By Roeland Ordelman
Alignment (officially ‘forced alignment’) is the process of synchronizing a text transcription of speech to the audio recording that contains the speech, by automatically adding time labels to every word in the transcript using a specific form of speech recognition technology. This technology is sometimes called ‘informed speech recognition’: the words to be recognized are already known — the task is to find the exact positions in the audio where the words occur. In practice, the speech transcripts are usually not an exact representation of the speech. For example, repetitions and filler words such as ‘uh’ and ‘ah’ are often omitted, and ungrammatical sentences reformulated. The further the text transcripts depart from the verbatim speech, the more difficult the alignment process for the speech recognition technology will be.
Alignment is a very valuable tool to enable word level access to spoken word recordings (jump to positions where a particular word is mentioned) in cases where text transcripts are available such as in Oral History research where scholars are used to making verbatim transcripts of interviews in word processors that do not capture time information. Examples of text transcripts in other domains that are deployed for alignment are subtitles for the hearing impaired, auto-cues, production scripts, and court reports.
An alignment tool typically takes as input (i) a plain text file with the speech transcript, and (ii) the audio recording that contains the speech, and provides as output a file where time labels are added to each of the words in the transcript, for example in a list format with on each line the start time and a word:
This output can be used in a transcription viewer to jump immediately to the corresponding position in the audio file by clicking on a word while viewing a single interview. It can also be used to jump to positions in multiple audio files by indexing the aligned text (of multiple interviews) in a search engine.
See more at this blog post about the CLARIN-Plus workshop on oral history .