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V.17 Modem DSP C Code

Description: implements the ITU Data Modem function V.17, which provides half-duplex data transmission at rates of 14.4, 12, 9.6, & 7.2kb/s over the telephone network for use in Group 3 Fax equipment. Good error performance is assured by the use of trellis coding and adaptive equalisation. A fast-train algorithm is implemented for resynchronisation after initial training.
         The C code is suitable for any processor with an ANSI-compliant C compiler. It is designed especially for efficient operation on low-cost fixed-point DSP- and general-purpose micro-processors. All data memory is specified as 16-bit integer words. Multiplies are integer 16x16-bit with 32-bit products.
         The code is operated by one callable subroutine for initialisation and one for main-routine processing of input/output sample blocks and data. The user is responsible for interfacing samples and data to hardware. The code is re-entrant to facilitate multi-channel operation. A comprehensive user's guide is provided with the code, together with example application files. Test results are available.

Interfaces: at the sample interface, the code can operate at 8.0kHz and 9.6kHz sample rates. Transmit and receive samples are transferred on a common clock, to which the transmit clock is synchronised. The receive clock is interpolated in the code for synchronisation to the far-end modem. At the data interface, data is transferred in buffers under pointer control via subroutine calls.
         At the control interface, the code has one 16-bit control word to sequence operation and to set operating parameters, and two diagnostic words to convey data transfer status and signal quality.

Performance: sophisticated techniques have been incorporated into the software to give excellent performance under noisy or poor line conditions. Over flat channels, near matched-filter performance is obtained. Over very severe channels such as the TAS Telephone Network Emulator ConUS Poor Voice channel – 30dB attenuation across the 3kHz band – the modem code will still operate at 14.4kb/s. SNR for 10-6 Error Rate at 14.4kb/s (4kHz BW noise):

  • 22dB Flat Channel ; 26dB Bell 3002 Channel

Processor Load:

Sample rate MIPS# Data memory (words) Prog memory (bytes)#
8.0kHz 12.8 425 12.0k
9.6kHz 11.6 406 10.6k

- #example figures from compiling for the TI TMS320C5000(C55x) DSP processor

Availability: NOW - sale is under licence - integration support offered

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