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Bell 202 Modem DSP C Code

Description: implements the data pump function of the modem Bell 202. This provides half-duplex data transmission over 2 wires or full-duplex over 4 wires at two rates, one 1800b/s and the other 1200b/s, over the telephone network.
        
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 samples 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. A stripped down version is available for Caller Identification (CID) use.

Interfaces: at the sample interface, the code can operate at 8.0kHz and 9.6kHz sample rates, tx and rx sample transfer being simultaneous. At the data interface, operation can be in either Asynchronous or Synchronous mode. In Asynchronous mode, data is transferred with start & stop bits auto-inserted/-deleted. In Synchronous mode, data is transferred transparently, with the receiver performing clock recovery on bit transitions. In both cases, the data can be passed in multiple-bit blocks, the nominal bit clock being 300Hz.
         At the control interface, the code has one 16-bit control word to sequence operation and to set operating parameters such as data size, and one diagnostic word to convey data transfer status such as ready-to-send and line-signal-detected.

Performance:

  • SNR for 10-6 error rate at 1200b/s (4kHz BW noise): 12.0dB - Flat Channel
  • SNR for 10-4 error rate at 1800b/s (4kHz BW noise): 23.5dB - Flat Channel

Processor Load:

Sample rate MIPS# Data memory (words) Prog memory (bytes)#
8.0kHz 4.8 39 1.6k
9.6kHz 5.8

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

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

 
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