1. A system comprising: a data processing apparatus; and a computer storage medium encoded with a computer program, the program comprising instructions that when executed by the data processing apparatus cause the data processing apparatus to perform operations comprising: receiving a digital video comprising video data; processing the video data to detect features indicative of a human face in the digital video; determining, from the video data, a first frame, in which the features indicative of the human face are detected in the digital video; determining first timestamp data corresponding to the first frame; determining, from the video data, a second frame, in which the features indicative of the human face are detected in the digital video; determining second timestamp data corresponding to the second frame; generating an identifier corresponding to the human face; generating a data set comprising the identifier, the first timestamp data and the second timestamp data; and appending the data set to the video data to provide annotated video data.