TIA IS-54 VSELP Vocoder
for TMS320C5x
by DSP Software Engineering, Inc.

Software Overview

The VSELP vocoder software is an implementation of TIA IS-54 VSELP for the TMS320C5x. VSELP is a 
North American standard for encoding 8-kHz sampled speech signals for transmission over 8-kbps 
digital cellular channels.VSELP provides approximately 4 kHz of speech bandwidth and has an 
algorithmic delay of 7.5 ms.
VSELP encodes frames of 160 linear-PCM samples into ten 16-bit code words. The IS-54 VSELP standard 
is for digital-cellular communications but has numerous other applications that require higher 
speech quality at a low-digital bit rate. These applications include digital telephony and voice 
storage.

Features and Benefits

•	Operable at 7950 bps
•	Single TMS320C5x implementation
•	C-callable functions
•	In-band synchronization capable (at 8000 bps)
•	Fully-optimized TMS320C5x Assembly language code
•	SPOX compatible

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 
160 samples. All measurements were made using an executable demonstration built for a third-party 
PC plug-in board.
Function
Avg MIPS
Chip RAM**
Data
Program
Encoder (half duplex)
15.3
0.6 k
0.7 k
8.0 k
Decoder (half duplex)
3.3
0.6 k
0.9 k
2.7 k
Full Duplex*
18.6
0.6 k
1.1 k
8.7 k
*  VSELP encoder and decoder share significant portions of data and program memory.

User Functions

The VSELP 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 16-bit code words.
VSELP Encoder
create_vselp_analysis (...)	
Initializes data memory for the encoder
vselpa (...)
Encodes a frame of linear PCM samples into a frame of code words
VSELP Decoder
create_vselp_synth (...)
Initializes data memory for the decoder
vselps (...)
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