MULTIPLEXING OF AVS CHINA PART 2 VIDEO WITH AAC BIT STREAMS AND DE MULTIPLEXING WITH LIP SYNC DURING PLAYBACK Swaminathan Sridhar Multimedia Processing Lab University of Texas at Arlington Thesis outline What is multiplexing Applications of multiplexing The need for choosing AVS video and AAC audio codecs Video audio elementary stream formats Multiplexing process De multiplexing process Lip synchronization during playback Results and conclusions Future work References What is multiplexing A multimedia program is a combination of multiple elementary streams such as video and audio Multiplexing is the process of converting multiple elementary streams such as video an audio streams into a single transport stream for transmission It conserves the usage of transmission channels Applications of multiplexing Multiplexing is used in areas of applications such as ATSC DVB T DVB S DVB H IPTV The digital transmission reception process adopted in the ATSC standard 22 The need for video and audio compression With the advent of high definition television transmission schemes high quality video and audio data are transmitted which occupy a lot of bandwidth over a transmission channel To address this issue the video and audio data are compressed using efficient compression schemes such as AVS China video codec and AAC audio codec Why AVS China video AVS audio video coding standard China is the latest digital video coding standard developed by the AVS work group of China AVS video codec employs the latest video coding tools which primarily targets standard definition SD and high definition HD video compression Compared to the previous video coding standards such as MPEG 2 and MPEG 4 AVS achieves the same quality of video with significantly lower bit rates or vice Overview of AVS China standard 5 AVS video profiles their applications 4 Coding tools of AVS part 2 video codec Intra prediction 8x8 block based intra prediction 5 modes for the luminance component namely the DC horizontal vertical down left and down right and 4 modes for the chrominance component namely the DC horizontal vertical and plane mode are specified Motion compensation 16x16 16x8 8x16 and 8x8 block sizes Motion vector resolution pixel accuracy with 4 tap interpolation filter Transform 8x8 integer cosine transform Quantization and scaling with scaling only in the encoder Entropy coding context based 2D VLC De blocking filter performed around the 8x8 boundaries AVS China part 2 video encoder 2 AVS video decoder 12 AVS video encoded bit stream format Start code It consists of start code prefix and start code value Start code prefix A string of 23 zero bits followed by a single bit with a value of 1 i e 0x000001 which are all byte aligned This is followed by start code value Start code value It is an 8 bit integer that identifies the start code type Start code types start code values used in the AVS video bit stream 8 Picture coding type used in AVS video bit stream Pb picture start code The bit string format is 0x000001B6 which indicates the start code of P or B picture Picture coding type It is a 2 bit unsigned integer which specifies the coding type of a Picture coding type Coding picture as shown in Table 1 type 00 Forbidden 01 Forward inter prediction P 10 Bidirectional inter prediction B 11 Reserved Table 1 Coding type of a picture 8 NAL unit NAL unit stands for network abstraction layer unit which is a type of packetization that prefixes certain headers to the encoded video bit stream It was designed to provide a network friendly environment for transmission of video data It mainly addresses video related applications such as video telephony video storage broadcast and streaming applications IPTV etc The syntax for NAL unit is defined in H 264 standard but AVS part 2 standard does not define any syntax for the NAL unit NAL unit mapping with the encoded AVS video stream The basic syntax for the NAL unit is shown in figure 1 Figure 1 NAL unit syntax 13 NAL unit consists of a 8 bit header followed by the payload The procedure for mapping AVS video stream with NAL unit is to map the data between every start code prefixes i e 0x000001 in the AVS NAL unit header description It is a 8 bit header consisting of the following parameters Forbidden zero bit which is a 1 bit value and it is always 0 Nal ref idc which is a 2 bit unsigned integer value It indicates the priority of the type of data carried in the NAL unit based upon the start code type This value should not be zero for I frames Nal unit type which is a 5 bit unsigned integer value and therefore 32 types of NAL units are allowed This value indicates the type of data carried in the NAL payload NAL unit type according to the start code values 14 Why AAC audio AAC codec showed superior performance at both low and high bit rates as compared to MP3 and AC3 Supports up to 48 audio channels with a wide variety of sampling frequencies from 8 KHz to 96 KHz The first codec to achieve ITU R broadcast quality at a bit rate of 128 Kb s for stereo The encoding efficiency is nearly 30 more than MP3 MPEG 1 2 audio layer 3 AAC audio Advanced audio coding is a standardized lossy compression scheme for coding the digital audio It has been standardized under the ISO IEC as part 7 of the MPEG 2 standard and part 3 of the MPEG 4 standard AAC profiles Main profile Provides the highest audio quality and is the most complex Low complexity profile Achieves nearly the same audio quality as the main profile but with significant savings on the memory and process requirements Scalable sampling rate profile It provides flexibility for scalable and low complexity applications It is more appropriate in applications where bandwidth is a constraint AAC audio stream format ADIF Audio Data Interchange Format This format uses only one header in the beginning of the file followed by the raw audio data blocks It is generally used for storage applications ADTS Audio Data Transport Stream This format uses separate header for each frame enabling decoding from any frame This format is mainly used for transport applications ADTS header format 18 Factors to be considered for multiplexing and transmission The audio and video coded bit streams are split into smaller data packets The frame wise arrangement of the coded video and audio streams help in forming small data packets While multiplexing equal priority is given to all the elementary streams Additional information to help synchronize the audio and video at the de
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