FIPS 137

FIPS 137, originally issued as FED-STD-1015, is a secure telephony speech encoding standard for Linear Predictive Coding vocoder developed by the United States Department of Defense and finished on November 28, 1984.^[1] It was based on the earlier STANAG 4198^[2] promulgated by NATO on February 13, 1984.

FED-STD-1015 was re-designated as Federal Information Processing Standard (FIPS) Publication 137, (FIPS PUB 137) on October 20, 1988.^[3]

It is also known as "LPC-10".

The codec uses a bit rate of 2.4 kbit/s, requiring 20 MIPS of processing power, 2 kilobytes of RAM and features a frame size of 22.5 ms. Additionally, the codec requires a large lookahead of 90 ms.

In 1998, an improved version^{[according to whom?]} of the standard was introduced. With a longer super frame structure and better VQ quantizer, the bit rate is reduced to 800 bit/s.^{[4][unreliable source?]}