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V.25 Modem Function DSP C Code

Description: implements the ITU function V.25 for data modems. This is the 2100Hz auto-answer sequence sent from the answering end at the start of a PSTN modem connection. As well as audibly denoting a far-end modem in answer mode, the 2100Hz signal automatically disables network echo suppressors and, if periodically phase reversed as with the code here, also network echo cancellers.
         The supplied V.25 code is intended to be used as an adjunct to a V- (or Bell-) series modem data pump function. Segment timings of the initial zero signal and the 2100Hz tone can be re-programmed. Note that some Algotron data pumps such as V.22(bis) and V.32(bis) can for convenience implement the V.25 sequence as part of the data pump handshake, so do not require this additional code module unless for its programmability.
         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. The user is responsible for interfacing samples 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, tx and rx sample transfer being simultaneous. At the control interface, the code has control word access to set V.25 segment durations, to sequence operation and to set operating parameters such as data size, and one diagnostic word to convey data transfer status such as V.25 progress.

Processor Load:

Sample rate MIPS# Data memory (words) Prog memory (bytes)#
8.0kHz 2.8 22 0.9k
9.6kHz 3.4

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

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

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