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Dolby AC-3 Audio Encoding & THX: Multichannel Audio in Film & Entertainment

Dive into the world of Dolby AC-3 audio encoding and THX technology, exploring the core principles, bit allocation strategies, spectral envelope coding, 5.1 channel surround sound, recommended setups, and the role of THX certification.

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Dolby AC-3 Audio Encoding & THX: Multichannel Audio in Film & Entertainment

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

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. Forward Adaptive

  9. Backward Adaptive

  10. Parametric Bit Allocation • Hybrid of Forward and Backward strategies

  11. Parametric Bit Allocation • Core is Backward Adaptive • Primary Modifications • Psychoacoustic model parameter adjustment • Delta Bit Allocation

  12. Filter Banks • Filter Bank construction • Overlapping blocks of 512 windowed samples • Block 10.66 msec long (@48kHz) • Adjustments for transients

  13. 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

  14. 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

  15. Spectral Envelope

  16. Bit Allocation • Psychoacoustic Reductions • Pool of bits for all channels • Adjustments made to SNR • Coping with Bit rate Conditions • coupling

  17. 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

  18. 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

  19. 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

  20. 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

  21. Decoder Downmixing

  22. 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

  23. 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

  24. 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

  25. 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.

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