17. The system of claim 11 , wherein each of the one or more run-time predicted feature vectors corresponds to a respective temporal frame of acoustic data in the run-time speech waveform, and wherein the data indicating time duration of the identified phonetic speech unit comprises a number that specifies how many consecutive temporal frames of acoustic data over which an acoustic rendering of the identified phonetic speech unit in the run-time speech waveform should last.
11. A system comprising: one or more processors; memory; and machine-readable instructions stored in the memory, that upon execution by the one or more processors cause the system to carry out operations comprising: training a neural network implemented by the system to map one or more training-time sequences of phonetic-context descriptors received by the neural network into training-time predicted feature vectors that correspond to acoustic properties of predefined speech waveforms, wherein the one or more training-time sequences of phonetic-context descriptors correspond to phonetic transcriptions of training-time text strings, and the training-time text strings correspond to written transcriptions of speech carried in the predefined speech waveforms, receiving, by a text analysis module implemented by the system, a run-time text string, processing the received run-time text string with the text analysis module to generate a run-time sequence of phonetic-context descriptors that corresponds to a phonetic transcription of the run-time text string, wherein each phonetic-context descriptor of the run-time sequence includes a respective label identifying a phonetic speech unit of a plurality of phonetic speech units, data indicating phonetic context of the identified phonetic speech unit, and data indicating time duration of the identified phonetic speech unit, processing the run-time sequence of the phonetic-context descriptors with the trained neural network in a corresponding sequence of neural network time steps to generate one or more run-time predicted feature vectors, and processing the one or more run-time predicted feature vectors with a signal generation module to produce and output a run-time speech waveform corresponding to a spoken rendering of the received run-time text string, wherein processing the received run-time text string with the text analysis module to generate the run-time sequence of phonetic-context descriptors comprises: generating a run-time transcription sequence of phonetic speech units that corresponds to the phonetic transcription of the run-time text string; and determining a respective number of consecutive phonetic-context descriptors to generate for each of the phonetic speech units of the run-time transcription sequence.