In the last few years, there has been an increasing interest in developing systems for Automatic LipReading (ALR). Similarly to other computer vision applications, methods based on Deep Learning (DL) have become very popular and have permitted to substantially push forward the achievable performance. In this survey, we review ALR research during the last decade, highlighting the progression from approaches previous to DL (which we refer to as traditional) toward end-to-end DL architectures. We provide a comprehensive list of the audiovisual databases available for lipreading, describing what tasks they can be used for, their popularity and their most important characteristics, such as the number of speakers, vocabulary size, recording settings and total duration. In correspondence with the shift toward DL, we show that there is a clear tendency toward large-scale datasets targeting realistic application settings and large numbers of samples per class. On the other hand, we summarize, discuss and compare the different ALR systems proposed in the last decade, separately considering traditional and DL approaches. We address a quantitative analysis of the different systems by organizing them in terms of the task that they target (e.g. recognition of letters or digits and words or sentences) and comparing their reported performance in the most commonly used datasets. As a result, we find that DL architectures perform similarly to traditional ones for simpler tasks but report significant improvements in more complex tasks, such as word or sentence recognition, with up to 40% improvement in word recognition rates. Hence, we provide a detailed description of the available ALR systems based on end-to-end DL architectures and identify a tendency to focus on the modeling of temporal context as the key to advance the field. Such modeling is dominated by recurrent neural networks due to their ability to retain context at multiple scales (e.g. short-and longterm information). In this sense, current efforts tend toward techniques that allow a more comprehensive modeling and interpretability of the retained context.

Speech is the most used communication method between humans and it involves the perception of auditory and visual channels. Automatic speech recognition focuses on interpreting the audio signals, although the video can provide information that is complementary to the audio. Exploiting the visual information, however, has proven challenging. On one hand, researchers have reported that the mapping between phonemes and visemes (visual units) is one-to-many because there are phonemes which are visually similar and indistinguishable between them. On the other hand, it is known that some people are very good lip-readers (e.g: deaf people). We study the limit of visual only speech recognition in controlled conditions. With this goal, we designed a new database in which the speakers are aware of being read and aim to facilitate lip-reading. In the literature, there are discrepancies on whether hearing-impaired people are better lip-readers than normal-hearing people. Then, we analyze if there are differences between the lip-reading abilities of 9 hearing-impaired and 15 normal-hearing people. Finally, human abilities are compared with the performance of a visual automatic speech recognition system. In our tests, hearing-impaired participants outperformed the normal-hearing participants but without reaching statistical significance. Human observers were able to decode 44% of the spoken message. In contrast, the visual only automatic system achieved 20% of word recognition rate. However, if we repeat the comparison in terms of phonemes both obtained very similar recognition rates, just above 50%. This suggests that the gap between human lip-reading and automatic speech-reading might be more related to the use of context than to the ability to interpret mouth appearance.

Abstract:Speech is the most common communication method between humans and involves the perception of both auditory and visual channels. Automatic speech recognition focuses on interpreting the audio signals, but it has been demonstrated that video can provide information that is complementary to the audio. Thus, the study of automatic lip-reading is important and is still an open problem. One of the key challenges is the definition of the visual elementary units (the visemes) and their vocabulary. Many researchers have analyzed the importance of the phoneme to viseme mapping and have proposed viseme vocabularies with lengths between 11 and 15 visemes. These viseme vocabularies have usually been manually defined by their linguistic properties and in some cases using decision trees or clustering techniques. In this work, we focus on the automatic construction of an optimal viseme vocabulary based on the association of phonemes with similar appearance. To this end, we construct an automatic system that uses local appearance descriptors to extract the main characteristics of the mouth region and HMMs to model the statistic relations of both viseme and phoneme sequences. To compare the performance of the system different descriptors (PCA, DCT and SIFT) are analyzed. We test our system in a Spanish corpus of continuous speech. Our results indicate that we are able to recognize approximately 58% of the visemes, 47% of the phonemes and 23% of the words in a continuous speech scenario and that the optimal viseme vocabulary for Spanish is composed by 20 visemes.

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