Low-Memory CELP Vocoder
for TMS320C5x
by DSP Software Engineering, Inc.

Software Overview

Low Memory CELP (LM-CELP) is a speech encoding/decoding algorithm developed by DSPSE that provides 
4-kHz speech bandwidth at either 4875 or 7458 bps. The algorithm has been fully implemented on a 
single TMS320C2x or TMS320C5x DSP. This implementation incorporates voice-activity detection, 
codebook search every subframe (7458 bps), and optimal postfiltering to provide near-toll- quality 
speech at maximum bit rates of 4875 and 7458 bps. Enabling voice-activity detection (VAD) can 
reduce the average bit rate.  The bit rate of VAD frames is 1083 bps. The algorithmic delay of 
LM-CELP is 9 ms.
LM-CELP encodes 192 sample frames of 16-bit linear-PCM data into frames of 117 or 179-bit 
codewords. VAD frames are 26 bits long. LM-CELP is ideally suited for applications requiring 
near-toll-quality speech coding while minimizing total system costs.  Typical applications include 
high-compression digital tape recorders, secure communications, personal-communications networks, 
land-mobile radios, tactical communications, and voice mail systems.

Features and Benefits

•	High-quality voice coding on a single fixed-point DSP using no external data memory
•	Provides 4875- or 7458-bps voice compression
•	Capable of multichannel operation
•	Constant bit rates of 4833 or 7417 bps possible with VAD removed
•	Voice-Activity Detection (VAD) to lower average bit rate
•	SPOX compatible
•	Comfort noise modeling improves perceptual quality during unvoiced segments

Processor and System Requirements

•	Devices supported: TMS320C5x
•	Algorithm category: Vocoder
•	Requirements:
All memory requirements are in units of 16-bit words. The MIPs ratings presented require the listed 
amounts of dual-access RAM, zero-wait-state program and data memory, and a processing frame size of 
192 samples compressed to 4875 bps. All measurements were made using an executable demonstration 
built for a third-party PC plug-in board.
Function
Avg MIPS
DARAM
Data
Program
Encoder (half duplex)
11.0
0.8 k
0
3.9 k
Decoder (half duplex)
2.6
0.5 k
0
2.4 k
Full Duplex*
13.6
1.0 k
0
5.6 k
* LMCELP encoder and decoder share significant portions of data memory and program.

User Functions

The LMCELP implementation consists of two C-callable functions that perform encoding and decoding 
operations. It also includes functions for initializing the encoder and decoder. The encoder and 
decoder interface with arrays of linear-PCM samples and 117- or 179-bit code words (rate 
dependent).
LMCELP Encoder
create_analysis (...)
Initializes data memory for the encoder
analysis (...)
Encodes a frame of linear-PCM samples into a frame of code words
LMCELP Decoder
create_synthesis (...)
Initializes data memory for the decoder
synthesis (...)
Decodes a frame of code words into a frame of linear-PCM samples

Algorithm Verification

•	Call DSPSE for verification details

Availability

•	Source and object code currently available for licensing.

Company Background and Contact Information

DSP Software Engineering, Inc. is a leading provider of highly-complex digital-signal-processing 
software.

Contact:	DSP Software Engineering, Inc
		175 Middlesex Turnpike
		Bedford, MA 01730  USA
		(617) 275-3733
		Fax: (617) 275-4323
		e-mail:  info@dspse.com
		www:  http://www.dspnet.com