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V.61 Modem DSP C Code

Description: implements the ITU Data Modem function V.61. This operates in two full-duplex modes over the telephone network: audio+data for Simultaneous Voice Data (SVD) where the data rate is 4.8kb/s, and data-only for rates of 14.4, 12, 9.6, 7.2, & 4.8kb/s. In the data-only mode, an auxiliary channel of up to 343b/s is available. Operation can be switched at the user interface during data mode and can be independent in each direction, though both ends send at the same rate in data-only mode. Duplex channel separation is achieved by echo-cancelling for both land-based and satellite calls. Good error performance is assured by the use of adaptive equalisation.
         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 DSP code for synchronisation to the far-end modem. At the data interface, data is transferred in buffers under pointer control.
         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. The software can automatically set the data rate according to line conditions and can renegotiate the data rate during data transmission as conditions change to maintain good throughput/error performance. The decision criteria can be adjusted at the user interface. SNR for 10-6 Error Rate at data-only 14.4kb/s (4kHz BW noise):

  • 26.0dB Flat Channel

Processor Load:

MIPS# Data memory (words) Prog memory (bytes)#
33.5 6.3k$ 23.1k

- #example figures from compiling for the TI TMS320C5000(C55x) DSP processor
- they apply to both 8.0 & 9.6kHz sample rates
- $allows for 1.3secs. round-trip-delay and is selectable

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

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