12. A method to pre-render an audio representation of textual content for subsequent playback, the method comprising: downloading, by a first device, first textual content of a content type during a first period from a server remote from the first device; converting, by the first device, the first textual content to first speech; computing, by the first device, a first signature from the first textual content that identifies the first textual content; downloading, by the first device, second textual content for the same content type from the server during a second period after the first period; computing, by the first device, a second signature from the second textual content that identifies the second textual content; converting, by the first device, the second textual content to second speech only when the first signature differs from the second signature; and when resources of the first device are limited, transferring the first or second speech from the first device to the server and removing the transferred speech from the first device.
12. A method to pre-render an audio representation of textual content for subsequent playback, the method comprising: downloading, by a first device, first textual content of a content type during a first period from a server remote from the first device; converting, by the first device, the first textual content to first speech; computing, by the first device, a first signature from the first textual content that identifies the first textual content; downloading, by the first device, second textual content for the same content type from the server during a second period after the first period; computing, by the first device, a second signature from the second textual content that identifies the second textual content; converting, by the first device, the second textual content to second speech only when the first signature differs from the second signature; and when resources of the first device are limited, transferring the first or second speech from the first device to the server and removing the transferred speech from the first device. 15. The method of claim 12 , further comprising, during a subsequent download period when the transferred speech is present on the server, the first device downloading the transferred speech from the server instead of third textual content of the content type to play the transferred speech.