Cellular Text Telephone Modem (CTM)

Software Application

ILLICO is offering Digital Signal Processing software for the 3rd Generation Partnership Project cellular text modem (CTM). This modem is described in Technical Specification 3GPP TS 26.226. In North America, hearing and speech-impaired persons have traditionally used specific Text Telephones (called TTY) to communicate over the fixed network. These devices use a simple Baudot Code modulation which can experience unsatisfactory error rates over the voice codecs of cellular systems. A Cellular Text Modem intercepts Baudot modulated signals and re-formats the modulation to a 4 tone FSK format which includes sophisticated features such as interleaving and convolutional coding. After passing through the cellular system, this signal is decoded and the information is re-modulated back into the simple Baudot Code modulation for PSTN transmission toward the receiving subscriber or emergency service. The CTM can therefore be viewed as a real-time relay system for traditional TTY communications.

Software Overview

The 3rd Generation Partnership Project has published a C-code demonstration version of the CTM including test specifications. ILLICO has re-structured, optimized, and re-qualified this software for the T.I. TMS320C54x DSP family. The result is a mixed 'C' and assembly language product which significantly reduces DSP resource loading including MIPs, stack and data memory usage. ILLICO has also provided a calling interface which observes the C54x 'C' calling conventions and provides ease of use in multi-channel processor environments. A user's guide aids installation of the software.

Future Extensions

The Baudot code modulation is only one several text telephone modes outlined in the 1998 ITU Recommendation V.18. The current FCC requirement for CTM calls for support of only Baudot formatted signals. U.S. and international requirements will likely evolve to include other modulations as well. ILLICO's modem design experience will enable us to track and support these new requirements as they emerge.

CTM program resource usage for TMS320C54x / C55x:

- Internal data memory: 3.5K words/channel
- Program memory: 10K words
- MIPs (millions instructions/sec): 35 MIPs for restructured bit-exact version. 
- MIPs (millions instructions/sec): 10 MIPs with ILLICO proprietary DSP routines. 

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