardware may be employed, either for the complete encoding or decoding process, or for acceleration assistance within a CPU-controlled environment. CPU based solutions are known to be much more flexible, particularly when encoding must be done concurrently in multiple formats, multiple bit rates and resolutions multi-screen video , and possibly with additional features on container format support, advanced integrated advertising features, etc.
CPU based software solution generally makes it much easier to load balance multiple concurrent encoding sessions within the same CPU. A hardware H. An FPGA is a general programmable chip. A full HD H. ASIC encoders with H. Chat WhatsApp. Retrieved
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