Dolby AC-3 Audio Encoding&

THX

Wai Kam (Winnie)Henele AdamsPeter Boettcher

Multichannel Audio Audio used in film and various entertainment

ranges from mono to stereo to 5.1 system Wideband Audio: 48 KHz *16 bits * 6

Channels= 46.08 Mbits/sec

AC-3: Digital Audio Coding

Flexible Multi-channel coding: encode up to 6 channels and downmix according to the listener’s available resources

Current Standard for HDTV audio compression; also used in LD, DVD

Sample rates: 32 KHz, 44.1KHz, 48KHz Data rates: 32 Kbps to 640 Kbps 1st AC-3 coded film: Star Trek VI in Dec. 91

AC-3 Encoder

Frequency Domain Transform:– 512 point MDCT with 50% overlap– 2 x 256 point MDCT in transient signal

Bit allocation in floating point representation:– Mantissa and Exponent are represented differently – Bits of Mantissa are adaptively allocated

AC-3 Decoder

Recover Mantissa according to Psychoacoustic model, inverse transform and de-mux to different output channels

2nd Step

1st Step

Recover PCM values for each channel

Bit-stream Syntax

1 Frame represents 1536 PCM samples for all channels

1 Block represents 256 PCM for each channel

SI=Sync. Info BSI=Bitstream Info CRC for error correction Aux Data for private control

Frame Structure

Block Structure

Encoding Philosophy Encoding Goals

– Minimize perceived sound quality degradation

– Keep the bit-rate within set value Bit allocation strategies

– forward-adaptive– backward-adaptive– parametric bit allocation

Forward Adaptive

Backward Adaptive

Parametric Bit Allocation

Hybrid of Forward and Backward strategies

Parametric Bit Allocation

Core is Backward Adaptive– Primary Modifications

• Psychoacoustic model parameter adjustment• Delta Bit Allocation

Filter Banks

Filter Bank construction– Overlapping blocks of 512 windowed

samples– Block 10.66 msec long (@48kHz)– Adjustments for transients

Spectral Envelope

Coefficients coded into an exponent and a mantissa– Allows wide dynamic range– Precision limitations in mantissa introduce

noise Exponents characterize the spectrum

– Spectral envelope

Spectral Envelope

Coding– DC sent as absolute– Exponents sent as differentials

• Groups of 3 encoded into 7 bit blocks• Four exponent encoding strategies

– D15, D25, D45, and REUSE

Spectral Envelope

Bit Allocation

Psychoacoustic Reductions– Pool of bits for all channels– Adjustments made to SNR

Coping with Bit rate Conditions– coupling

5.1 Channel Surround Sound

5 main channels - L, C, R, LS, RS 1 LFE (low-frequency effects) channel

– bandwidth is approximately 1 tenth, thus .1 Unlike analog Dolby Surround, AC-3 5.1

format includes 6 discrete channels, but coded together for efficiency

Recommended Setup Left and Right stereo pair 45-60 degrees off

center Surround speakers to side and slightly to rear

(110 degrees) All speakers equidistant from listener, or use

appropriate time delays in system. Subwoofer anywhere

Bass Management

The subwoofer channel is NOT the same as the LFE channel

LFE channel is created by the sound engineer at mixing time

The subwoofer channel is produced by the decoder at playback time

Dolby recommends caution in creating LFE channels at all

Use of the Channels

Front channels:– Phantom center (like stereo), center channel

alone, or both Surround channels:

– Enhance depth and immersion– Not well established

LFE:– Only special bass effects

Decoder Downmixing

Decoder Downmixing

Decoder adjusts playback for equipment 5.1 audio track can be mixed down to 4

channel surround or stereo Dynamic range reduction, LFE channel

elimination

THX Two needs identified:

– A need to correct the audible tonal and spatial errors caused by the playback of soundtracks designed in and for large theatres in the smaller environment of a home.

– A need to more accurately reproduce the complex and competing sound fields present in multi-channel sound playback.

Plus, George Lucas wants to make money

THX Certification

“THX is like the Good Housekeeping Seal of Approval. It certifies that the product performs to a rigorously high standard that makes sense. THX provides the confidence and reassurance a consumer needs in a purchasing decision.”

-- Steve Baker, V.P. Sales & Marketing, Denon Electronics

Questions What are the primary differences

between MPEG audio and AC-3 coding?– MPEG audio uses the forward adaptive bit

allocation scheme whereas the AC-3 method uses the hybrid (parametric) scheme discussed earlier.

– The bitstream formats are different and allow different extensions and capabilities.