Transcript Dolby AC-3 Audio Encoding & THX Wai Kam (Winnie) Henele Adams Peter Boettcher Multichannel Audio   Audio used in film and various entertainment ranges from mono to stereo.

Dolby AC-3 Audio Encoding
&
THX
Wai Kam (Winnie)
Henele Adams
Peter Boettcher
Multichannel Audio
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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
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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
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Frequency Domain Transform:
– 512 point MDCT with 50% overlap
– 2 x 256 point MDCT in transient signal
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Bit allocation in floating point representation:
– Mantissa and Exponent are represented differently
– Bits of Mantissa are adaptively allocated
AC-3 Decoder
1st Step
Recover Mantissa according to Psychoacoustic model, inverse
transform and de-mux to different output channels
2nd Step
Recover PCM values
for each channel
Bit-stream Syntax
1 Frame represents 1536 PCM samples for all channels
Frame Structure
SI=Sync. Info BSI=Bitstream Info CRC for error correction Aux Data for private control
1 Block represents 256 PCM for each channel
Block Structure
Encoding Philosophy
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Encoding Goals
– Minimize perceived sound quality
degradation
– Keep the bit-rate within set value
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Bit allocation strategies
– forward-adaptive
– backward-adaptive
– parametric bit allocation
Forward Adaptive
Backward Adaptive
Parametric Bit Allocation
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Hybrid of Forward and Backward
strategies
Parametric Bit Allocation
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Core is Backward Adaptive
– Primary Modifications
• Psychoacoustic model parameter adjustment
• Delta Bit Allocation
Filter Banks
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Filter Bank construction
– Overlapping blocks of 512 windowed
samples
– Block 10.66 msec long (@48kHz)
– Adjustments for transients
Spectral Envelope
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Coefficients coded into an exponent and
a mantissa
– Allows wide dynamic range
– Precision limitations in mantissa introduce
noise
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Exponents characterize the spectrum
– Spectral envelope
Spectral Envelope
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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
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Psychoacoustic Reductions
– Pool of bits for all channels
– Adjustments made to SNR
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Coping with Bit rate Conditions
– coupling
5.1 Channel Surround Sound
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5 main channels - L, C, R, LS, RS
1 LFE (low-frequency effects) channel
– bandwidth is approximately 1 tenth, thus .1
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Unlike analog Dolby Surround, AC-3 5.1
format includes 6 discrete channels, but
coded together for efficiency
Recommended Setup
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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
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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
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Front channels:
– Phantom center (like stereo), center channel
alone, or both
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Surround channels:
– Enhance depth and immersion
– Not well established
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LFE:
– Only special bass effects
Decoder Downmixing
Decoder Downmixing
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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
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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 multichannel sound playback.
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Plus, George Lucas wants to make money
THX Certification
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“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
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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.