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V.27 Modem DSP C Code

Description: implements the data pump function of the ITU Modems V.27bis & V.27ter, which provide half-duplex data transmission at rates of 4.8 & 2.4kb/s over the telephone network for applications such as Group 3 Fax. Also provided is full-duplex transmission for 4-wire leased line operation. In both cases, an optional independently-controlled backward channel can be incorporated for rates of up to 75b/s. Good error performance is assured by the use of adaptive equalisation. The code features train-on-data (V.27bis) & fast resync. (V.27ter) from 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: over flat channels, near matched-filter performance is obtained. SNR for 10-6 Error Rate (4kHz BW noise):

  • 4.8kb/s: 15.5dB Flat Channel; 17dB Bell 3002 Channel
  • 2.4kb/s: 9dB Flat Channel; 10dB Bell 3002 Channel

Processor Load:

Sample rate MIPS# Data memory (words) Prog memory (bytes)#
8.0kHz 6.8(10.1) 237(277) 8.4k(9.6k)
9.6kHz 5.5(9.5) 218(258) 7.3k(8.5k)

- #example figures from compiling for the TI TMS320C5000(C55x) DSP processor
- figures in ( ) are with the backwards channel

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

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