Monaural Speech Segregation: Representation, Pitch, and Amplitude Modulation DeLiang Wang The Ohio State University

Outline of Presentation l Introduction l Speech segregation problem l Auditory scene analysis (ASA) approach l A multistage model for computational ASA l On amplitude modulation and pitch tracking l Oscillatory correlation theory for ASA

Speech Segregation Problem l In a natural environment, target speech is usually corrupted by acoustic interference. An effective system for speech segregation has many applications, such as automatic speech recognition, audio retrieval, and hearing aid design l Most speech separation techniques require multiple sensors l Speech enhancement developed for the monaural situation can deal with only specific acoustic interference

Auditory Scene Analysis (Bregman’90) l Listeners are able to parse the complex mixture of sounds arriving at the ears in order to retrieve a mental representation of each sound source l ASA would take place in two conceptual processes: l Segmentation. Decompose the acoustic mixture into sensory elements (segments) l Grouping. Combine segments into groups, so that segments in the same group are likely to have originated from the same environmental source

Auditory Scene Analysis - continued l The grouping process involves two aspects: l Primitive grouping. Innate data-driven mechanisms, consistent with those described by Gestalt psychologists for visual perception (proximity, similarity, common fate, good continuation, etc.) l Schema-driven grouping. Application of learned knowledge about speech, music and other environmental sounds

Computational Auditory Scene Analysis l Computational ASA (CASA) systems approach sound separation based on ASA principles l Weintraub’85, Cooke’93, Brown & Cooke’94, Ellis’96, Wang’96 l Previous CASA work suggests that: l Representation of the auditory scene is a key issue l Temporal continuity is important (although it is ignored in most frame-based sound processing algorithms) l Fundamental frequency (F0) is a strong cue for grouping

A Multi-stage Model (Wang & Brown’99)

Auditory Periphery Model l A bank of fourth-order gammatone filters (Patterson et al.’88) l Meddis hair cell model converts gammatone output to neural firing

Auditory Periphery - Example l Hair cell response to utterance: “Why were you all weary?” mixed with phone ringing l 128 filter channels arranged in ERB

l Mid-level representations form the basis for segment formation and subsequent grouping l Correlogram extracts periodicity information from simulated auditory nerve firing patterns l Summary correlogram is used to identify F0 l Cross-correlation between adjacent correlogram channels identifies regions that are excited by the same frequency component or formant Mid-level Auditory Representations

Mid-level Representations - Example l Correlogram and cross-channel correlation for the speech/telephone mixture

Oscillator Network: Segmentation Layer l Horizontal weights are unity, reflecting temporal continuity, and vertical weights are unity if cross-channel correlation exceeds a threshold, otherwise 0 l A global inhibitor ensures that different segments have different phases l A segment thus formed corresponds to acoustic energy in a local time-frequency region that is treated as an atomic component of an auditory scene

Segmentation Layer - Example l Output of the segmentation layer in response to the speech/telephone mixture

Oscillator Network: Grouping Layer l At each time frame, an F0 estimate from the summary correlogram is used to classify channels into two categories; those that are consistent with the F0, and those that are not l Connections are formed between pairs of channels: mutual excitation if the channels belong to the same F0 category, otherwise mutual inhibition l Strong excitation within each segment l The second layer embodies the grouping stage of ASA

Grouping Layer - Example l Two streams emerge from the grouping layer at different times or with different phases l Left: Foreground (original mixture ) l Right: Background

l Previous systems, including the Wang-Brown model, have difficulty in l Dealing with broadband high-frequency mixtures l Performing reliable pitch tracking for noisy speech l Retaining high-frequency energy of the target speaker l Our next step considers perceptual resolvability of various harmonics Challenges Facing CASA

Resolved and Unresolved Harmonics l For voiced speech, lower harmonics are resolved while higher harmonics are not l For unresolved harmonics, the envelopes of filter responses fluctuate at the fundamental frequency of speech l Hence we apply different grouping mechanisms for low-frequency and high-frequency signals: l Low-frequency signals are grouped based on periodicity and temporal continuity l High-frequency signals are grouped based on amplitude modulation (AM) and temporal continuity

Proposed System (Hu & Wang'02)

Envelope Representations - Example (a) Correlogram and cross-channel correlation of hair cell response to clean speech (b) Corresponding representations for response envelopes

Initial Segregation l The Wang-Brown model is used in this stage to generate segments and select the target speech stream l Segments generated in this stage tend to reflect resolved harmonics, but not unresolved ones

Pitch Tracking l Pitch periods of target speech are estimated from the segregated speech stream l Estimated pitch periods are checked and re- estimated using two psychoacoustically motivated constraints: l Target pitch should agree with the periodicity of the time- frequency (T-F) units in the initial speech stream l Pitch periods change smoothly, thus allowing for verification and interpolation

Pitch Tracking - Example (a) Global pitch (Line: pitch track of clean speech) for a mixture of target speech and ‘cocktail-party’ intrusion (b) Estimated target pitch

T-F Unit Labeling l In the low-frequency range: l A T-F unit is labeled by comparing the periodicity of its autocorrelation with the estimated target pitch l In the high-frequency range: l Due to their wide bandwidths, high-frequency filters generally respond to multiple harmonics. These responses are amplitude modulated due to beats and combinational tones (Helmholtz, 1863) l A T-F unit in the high-frequency range is labeled by comparing its AM repetition rate with the estimated target pitch

AM - Example (a) The output of a gammatone filter (center frequency: 2.6 kHz) to clean speech (b) The corresponding autocorrelation function

AM Repetition Rates l To obtain AM repetition rates, a filter response is half-wave rectified and bandpass filtered l The resulting signal within a T-F unit is modeled by a single sinusoid using the gradient descent method. The frequency of the sinusoid indicates the AM repetition rate of the corresponding response

Final Segregation l New segments corresponding to unresolved harmonics are formed based on temporal continuity and cross-channel correlation of response envelopes (i.e. common AM). Then they are grouped into the foreground stream according to AM repetition rates l The foreground stream is adjusted to remove the segments that do not agree with the estimated target pitch l Other units are grouped according to temporal and spectral continuity

Ideal Binary Mask for Performance Evaluation l Within a T-F unit, the ideal binary mask is 1 if target energy is stronger than interference energy, and 0 otherwise l Motivation: Auditory masking - stronger signal masks weaker one within a critical band l Further motivation: Ideal binary masks give excellent listening experience and automatic speech recognition performance l Thus, we suggest to use ideal binary masks as ground truth for CASA performance evaluation

Monaural Speech Segregation Example Left: Segregated speech stream (original mixture: ) Right: Ideal binary mask

Systematic Evaluation l Evaluated on a corpus of 100 mixtures (Cooke’93): 10 voiced utterances x 10 noise intrusions l Noise intrusions have a large variety l Resynthesis stage allows estimation of target speech waveform l Evaluation is based on ideal binary masks

Signal-to-Noise Ratio (SNR) Results Average SNR gain: 12.1 dB; average improvement over Wang-Brown: 5 dB Major improvement occurs in target energy retention, particularly in the high-frequency range

Segregation Examples Mixture Ideal Binary Mask Wang-Brown New System

How Does Auditory System Perform ASA? l Information about acoustic features (pitch, spectral shape, interaural differences, AM, FM) is extracted in distributed areas of the auditory system l Binding problem: How are these features combined to form a perceptual whole (stream)? l Hierarchies of feature-detecting cells exist, but do not seem to constitute a solution to the binding problem

Oscillatory Correlation Theory (von der Malsburg & Schneider’86; Wang’96) l Neural oscillators are used to represent auditory features l Oscillators representing features of the same source are synchronized (phase-locked with zero phase lag), and are desynchronized from oscillators representing different sources l Supported by growing experimental evidence, e.g. oscillations in auditory cortex measured by EEG, MEG and local field potentials

Oscillatory Correlation Representation FD: Feature Detector

Oscillatory Correlation for ASA l LEGION dynamics (Terman & Wang’95) provides a computational foundation for the oscillatory correlation theory l The utility of oscillatory correlation has been demonstrated for speech separation (Wang- Brown’99), modeling auditory attention (Wrigley-Brown’01), etc.

Issues l Grouping is entirely pitch-based, hence limited to segregating voiced speech l How to group unvoiced speech? l Target pitch tracking in the presence of multiple voiced sources l Role of segmentation l We found increased robustness with segments as an intermediate representation between streams and T-F units

Summary l Multistage ASA approach to monaural speech segregation l Performs substantially better than previous CASA systems l Oscillatory correlation theory for ASA l Key issue is integration of various grouping cues

Collaborators l Recent work with Guoning Hu- Ohio State University l Earlier work with Guy Brown - University of Sheffield