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. – 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.