HSR Intro

From last time …

ASR System Architecture Grammar

Cepstrum

Speech Signal

Signal Processing

Recognized Words “zero” “three” “two”

Probabilities “z” -0.81 “th” = 0.15 “t” = 0.03

Probability Estimator

Decoder

Pronunciation Lexicon

A Few Points about Human Speech Recognition (See Chapter 18 for much more on this)

Human Speech Recognition • Experiments dating from 1918 dealing with noise, reduced BW (Fletcher) • Statistics of CVC perception • Comparisons between human and machine speech recognition • A few thoughts

The Ear

The Cochlea

Assessing Recognition Accuracy • Intelligibility • Articulation - Fletcher experiments – CVC, VC, CV, syllables in carrier sentences – Tests over different SNR, bands – Example: “The first group is `mav’ (forced choice between mav and nav) – Used sharp lowpass and/or highpass filtered. For equal energy, crossover is 450 Hz; for equal articulation, 1550 Hz.

Results • S = vc2 • Articulation Index (the original “AI”) • Error independence between bands – – – – –

Articulatory band ~ 1 mm along basilar membrane 20 filters between 300 and 8000 Hz A single zero error band -> no error! Robustness to a range of problems AI = ∑k 1/K (SNRk / 30) where SNR saturates at 0 and 30

AI additivity • s(a,b) = phone accuracy for band from a to b, a