15. A non-transitory computer-readable medium storing software comprising instructions executable by one or more computers which, upon such execution, cause the one or more computers to perform operations comprising: receiving data encoding sounds produced by a human voice; extracting a monophonic melody line from an audio channel of the data encoding the sounds produced by the human voice; providing, to a recognizer that recognizes songs from sounds produced by a human voice, the monophonic melody line extracted from the audio channel of the data encoding the sounds produced by the human voice; comparing, at the recognizer that recognizes songs from sounds produced by a human voice, the monophonic melody line extracted from the audio channel of the data encoding the sounds produced by the human voice with one or more monophonic melody lines of candidate videos of a subset of a set of candidate videos that are each (i) identified as being associated with a particular song, and (ii) are classified as a cappella video recordings; determining, based on the comparison, that the monophonic melody line extracted from the audio channel of the data encoding the sounds produced by the human voice matches one or more of the one or more monophonic melody lines of the candidate videos of the subset; and in response to determining that the monophonic melody line extracted from the audio channel of the data encoding the sounds produced by the human voice matches one or more of the one or more monophonic melody lines of the candidate videos of the subset, providing an identifier of the particular song for output.
15. A non-transitory computer-readable medium storing software comprising instructions executable by one or more computers which, upon such execution, cause the one or more computers to perform operations comprising: receiving data encoding sounds produced by a human voice; extracting a monophonic melody line from an audio channel of the data encoding the sounds produced by the human voice; providing, to a recognizer that recognizes songs from sounds produced by a human voice, the monophonic melody line extracted from the audio channel of the data encoding the sounds produced by the human voice; comparing, at the recognizer that recognizes songs from sounds produced by a human voice, the monophonic melody line extracted from the audio channel of the data encoding the sounds produced by the human voice with one or more monophonic melody lines of candidate videos of a subset of a set of candidate videos that are each (i) identified as being associated with a particular song, and (ii) are classified as a cappella video recordings; determining, based on the comparison, that the monophonic melody line extracted from the audio channel of the data encoding the sounds produced by the human voice matches one or more of the one or more monophonic melody lines of the candidate videos of the subset; and in response to determining that the monophonic melody line extracted from the audio channel of the data encoding the sounds produced by the human voice matches one or more of the one or more monophonic melody lines of the candidate videos of the subset, providing an identifier of the particular song for output. 20. The computer-readable medium of claim 15 , wherein the data encoding the sounds produced by the human voice is data encoding humming produced by a human voice.