MELP enhanced codec
MELPe codec supports variable low bitrate multichannel mobile, VoIP and radio voice communications

MELPe (Mixed-Excitation Linear Predictive enhanced) codec with highly optimized source code is used in a range of low power and low bandwidth (600 / 1200 / 2400 bps) mobile, VoIP and radio applications. VOCAL’s MELPe codec is a scalable real-time implementation for multi-channel applications that is run time configurable. Support is also available for the optional Secure 2400bps Blank and Burst MELP voice transmission features in accordance with the Secure Communication Interoperability Protocol (SCIP) 210. Contact us to discuss your voice application performance requirements.

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VOCAL’s expertise in the design, implementation, and configuration of mobile, radio, and telephony solutions will help you meet your low bandwidth communications requirements. For example, the three different speech encoding rates and sample frames sizes, comfort noise procedures and packet loss concealment as well as the use of MELP with SDP are detailed in RTP Payload Format for MELPe Codec.

VOCAL’s MELPe codec source code is optimized for RISC/CISC processors and DSPs from TI, ADI, ARM, Intel and other leading vendors. Our MELPe voice compression algorithm is available either standalone, as part of an embedded software library, or with a VoIP stack easily integrated with your application. Custom designs are also available to meet your specific digital signal processing requirements.

Features

Applications

Performance

MELPe

The MELPe vocoder was developed using several enhancements to the original MELP 2400 bps specification which itself was derived from modifications to another military coder, LPC-10. MELPe speech compression implements a variable low bit rate vocoder that supports multiple rates of 600 bps, 1200 bps, and 2400 bps. It uses the existing MELP sampling rate of 8,000 Hz as well as the original frame size of 22.5 ms containing 180 digital voice samples. A nominal bandwidth ranging from 100 Hz – 3800 Hz is used for recommended analog requirements. Like the original, MELPe can operate with a limited low bandwidth signal; although with some degradation in performance.

In addition to the 600 and 1200 bps rates, the MELPe vocoder also supports compressed bit-stream transcoding between the different rates. Noise Preprocessing (NPP) has been implemented to reduce background noise while the original postfilter has been improved to further increase the quality of speech reproduction.

MELPe encoding is robust in difficult background noise environments such as those frequently encountered in commercial and military communication systems. The speech compression algorithm is very efficient in its computational requirements which translates into relatively low power consumption, an important consideration for portable systems.

Like its predecessor, the MELPe vocoder uses extensive lookup tables and models of the human voice to extract and regenerate speech; furthermore, the decoder is tuned primarily for the English language. As a result, speakers of non-Germanic languages generally rate the coder more poorly than English speakers.

Traditional pitched-excited LPC vocoders use either a periodic pulse train or white noise as the excitation for an all-pole synthesis filter. These voice codecs produce intelligible speech at very low bit rates, but they sometimes sound mechanical or buzzy and are prone to annoying thumps and tonal noises. These problems arise from the inability of a simple pulse train to reproduce all kinds of voiced audio. Because the vocoder provides a richer selection of possible speech characteristics, the MELPe voice quality is achieved using a mixed-excitation model that produces more natural sounding speech.

Configurations

Platforms

supported-platforms

VOCAL’s optimized vocoder software is available for the following platforms. Please contact us for specific MELPe codec supported platforms and performance information.

Processors Operating Systems
  • Texas Instruments – C6xx (TMS320C62x, TMS320C64x, TMS320C645x, TMS320C66x, TMS320C67x), DaVinci, OMAP, C5xx (TMS320C54x, TMS320C55x)
  • Analog Devices – Blackfin, ADSP-21xx, TigerSHARC, SHARC
  • PowerPC, PowerQUICC
  • MIPS – MIPS32, MIPS64, MIPS4Kc
  • ARM – ARM7, ARM9, ARM9E, ARM10E, ARM11, StrongARM, ARM Cortex-A8/A9/A15, Cortex-M3/M4
  • Intel / AMD – x86, x64 (both 32 and 64 bit modes)
  • Linux, uClinux, BSD, Unix
  • Microsoft Windows ACM / RTC / CE / Mobile
  • Apple iOS / iPhone / iPad & MacOS
  • eCOS / eCOSPro
  • Google Android
  • Green Hills Integrity
  • Micrium μCOS
  • Symbian
  • Wind River VxWorks
  • VOCAL LANsEND

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