Varying data rate (VDR) capability is essential for adapting to varying channel conditions. However, the standard reference STANAG-4591 MELPe implementation was designed to operate at one of three fixed independent rates (2400/1200/600 bps), and did not enable varying the rate on the fly. Each rate’s vocoder was designed to operate and implemented in isolation from the other rates’s. In other words, the STANAG-4591 (and also TCVCIS Narrowband (NB) vocoder which is basically STANAG-4591) reference implementation is multi-rate but not VDR.
However, frequently varying the rate in the STANAG-4591 standard reference implementation (with or without re-initialization) may generate artifacts that can render the speech momentarily unintelligible. To resolve this limitation, Compandent developed and implemented an excellent solution that solved this problem and allows for smooth transitions from one vocoder rate to another on the fly. While maintaining full STANAG-4591 compliance for fixed-rate operation, this enhanced suite enables dynamic, on-the-fly transitions across the 2400, 1200, and 600 bps rates, providing reliable performance in changing channel conditions with remarkable speech quality and no noticeable artifacts.
Note: Neither the standard STANAG-4591 reference implementation nor the TSVCIS NB vocoder natively supports seamless, artifact-free VDR transitions. Compandent’s enhanced MELPe suite maintains full STANAG-4591 compliance during fixed-rate operation while enabling dynamic, on-the-fly rate switching across 2400, 1200, and 600 bps to accommodate fluctuating channel conditions.