# Fast Convolutional Dictionary Learning off the Grid

## TL;DR

This paper introduces a novel convolutional dictionary learning framework that accurately estimates continuous-time events from sampled signals, using interpolated dictionaries and efficient algorithms, significantly improving off-the-grid event detection and template learning.

## Contribution

The paper proposes a new CDL framework with interpolated dictionaries and a fast CSC algorithm, COMP-INTERP, enabling accurate off-the-grid event detection and improved template learning.

## Key findings

- COMP-INTERP achieves similar accuracy to state-of-the-art CBP
- COMP-INTERP is two orders of magnitude faster
- Dictionary update with overcomplete dictionaries yields more accurate templates

## Abstract

Given a continuous-time signal that can be modeled as the superposition of localized, time-shifted events from multiple sources, the goal of Convolutional Dictionary Learning (CDL) is to identify the location of the events--by Convolutional Sparse Coding (CSC)--and learn the template for each source--by Convolutional Dictionary Update (CDU). In practice, because we observe samples of the continuous-time signal on a uniformly-sampled grid in discrete time, classical CSC methods can only produce estimates of the times when the events occur on this grid, which degrades the performance of the CDU. We introduce a CDL framework that significantly reduces the errors arising from performing the estimation in discrete time. Specifically, we construct an expanded dictionary that comprises, not only discrete-time shifts of the templates, but also interpolated variants, obtained by bandlimited interpolation, that account for continuous-time shifts. For CSC, we develop a novel computationally efficient CSC algorithm, termed Convolutional Orthogonal Matching Pursuit with interpolated dictionary (COMP-INTERP). We benchmarked COMP-INTERP to Contiunuous Basis Pursuit (CBP), the state-of-the-art CSC algorithm for estimating off-the-grid events, and demonstrate, on simulated data, that 1) COMP-INTERP achieves a similar level of accuracy, and 2) is two orders of magnitude faster. For CDU, we derive a novel procedure to update the templates given sparse codes that can occur both on and off the discrete-time grid. We also show that 3) dictionary update with the overcomplete dictionary yields more accurate templates. Finally, we apply the algorithms to the spike sorting problem on electrophysiology recording and show their competitive performance.