In this paper, we describe the real-time implementation effort of MPEG-4 audio HVXC (Harmonic Vector eXcitation Coding) encoder algorithm for very low bitrates, which has target applications from mobile communications to Internet telephony, on current high performance floating point DSP. Using the C-language and assembly-language level optimization for time-critical functional codes, utilizing the internal program memory of the DSP as the program cache, and further utilizing the internal data memory operation and DMA functionality we could get a goal of realtime operation of HVXC encoder both at 2 kbit/s and at 4 kbit/s.