Contextual Biasing and Custom Vocabulary in ASR

Speech recognizers tend to struggle with uncommon names, product codes and specialized words because examples may be sparse in their training data. Contextual biasing supplies likely phrases for the current task so the decoder can consider them more strongly. A person dictating a message might expect contact names; a meeting may have an attendee list; a technician may need a set of equipment terms. Google’s Contextual Listen, Attend and Spell research is one example of a model that incorporates context phrases during recognition. Other systems can bias search scores or rerank candidates without using the same architecture. Context is useful only when it fits the moment. A large list of unrelated terms creates confusion, and a high boost can make a rare name appear even when the speaker said an ordinary word. A short, relevant list can be safer than a permanent global vocabulary. Pronunciation matters: a spelling alone may not tell the model how a name sounds, particularly across languages. Context is prior information, not proof from the waveform. The acoustic evidence still needs to support the chosen transcript. Evaluation should include both recall of intended custom words and false insertions when those words were not spoken. Report ordinary-word accuracy too, because improving names while degrading common speech may be a poor trade. Test different accents, microphones and phrase positions. Keep development audio separate from the final test set when choosing a boost. For medical or legal terms, human verification is important because a fluent but wrong name, amount or negation may be consequential. Custom vocabularies can contain private contacts, patient names or business plans. Collect only the terms necessary for the session, control who can access them and set retention to the actual purpose. Let users review or remove entries. A good interface presents contextual suggestions as aids to recognition, not as authenticated facts about the person speaking or the topics discussed.