TIA IS-54 VSELP Vocoder
for TMS320C3x/'C4x
by DSP Software Engineering, Inc.

Software Overview

The VSELP vocoder software is an implementation of TIA IS-54 VSELP for the TMS320C3x. 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
•	In-band synchronization capable (at 8000 bps)
•	Object-based interface
•	SPOX compatible
•	C-callable functions

Processor and System Requirements

•	Devices supported: TMS320C3x, TMS320C4x
•	Algorithm category: Vocoder
•	Requirements:
All memory requirements are in units of 32-bit words. The MIPs ratings presented requires the 
listed amounts of on-chip, 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)
12.8
1.8 k
6.6 k
8.3 k
Decoder (half duplex)
2.2
1.8 k
6.6 k
8.3 k
Full Duplex*
15
1.8 k
6.6 k
8.3 k
*  VSELP encoder and decoder software are included in one module that is operable in half duplex.
** Includes 1.5 k on-chip scratch memory required during function execution only.

User Functions

The VSELP implementation consists of two C-callable functions that perform encoding and decoding 
operations. It also includes functions that reflect the implementation''s object-based interface. 
The encoder and decoder interface with arrays of linear PCM samples and 16-bit code words.
VSELP Encoder/Decoder
VLP_create (...)
Initializes data memory for the encoder and decoder
VLP_encode (...)
Encodes a frame of linear PCM samples into a frame of code words
VLP_decode (...)
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 used in telecommunications and multimedia applications such as computer telephony, digital 
wireless, PSTN, and satellite communications, and videoconferencing. 

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