Perceptual Audio Coding The AT&T/Bell Labs view. James D. Johnston Chief Scientist Neural Audio, Kirkland, Wa. The early work. Harvey Fletcher, et al. Loudness curves Initial masking measurements Spatial hearing analysis Rabiner , Atal , Flanagan, Crochiere , Jayant et al.
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Perceptual Audio CodingThe AT&T/Bell Labs view James D. Johnston Chief Scientist Neural Audio, Kirkland, Wa
The early work • Harvey Fletcher, et al. • Loudness curves • Initial masking measurements • Spatial hearing analysis • Rabiner, Atal, Flanagan, Crochiere, Jayant et al. • Digital Signal Processing advancements • Lpc • A(d)PCM • Resampling • Filtering • CELP
My early work • The “commentary grade codec” • 56 kb/s, 7kHz bandwith, 16kHz sampling rate (pre-G-72x) 2-band SBC • Good on most material • Sounds awful on high passed material. • Ok, what and why? • Masking, or actually, a lack thereof.
PXFM • A testbed for the Alliant FX8 computers • Tonality metric in psychoacoustic model • FFT overlap/add filterbank • Bitstream compression • First, used multiple-radix encoding • Evolved into multiple Huffman codebooks for compression • Simple M/S stereo (all or nothing) • Ancestor of ASPEC • And thus MP3
PAC • Multichannel bitstream • MDCT coder, 128/1024 size • Pairwise channel coding for noise imaging control • Huffman coding for bitstream, • Sectioning • Zero codebook • Each channel pair has M/S coding on or off per scalefactor band • The predecessor to AAC coding • Won the “bake off” between BC and NBC codecs • All features adapted into MPEG-2 AAC • 1998 – a2b Music from AT&T – Shut down for perceived lack of market.