cantaba has a similar meaning with "I used to sing", altough the best meaning is " yo solía cantar" it is a tense called "copretérito" see you in skype and I'll show you more examples using all the tenses to know when you can use them.

Hi Bill, I believe in this case you could say, for example: "Yo estaba cantando ayer en la mañana": "I was singing yesterday in the morning" to refer to one specific time or "Cuando era niño, yo cantaba (solía cantar) if the action was made several times" When I was a child, I used to sing",but I am not expert, is only an opinion.