MPEG-1 / MPEG-2 BC Audio. Prof. Dr.-Ing. K. Brandenburg, bdg@idmt.fraunhofer.de Dr.-Ing. G. Schuller, shl@idmt.fraunhofer.de

Transcription

1 MPEG-1 / MPEG-2 BC Audio

2 The Basic Paradigm of T/F Domain Audio Coding Digital Audio Input Filter Bank Bit or Noise Allocation Quantized Samples Bitstream Formatting Encoded Bitstream Signal to Mask Ratio Psychoacoustic Model

3 MPEG Audio Standardization Philosophy (1) Definition of a complete transmission chain consists of specification of Encoding algorithm Bitstream format Decoding algorithm ITU-T Approach MPEG Approach ITU-T standardizes all three parts Encoder output predictable MPEG standardizes only bitstream format and decoder, not the encoder ( informative part )

4 MPEG Audio Standardization Philosophy (2) Motivation: open for further improvements, room for specific corporate know-how But: No sound quality guaranteed!

5 MPEG-1/2 Audio MPEG-1 Audio Audio coding khz, mono/stereo Layer 1, 2, 3 Layer-3 (aka.mp3) optimized for lower bit-rates Copy protection via SCMS included MPEG-2 Audio Low sampling frequencies audio add khz to Layer 1, 2, 3 Multichannel audio, BC

6 MPEG-1 Development History IRT (MASCAM) Philips CCETT AT&T (PXFM) Fraunhofer-IIS UNI-Erl.(OCF) Thomson Brandt (MSC) CNET MUSICAM ASPEC simplified version Psychoacoustic model 2 (optional) LAYER 1 LAYER 2 LAYER 3

7 MPEG-1 Audio Developed Dec. 88 to Nov. 92 Coding of mono and stereo signals Bitrates from 32 kbit/s to 448 kbit/s Three "Layers": Layer 1: lowest complexity Layer 2: increased complexity and quality Layer 3: highest complexity and quality at low bit-rates Target bitrates 384 kbits/s, 256 kbit/s, < 192 kbit/s for Layers 1, 2, 3 respectively Sampling frequencies supported: 48 khz, 44.1 khz, 32 khz

8 The main building blocks Perceptual model using psychoacoustics, mostly proprietary Filter bank subdividing the input signal into spectral components more lines more coding gain longer impulse response pre-echo artifacts Quantization and coding this is the step introducing quantization noise spectral shape of quantization noise determines the audibility can be designed to leave encoding methods optional

9 MPEG Audio - Short Description of the Layers (1) Layer I Frame length: 384 samples (8 ms@ 48 khz) Frequency resolution: 32 subbands Quantization: Block-companding (12 samples), amplitude of subband samples indicated by scalefactors (SCF); 2 db resolution Layer II Frame length: 1152 samples (24 ms@ 48 khz) Frequency resolution: 32 subbands Quantization: Block-companding (12 samples) Use of Scalefactor select information

10 MPEG Audio - Short Description of the Layers (2) Layer III Standard frame length: 1152 samples (24 48 khz) Frequency resolution: 576/192 subbands Quantization: non-uniform with Huffman coding Use of Scalefactor Select Information

11 MPEG - Layer-1, -2 and -3 Compression: Header

12 MPEG-1 Layer 1

13 Block Diagram MPEG-1 Layer 1

14 MPEG Audio - Layer-1

15 MPEG Audio - Layer-1 (2)

16 MPEG Audio Layer-1 Signal-to-Noise Ratios

17 MPEG-1 Audio Layer 1 Bitstream Syntax

18 MPEG-1 Layer 2

19 MPEG Audio - Layer-2 (1) Processing of 1152 sample long frames 32 Sub-bands used; grouping in 3 granules of 12 subband samples Layer II additionally offers coding of bit allocation, scalefactors and samples The number of necessary scalefactors can vary Use of different windows Theoretically Minimum of the Coder-/Decoder- Delays around 35 ms

20 MPEG-1 Audio Layer 2 Bitstream Syntax

21 MPEG-1 Audio Layer 2 Bitstream Syntax (2)

22 MPEG-1 Layer 3

23 MPEG Audio Layer 3 MP3 is the nickname of the ISO/IEC-Standard for audio compression described by ISO/IEC IS (MPEG-1 Layer-3) and (MPEG-2 Layer-3) The benefits of MP3-formats is that it is a headerless file format, which means that it is not necessary to have the header to play the music Allows MP3 streaming Theoretic minimum delay of the Coder/Decoder is around 59 ms

24 Block diagram Layer-3 32 MDCT s

25 MPEG Layer-3 Same basic configuration as Layer-2: Frame length 24 ms at 48 khz Polyphase filter bank Specifics of Layer-3: Hybrid filter bank (32*18 = 576 subbands or 32*6=192 subbands) nonuniform quantization (implicit noise shaping) with a power law ( ^.75) Huffman coding Analysis-by-Synthesis structure Bitreservoir (short time buffer) Support for variable bitrate (mandatory)

26 MPEG-1 Layer 3 Psychoacoustic mode Block Size often used: Psychoacoustic model 2 1 Frame = 1152 samples = 36 samples per Polyphase filter bank subband Hybrid Filter bank Additional Modified Discrete Cosine Transform (MDCT), and aliasing reduction stage

27 Hybrid Filter Bank & Aliasing (1) e.g. 32kHz 32 bands Aliasing in the subbands e.g. 32Hz 32*18=576 bands 6-18 bands 32 bands 6 or 18 pre-echo avoidance better compression Filter bank is critically sampled Problem of aliasing in the analysis filter bank

28 Hybrid Filter Bank & Aliasing (2) Nyquist frequencies/channel boundaries 3dB y k signal mirrored inside aliasing cannot be neglected

29 Hybrid Filter Bank & Aliasing (3) mirrored original Nyquist frequency f p Mirroring of the original signals greater than the nyquist frequency occurs on downsampling

30 Problem of Aliasing in a Cascaded Filter Bank (1) Result of cascading: Far off frequencies are aliased into the SB of the 2nd filter bank Observe: Aliasing spreads over several subbands of second stage! Not the case for only single stage FB (practically) attenuation of the filter Nyquist frequency subbands of 2nd FB first stage filter bank signal aliased signal subband of 1st FB

31 Problem of Aliasing in a Cascaded Filter Bank (2) Frequency response contains peaks from the aliasing: passband peaks continue aliasing aliasing not just one passband, but several, only slightly attenuated Reduced by alias reduction Signal in many subbands Must be coded in many subband Worse coding efficiency

32 Aliasing Reduction Structure (MP3) similar approach as synthesis FB uses to reduce/cancel aliasing 32 bands 6/18 bands 6/18 bands - - (-1) n (1,-1,1,-1, ), high / low frequencies n are mirrored every second subband suitable factors (fixed), obtained from frequency response of filters, for equal level of aliasing

33 Problem of Aliasing in a Cascaded Filter Bank (3) Solution: Less downsampling in first stage (non-critical sampling) better filter Aliasing reduction with subtraction from neighboring bands (e.g.mp3) no cascaded filter bank (e.g. AAC)

34 Huffman Coding Details Vector coding One codeword for 2 or 4 subbands Escape sequence Large words are coded as sum of ESC-pointer + difference Adaptive table selection (signal table number) Table selection according to maximum Table selection according to local statistics Adaptive sectioning (4 sections) Each section defines Huffman table Coding efficiency approaches theoretical limits!

35 Layer-3 Iteration Loops Iteration Loops Scale Factors Rate/Distortion Control Process Quantizer Noiseless Coding

36 Possibilities to control the Quantization Two basic ways Bit Allocation Noise Allocation Detailed possibilities: Bit allocation between fixed Worst-Case and Maximum-SNR situations Bit allocation is calculated from the Threshold Estimation Direct calculation of the allowed noise (Noise Allocation) Simplified Noise Allocation

37 Layer-3 : Outer Loop Distortion Loop (control of the distortion) Saves the unquantized spectral values Compares the reconstructed values with the original Builds the actual distortion in the frequency domain Scaling by frequency groups with the amount of distortion Convergence of the iteration is not guaranteed

38 Layer-3 : Inner Loop Rate Loop (Data rate controller) Entropy coding: Data rate depends on actual data set Buffer Control: Controls the necessary bits Convergence through iterations: is always possible Beginning level: Calculated from SFM (Spectral Flatness Measure)

39 MPEG Audio - Layer-3: Bitstream Organization of the bit streams Fixed length of bytes: 17 at mono, 32 at stereo, independent of the bitrate Constant Section Header (ISO Standard, like with Layer-1 and -2) Additional information for a frame (e.g. Pointer to the variable section) Additional information for each granule (e.g. Number of the Huffman-Code table) Variable Section Scalefactors Huffman-coded frequency lines Additional Data

40 MPEG Audio - Layer-3: Bitstream (2)

41 MPEG Audio - Layer-3: Bitstream (3)

42 MPEG Audio - Layer-3: Bitstream (4)

43 MPEG-1 Audio Decoder

44 MPEG Audio General Decoder Structure

45 MPEG - Audio Decoder Process (1) Layer-1 and -2 Decoder flow chart

46 MPEG - Audio Decoder Process (2) Layer-3 Decoder flow chart

47 MPEG - Audio Decoder Process (3) Synthesis of the subband filter

48 MPEG - Audio Decoder Process (4) Layer-3 Decoder Diagramm

49 Stereo Coding in MPEG-1/-2

50 MPEG Audio - Layer-1 and Layer-2 For better stereo performance, Intensity Stereo (IS) can be applied With IS-Coding, a mono signal is transmitted in higher frequency bands and the decoder places it close to the original stereo position

51 MPEG Audio - Layer-3 (3) "Joint Stereo"-Coding to additionally improve compression: Mid/Side coding (M/S) and Intensity Stereo (IS) M/S coding: two channels are coded where left is actually the sum of the original left and right and the right channel is the difference Either both channels are separately coded ("stereo"- mode) or "Intensity Stereo"-Coding is used

52 MPEG-2 Audio

53 MPEG-2 Audio LSR extensions: Technical Background Implications Run coder at half the sampling rate Only minor changes required (e.g. tables) Increased frequency resolution Less bitrate for side information Better coding efficiency Only one half of the workload required (SW implementations!) But: Transparent coding not possible due due restriction in bandwidth (max. 12 khz) All current MP3 decoders support this mode

54 MPEG-2 MC Features Sound Formats L C R Supports 3/2+1 multi-channel sound (ITU-R): Left, right, center, left surround, right surround Low frequency enhancement channel (LFE) LS RS Downward compatibility : Also 3/1, 3/0, 2/2, 2/1, 1/0 supported Multi-Lingual capability Up to 7 channels e.g. for different languages, commentary channel, clean dialog etc.

55 MPEG - 2 Audio Multichannel Structure of the ISO Layer II multichannel extension, backwards compatible with ISO Layer II

56 Annex: Abbreviations and Companies

57 Abbreviations and Companies (1) AAC: Advanced Audio Coding ASPEC: Adaptive Spectral Perceptual Entropy Coding AT&T:American Telephone and Telegraph Company CCETT: Centre Commun d Etudes de Télédiffusion et Télécommunication CNET: Research and Development Center of France Télécom FhG-IIS: Fraunhofer Gesellschaft/Institut für Integrierte Schaltungen (Erlangen)

58 Abbreviations and Companies (2) IRT: Institut für Rundfunktechnik GmbH, München, Research and Development Institute of ARD, ZDF, DLR, ORF and SRG ITU-R: International Telecommunication Union Radio Communication Sector MASCAM: Masking-pattern Adapted Subband Coding and Multiplexing AT&T:American Telephone and Telegraph Company MUSICAM: Masking-pattern Universal Subband Integrated Coding and Multiplexing

59 Abbreviations and Companies (3) NTT: Nippon Telegraph and Telephone Corp./Human Interface Laboratories Thomson: Thomson, Telefunken, Saba, RCA, GE, ProScan TwinVQ: Transform-domain Weighted Interleave Vector Quantization