CS 414 - Spring 2012 CS 414 – Multimedia Systems Design Lecture 11 – MP3 and MP4 Audio (Part 7) Klara Nahrstedt Spring 2012

CS 414 - Spring 2012

CS 414 – Multimedia Systems Design Lecture 11 – MP3 and MP4 Audio (Part 7)

Klara Nahrstedt

Spring 2012


Administrative MP1 – deadline February 18

Outline MP3 Audio Encoding MP4 Audio Reading:

Media Coding book, Section 7.7.2 – 7.7.5 Recommended Paper on MP3: Davis Pan, “A Tutorial on MPEG/Audio

Compression”, IEEE Multimedia, pp. 6-74, 1995 Recommended books on JPEG/ MPEG Audio/Video Fundamentals:

Haskell, Puri, Netravali, “Digital Video: An Introduction to MPEG-2”, Chapman and Hall, 1996


Why Compression is Needed

Data rate = sampling rate * quantization bits * channels (+ control information)

For example (digital audio):44100 Hz; 16 bits; 2 channelsgenerates about 1.4M of data per second;

84M per minute; 5G per hour

MPEG-1 Audio

Lossy compression of audio In late 1980’s ISO’s MPEG group started to

standardize TV broadcastingUse of Audio on CD-ROM (later DVD)

MPEG-1 Audio – 1992 MPEG-2 Audio - 1994 MPEG-1 Audio Layer I, II, III


Criteria for A Good Standard

Achieve desired outcome Be comprehensible Allow efficient

implementation Support competition Give benchmark tests Be supported by industry Be good for end users ….

Two models: implement first, then

standardize standardize first, then

implement

MPEG-1 Audio Layer II Called MP2 Dominant standard for audio broadcasting

DAB digital radio and DVB digital television

Came out of MUSICAM codecs with bit rates 64-196 kbps MUSICAM audio coding - basis for MPEG-1 and MPEG-2 audio

Sampling rates: 32, 44.1, 48 kHz Bit rates: 32, 48, 56, 64, 80, 96, … 384 kbps Format: mono, stereo, dual channel, …

MP2 – sub-band audio encoder in time domain

MPEG-1 Audio Layer III

MPEG-1 Layer III is called MP3 format Popular for PC and Internet applicationsGoal to compress to 128 kbps, but can be

compressed to higher or lower resulting quality

Utilization of psychoacoustics Scientific study of sound perception .


MPEG Audio – MP3 First psychoacoustic masking code was

proposed in 1979 in AT&T – Bell Labs, Murray Hill.

MP3 based on OCF (optimum coding in frequency domain) and PXFM (Perceptual transform coding)

MPEG-1 Audio Layer III – public release 1993 MPEG-2 Audio III – public release 1995


MPEG Audio – MP3

1997 – mp3.com – offering thousands of MP3s created by independent artists for free

1999 – Napster MP3 peer-to-peer file sharing

Problem: copyright infringement Authorized services: Amazon.com,

Rhapsody, Juno Records, ..


MPEG-1 Audio Encoding

CharacteristicsPrecision 16 bitsSampling frequency: 32KHz, 44.1 KHz, 48 KHz3 compression layers: Layer 1, Layer 2, Layer 3

(MP3) Layer 3: 32-320 kbps, target 64 kbps Layer 2: 32-384 kbps, target 128 kbps Layer 1: 32-448 kbps, target 192 kbps


MPEG Audio Encoding Steps


MPEG Audio Filter Bank Filter bank divides input into multiple sub-bands

(32 equal frequency sub-bands) Sub-band i defined

- filter output sample for sub-band i at time t, C[n] – one of 512 coefficients, x[n] – audio input sample from 512 sample buffer


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MPEG Audio Psycho-acoustic Model MPEG audio compresses by removing

acoustically irrelevant parts of audio signals Takes advantage of human auditory systems

inability to hear quantization noise under auditory masking

Auditory masking: occurs when ever the presence of a strong audio signal makes a temporal or spectral neighborhood of weaker audio signals imperceptible.


Loudness and Pitch (Review on Psychoacoustic Effects)

More sensitive to loudness at mid frequencies than at other frequencies intermediate frequencies at [500hz, 5000hz]Human hearing frequencies at [20hz,20000hz]

Perceived loudness of a sound changes based on frequency of that soundbasilar membrane reacts more to intermediate

frequencies than other frequencies


Fletcher-Munson Contours

Each contour represents an equal perceived sound


Perception sensitivity (loudness) is not linear across all frequencies and intensities

Masking Effects (Review of Psychoacoustic Effects)


Frequency masking

Temporal masking


MPEG/audio divides audio signal into frequency sub-bands that approximate critical bands. Then we quantize each sub-band according to the audibility of quantization noise within the band

MPEG Audio Bit Allocation This process determines number of code bits allocated to each sub-

band based on information from the psycho-acoustic model Algorithm:

1. Compute mask-to-noise ratio: MNR=SNR-SMR Standard provides tables that give estimates for SNR resulting from

quantizing to a given number of quantizer levels

2. Get MNR for each sub-band

3. Search for sub-band with the lowest MNR

4. Allocate code bits to this sub-band. If sub-band gets allocated more code bits than appropriate, look up new

estimate of SNR and repeat step 1


Audio Quality Bitrate

With too low bit rate, we get compression artifacts Ringing Pre-echo – sound is heard before it occurs. It is most noticeable

in impulsive sounds from percussion instruments such as cymbals Occurs in transform-based audio compression algorithms

Quality of encoder and encoding parametersConstant Bit rate encoding Variable Bit rate encoding


MP3 Audio Format


Source: http://wiki.hydrogenaudio.org/images/e/ee/Mp3filestructure.jpg

MPEG Audio Comments Precision of 16 bits per sample is needed to get good SNR

ratio Noise we are getting is quantization noise from the

digitization process For each added bit, we get 6dB better SNR ratio Masking effect means that we can raise the noise floor

around a strong sound because the noise will be masked away

Raising noise floor is the same as using less bits and using less bits is the same as compression


Successor of MP3 Advanced Audio Coding (AAC) – now part of

MPEG-4 Audio Inclusion of 48 full-bandwidth audio channels Default audio format for iPhone, iPad, Nintendo,

PlayStation, Nokia, Android, BlackBerry Introduced 1997 as MPEG-2 Part 7 In 1999 – updated and included in MPEG-4


AAC’s Improvements over MP3

More sample frequencies (8-96 kHz) Arbitrary bit rates and variable frame

length Higher efficiency and simpler filterbank

Uses pure MDCT (modified discrete cosine transform)

Used in Windows Media Audio


MPEG-4 Audio

Variety of applications General audio signalsSpeech signalsSynthetic audioSynthesized speech (structured audio)


MPEG-4 Audio Part 3

Includes variety of audio coding technologiesLossy speech coding (e.g., CELP)

CELP – code-excited linear prediction – speech coding

General audio coding (AAC)Lossless audio codingText-to-Speech interfaceStructured Audio (e.g., MIDI)


MPEG-4 Part 14 Called MP4 with Extension .mp4 Multimedia container format Stores digital video and audio streams and

allows streaming over Internet Container or wrapper format

meta-file format whose spec describes how different data elements and metadata coesit in computer file


MPEG-4 Audio Bit-rate 2-64kbps Scalable for variable rates MPEG-4 defines set of coders

Parametric Coding Techniques: low bit-rate 2-6kbps, 8kHz sampling frequency

Code Excited Linear Prediction: medium bit-rates 6-24 kbps, 8 and 16 kHz sampling rate

Time Frequency Techniques: high quality audio 16 kbps and higher bit-rates, sampling rate > 7 kHz


Conclusion

MPEG Audio is an integral part of the MPEG standard to be considered together with video

MPEG-4 Audio represents an major extension in terms of capabilities to MPEG-1 Audio


Documents

CS 414 - Spring 2012 CS 414 – Multimedia Systems Design Lecture 11 – MP3 and MP4 Audio (Part 7) Klara Nahrstedt Spring 2012