| Variable-Data-Rate Speech Encoder |
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| Naval Research Laboratory, Washington, DC | |
| Oct 01 2007 | |
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Advertisement: The VDR bit stream has an embedded structure in which higher-rate voice data frames contain successively lower-rate voice data frames as subsets. Deletion of a certain portion of the superset (higherrate frames typically representing higher audio frequencies) makes it possible to reduce the data rate, even in the presence of encryption. Because of this embedded data structure, any of the VDR data rates are interoperable and can be switched, as often as 44 times per second, even when speech is present. Because the speech waveforms of all the VDR rates are synchronous, switching of data rates does not introduce such undesirable sounds such as clicks or warbles. It must be emphasized that the multirate voice processor in the VDR voice encoder is a single processor running a single algorithm, in contradistinction to both (1) a collection of separate processors operating at different rates and (2) a processor running a multitude of speechcompression algorithms. Prior voice encoders that use multiple compression algorithms do not perform well when algorithms are switched while speech is present. Speech waveforms sometimes become cropped upon switching because different voice algorithms can have different internal delays. Such cropping degrades speech quality and is annoying to listeners. The VDR speech encoder exploits the variable nature of the speech waveform, utilizing higher or lower data rates as needed (e.g., higher rates for vowels, lower rates for consonants). Unlike some prior speech processors, the speech processor in the VDR speech encoder processor does not eliminate gaps in speech for the sake of efficiency. Elimination of speech gaps that contain ambient sounds could be harmful in military communications because speech gaps often contain sounds that help listeners gauge battlefield conditions at transmitter sites. In the VDR speech encoder, speech gaps are encoded at appropriately low data rates that still provide audible information. This work was done by Thomas M. Moran, David A. Heide, Yvette and T. Lee of the Naval Research Laboratory and George S. Kang of ITT Industries. This Brief includes a Technical Support Package (TSP).
Variable-Data-Rate Speech Encoder (reference NRL-0019) is currently available for download from the TSP library. Login first to download.
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