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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 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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Dolby AC-3 Audio Encoding & THX

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dolby ac 3 audio encoding thx
Dolby AC-3 Audio Encoding&THX

Wai Kam (Winnie)

Henele Adams

Peter Boettcher

multichannel audio
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
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
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
AC-3 Decoder

1st Step

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

Recover PCM values

for each channel

2nd Step

bit stream syntax
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
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
parametric bit allocation
Parametric Bit Allocation
  • Hybrid of Forward and Backward strategies
parametric bit allocation11
Parametric Bit Allocation
  • Core is Backward Adaptive
    • Primary Modifications
      • Psychoacoustic model parameter adjustment
      • Delta Bit Allocation
filter banks
Filter Banks
  • Filter Bank construction
    • Overlapping blocks of 512 windowed samples
    • Block 10.66 msec long (@48kHz)
    • Adjustments for transients
spectral envelope
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 envelope14
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
bit allocation
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
5.1 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
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
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
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 downmixing22
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
  • 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 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

  • 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.