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

Multichannel Audio n Audio used in film and various entertainment ranges from mono to stereo to 5.1 system n Wideband Audio: 48 KHz *16 bits * 6 Channels= Mbits/sec

AC-3: Digital Audio Coding n Flexible Multi-channel coding: encode up to 6 channels and downmix according to the listener’s available resources n Current Standard for HDTV audio compression; also used in LD, DVD n Sample rates: 32 KHz, 44.1KHz, 48KHz n Data rates: 32 Kbps to 640 Kbps n 1st AC-3 coded film: Star Trek VI in Dec. 91

AC-3 Encoder n Frequency Domain Transform: –512 point MDCT with 50% overlap –2 x 256 point MDCT in transient signal n 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 n Encoding Goals –Minimize perceived sound quality degradation –Keep the bit-rate within set value n Bit allocation strategies –forward-adaptive –backward-adaptive –parametric bit allocation

Forward Adaptive

Backward Adaptive

Parametric Bit Allocation n Hybrid of Forward and Backward strategies

Parametric Bit Allocation n Core is Backward Adaptive –Primary Modifications Psychoacoustic model parameter adjustment Delta Bit Allocation

Filter Banks n Filter Bank construction –Overlapping blocks of 512 windowed samples –Block msec long –Adjustments for transients

Spectral Envelope n Coefficients coded into an exponent and a mantissa –Allows wide dynamic range –Precision limitations in mantissa introduce noise n Exponents characterize the spectrum –Spectral envelope

Spectral Envelope n 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 n Psychoacoustic Reductions –Pool of bits for all channels –Adjustments made to SNR n Coping with Bit rate Conditions –coupling

5.1 Channel Surround Sound n 5 main channels - L, C, R, LS, RS n 1 LFE (low-frequency effects) channel –bandwidth is approximately 1 tenth, thus.1 n Unlike analog Dolby Surround, AC format includes 6 discrete channels, but coded together for efficiency

Recommended Setup n Left and Right stereo pair degrees off center n Surround speakers to side and slightly to rear (110 degrees) n All speakers equidistant from listener, or use appropriate time delays in system. n Subwoofer anywhere

Bass Management n The subwoofer channel is NOT the same as the LFE channel n LFE channel is created by the sound engineer at mixing time n The subwoofer channel is produced by the decoder at playback time n Dolby recommends caution in creating LFE channels at all

Use of the Channels n Front channels: –Phantom center (like stereo), center channel alone, or both n Surround channels: –Enhance depth and immersion –Not well established n LFE: –Only special bass effects

Decoder Downmixing

n Decoder adjusts playback for equipment n 5.1 audio track can be mixed down to 4 channel surround or stereo n Dynamic range reduction, LFE channel elimination

THX n 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. n Plus, George Lucas wants to make money

THX Certification n “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 n 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.