Subject : multimedia
Explain in detail about MPEG-1 audio
Download 435.21 Kb.
|
- Navigate this page:
- Explain about MPEG1 – Video. MPEG -1 VIDEO
Explain in detail about MPEG-1 audio. . MPEG-1 AUDIO: The MPEG-1 Audio standard, defined in ISO Recommendation, It provides compression algorithm for digital audio. The approximate values corresponding to the bit rates and compression ratio for the three layers are given below:
The rates are for a single channel. Altogether there are four different audio channel formats. These are monophonic where only one single channel corners audio signals, dual mono where the same mono signal is carried over two channels, stereophonic where two channels carry different signals and joint stereo where signals are combined and carried over a single channel.
It describes the least sophisticated method that requires relatively high data rates of about 192Kbps/channel. The compression ratio is about 4:1 and the quality is same as that of digital audio cassette.
In 1991 there were two proposals: MUSICAM (which latter came to known as Layer II) and ASPEC (Adaptive Spectral Perceptual Entropy Coding). MUSICAM was chosen because of its simplicity and error résistance. ASPEC and added their own ideas to create the MP# format which was designed to achieve the same quality as MP@ but at a lower bit rate. This CD was chosen because of its softness and simplicity making it easier to hear imperfection in the compression format during playback. In 1993 MP@ files first appeared on the internet and were often played back using the Xing MPEG player, and latter in a program in UNIX called MAPlay released in 1994. Initially the only encoder available for MP2 production was the Xing encoder.
The MPEG Layer III data reduction algorithm is widely used to compress files prior to electronic distribution. The Layer III algorithm is widely known as MP3. The MP3 CODEC like other MPEG compression algorithm is lossy in nature. This means that it discards data from the original uncompressed file to reduce its file size. The MP3 encoder first converts the input audio from the temporal domain to the frequency domain using DFT (Discrete Fourier Transform). The frame packing block adds error checking codes before storing the compressed data as an MP3 files. MP3 files can be stored on a hard disk, recorded to CD-ROM or store in solid state players using flash memory. Frame Packing Quantizer and Coding Psycho-acoustic model Mapping into Frequency sub-band (DFT) PCM audio compressed Input audio data (MPEG -1 Audio Encoder) Synthesis, i.e. frequency domain to temporal domain (IDFT) Dequantization Compressed PCM Audio Audio Data Samples
MPEG -1 VIDEO: To suit a range of applications the user can specify a set of input parameters including frame size and frame rate. Important feature provide by MPEG-1 include frame based random access of video. To achieve a high compression ratio, both intra- frame and inter-frame redundancy are exploited. The MPEG-1 video uses three different types of frames: I-frames, P-frames and B-frames.
These are coded without any reference to other images. MPEG makes use of JPEG coding for I frames. I-Frames are present as regular intervals in order to allow for the possibility of the contents of an I-Frame being corrupted during transmission. The collection of frames between successive I-frames is known as group of pictures or GOP.
P frames can be accessed only after the referenced I or P frames has been decoded. Encoding of P-frames is done using a combination of frame replenishment and motion compensation. The number of frames a between a P-frame and the proceeding I or P frame is called Prediction Span and typical values range from 1 to 3.
B frames are never used as reference for other frames. Reference frames must be transmitted first. Thus, transmission order and display order may differ. I-Frame B-Frame P-Frame (I, P and B Frames)
During the encoding phase, the digitized content of the Y array associated with each frame are first divided into a two dimensional array of 16 by 16 pixels known as Macroblack. If a match is not found, the search is extended to cover a number of macro blocks in the neighboring area. If a neighboring macro block is found to match with the current one, a motion vector is created encoding the offset between these two macroblocks. The difference in the pixel values between these two macroblocks are encoded in a set of arrays (for Y, Cb, and Cr) called the prediction error. 4(8*8) blocks of Y 1(8*8) blocks of Cb 1(8*8) blocks of Cr
(1 macroblock= 4 blocks of Y+1 block of Cb+1 block of Cr) (Macroblock) To encode a B-frame the motion is estimated with reference to both the immediately preceding I-or P-frame and the immediately succeeding P- or I-frame. In this case two motion vectors and two sets of prediction error array are computed. The B-frame the contents are used in combination with the decoded contents of the previous or following I- or P-frames. A typical sequence of stored frames can be as follows. IBBPBBPBBI…. When these frames are transmitted from one place to another, it can be easily seen that the first two B- frames cannot be decoded fully until the first P- frame arrives. Hence for transmission purpose the order of the frames are changed to: IPBBPBBIBBPBB… Hence the transmission order of frames is generally different from the storage order
This refers to the encoding to two neighbouring P and B frames as a single frame. This enables increasing the frame rate without significantly increasing the resulting bit rate.
These are used to display the content when the user rewinds or fast forwards through the movie and are ignored during the normal decoding of frames. The D-frames are highly compressed and inserted at regular intervals through the stream. Multimedia Page Directory: images images -> Pelan pentaksiran kursus images -> Military callsign list as of dec 2010 images -> For Date: 02/01/2013 Friday Call Number Time Call Reason Action 13-768 0008 motor vehicle stop warning Issued images -> New embedded S images -> Creating Buy-In – Building Relationships images -> Deadline: Saturday, March 12th, 2016 images -> News items from Raising Our Voices images -> Last year four of the best Afrikaans Christian singers joined forces to tour Down Under with the production ‘Manne wat Glo’ images -> World-renowned vocal ensemble sweet honey in the rock images -> Peoples Voice Café History Download 435.21 Kb. Share with your friends:
|