MPEG-1 MUMT-614 Jan.23, 2002 Wes Hatch. Purpose of MPEG encoding To decrease data rate How? –two choices: could decrease sample rate, but this would cause.

MPEG-1

MUMT-614

Jan.23, 2002

Wes Hatch

Purpose of MPEG encoding

• To decrease data rate

• How?– two choices:

• could decrease sample rate, but this would cause a decrease in available BW

• or, we could decrease word size. This will introduce noise into the signal (lower S:N ratio).

• Solution:– perceptual codingreduce word size based on signal

conditions

Quick Overview

• MPEG removes “irrelevancy” & statistical redundancy

• lossy (but not perceptibly so)

• 1.41 Mbps (cd audio) between 64 and 448 kbps. (95% to 68% reduction)

• ratios of 4:1, 6:1 can be transparent in advanced listening tests

• supports 32, 44.1, 48 kHz sampling rates

MPEG-1 Types

• 3 layers: I, II, and III– I is simplest III is most complex– a layer can play encodings of those beneath it

• eg. Layer III can play I, II, and III; layer II may only play I and II

Components

• There are two “components” to MPEG-1: the encoder and the decoder. – the decoder is what is actually described under

the specification; the encoder is not.– improvements to the encoder will have

immediate effects in quality without necessitating corresponding changes to the decoder

Encoder vs. Decoder

• Encoder– does all the work– forward adaptive encoding

• all allocation of bits is performed by the encoder

• the psychoacoustic model used to determine “irrelevant” data is contained here

• improving psychoacoustic models/changes to encoder doesn’t require changing the decoder

• Decoder does less work

Encoder Details

• audio (PCM) passes through a polyphase filter bank, splitting the signal into 32 bands– filter outputs one sample per band for every 32

samples in• layer I: after each band gets 12 samples the decoder

determines the bit allocation for that band

• layer II: operates on 12 x 3= 36 samples per band (larger frame). Lower bands may receive: 15 bits, middle: 7 bits, and high: 3 bits max

• layer III is different…we’ll come back to it.

Encoder Details

• FFT is performed (w/Hann window)– 512 point for layer I– 1024 point for layer II

• a psychoacoustic model compares the output and is used to calculate masking thresholds

• used to determine which are the audible components (ie. SMR)

More details...

How bits are allocated

• data in the band is coded, NOT the FFT data.

• more “audible” components (ie. those highest above the masking threshold) are assigned the most bits

Encoder Details

• Scale factor is calculated– largest sample value in the band for each frame

is found. Each of the 12 samples in the band are divided by this factor

– layer II has 3 scale factors (for 3 groups of 12 samples), but one may suffice if the differences are small

• Corresponds to max. SPL in each band

Encoder Schematic

Encoder Details (layer III)

• layer III:– each band is transformed into 18 spectral

coefficients with a MDCT (50% overlap)• gives 576 coefficients, each representing a BW of

41.67 at 48 kHz24ms

– window size of the MDCT is variable• long window for steady state signals (36 samples) to

small windows for transient (12 samples)

Encoder Details (layer III)

• framerate varies in layer III

• can also use a bit reservoir for if more accuracy is needed

• Huffman encoding employed

Encoder

• a portion of the data stream is consumed by coding info:– headers– bit allocation info– scale factors– samples from each band

Other Features

• stereo joint coding– stereophonic irrelevance/redundancy eliminated– sum and difference signals (layer III)– L/R high frequency band samples summed into

one channel, but scale factors remain independent

Decoder Details

• Put signal back together:– decode bit allocation info– samples multiplied by scale factors and run

through an inverse filterbank– delays typically range from 10 to 30ms

Decoder schematic

Summary

• Split signal into 32 bands

• determine max. SPL levels for each band

• FFT to calculate masking thresholds– determine global masking curve

• calculate SMR for each band and assign bits accordingly