Dolby AC-3 Audio Encoding & THX Wai Kam (Winnie) Henele Adams Peter Boettcher.
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Transcript of Dolby AC-3 Audio Encoding & THX Wai Kam (Winnie) Henele Adams Peter Boettcher.
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.