VOCAL’s Mixed-Excitation Linear Predictive (MELP) voice codec is optimized for leading DSPs as a real-time implementation that can be configured to support multichannel applications. The MELP vocoder specification, first released in 1999, is commonly referred to as MIL-STD-3005 (U.S.) or STANAG 4591 (NATO).
VOCAL’s MELP voice compression algorithm is available standalone, as part of an embedded library, or with a VoIP stack for integration with developer applications. MELP voice codec software may be licensed as binary or source code (C and assembly) for a flat fee and VOCAL does not charge a royalty. Custom designs are also available to meet specific application requirements. Contact us to discuss your application requirements.
The MELP codec evolved from improvements and modifications to another code excited linear predictive (CELP) coder known as LPC-10. An improved version of the algorithm is available, the mixed-excitation linear predictive enhanced (MELP enhanced or MELPe) vocoder.
The MELP voice compression algorithm specifies the 2400 bps mode of operation with an optional postfilter available for the decoder. The MELP frame size is 22.5 ms in duration with each frame containing 180 digital voice samples at a sampling rate of 8,000 kHz. Recommended analog requirements are for a nominal bandwidth ranging from 100 – 3800 Hz. Although the MELP encoder audio compression algorithm can operate with a limited low bandwidth signal, there is some degradation in performance.
MELP encoding is robust in difficult acoustic environments with significant background noise and reverberation such as those frequently encountered in commercial and military communication systems. The MELP encoder speech compression algorithm is very efficient in its computational requirements. This translates into relatively low power consumption, an important consideration for portable systems.
The MELP decoder uses extensive lookup tables and models of the human voice to extract and regenerate speech. The MELP algorithm is tuned to regenerate the English language and, as a result, speakers of non-Germanic languages generally rate MELP speech intelligibility more poorly than English speakers.
MELP audio quality is achieved using a mixed-excitation model that can produce more natural sounding speech than traditional pitched-excited LPC vocoders because the MELP algorithm can represent a richer ensemble of possible speech characteristics. The LPC vocoders use either a periodic pulse train or white noise as the excitation for an all-pole synthesis filter. These vocoders do 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 used in LPC algorithms to reproduce a complete range of voiced audio.
- DAA interface using linear codec at 8.0 kHz sample rate
- Direct interface to 8.0 kHz PCM data stream (A-law or μ-law)
- North American/International Telephony (including caller ID) support available
- Simultaneous DTMF detector operation available – (less than 10 talkoff hits on Bellcore test tape set)
- MF tone detectors, general purpose programmable tone detectors/generators available
- Data/Facsimile/Voice Distinction available
- Common compressed speech frame stream interface to support systems with multiple speech coders
- Dynamic speech coders selection if multiple speech codecs available
- Can be integrated with G.168 Echo Canceller and Tone Detection/Regeneration modules
- Multiple ports can be executed on a single DSP
- Available with VoIP stack
VOCAL’s speech coders are available for the following platforms:
- Texas Instruments - C6000 (TMS320C62x, TMS320C64x, TMS320C645x, TMS320C67x, DaVinci), OMAP, C5000 (TMS320C54x, TMS320C55x)
- Analog Devices – Blackfin, ADSP-21xx, TigerSHARC, SHARC
- AMCC – PowerPC
- MIPS – MIPS32, MIPS64, MIPS4Kc
- ARM – ARM7, ARM9, ARM9E, ARM10E, ARM11, StrongARM, ARM Cortex-A8, Cortex-M1
- iPhone / iPad
Please contact us for specific MELP voice codec supported platforms.
VOCAL’s MELP voice codec is available as source code or binary for the following Operating Systems:
- Unix, Linux, uClinux
- Microsoft Windows ACM / RTC / CE / Mobile
- Mac OS
- Green Hills Software
Please contact us for other supported Operating Systems for the MELP vocoder.