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DSP/CODEC Modem Implementation

Murton System Technologies have achieved an exceptional signal to noise ratio in the new OEM DSP-CODEC modem designed for telemetry applications, now available for general use.

“For most point-to-point telemetry applications, modems are used in a ‘polling’ environment or in a master-slave configuration.  The master polls each device or node connected to it periodically,” added Brent Maurer Murton System Technologies MD.

“Modern modems, the ones designed for connection to the Internet for example, require a considerable amount of preamble time to negotiate various communication parameters when they connect.  Internet modems embark on a complex set of negotiations at the start up to determine the best baud rate, error correction scheme and communications protocol.  This takes several seconds to accomplish,” explained Maurer.

In a telemetry application, the host needs an immediate response from any device it polls and no time can be spent on negotiating a set of communication standards. “With this in mind, we developed the modem,” said Maurer.

At the hardware level the modem consists of 60MHz internal DSP (digital signal processor) connected to an industry standard CD quality stereo CODEC (coder/decoder).  The DSP and CODEC connect to one another via a high-speed industry standard serial bus. 

On the user interface side of the modem both ‘left’ and ‘right’ signals are available.  Both CODEC channels are made available allowing the end user applications to implement a number of options such as main or standby configuration, data routing and dual channel modem configuration. 

Signals can be sampled at 44kHz making the CODEC ideal for medium to high-speed FSK applications. 

The host side connection is achieved by means of a standard MOTEL (Motorola/Intel) parallel interface.  “As most off-the-shelf modems have a serial interface for hooking up to the control equipment, it was decided to use a parallel interface,” comments Maurer.

This has several advantages for embedded applications.  Firstly, the need for a UART (universal asynchronous receiver transmitter) is eliminated.  Secondly, the processor overhead requirement and transfer speed between host and DSP, for most embedded controller types, is substantially reduced since the transfer of data between controller and modem is done on a byte wide basis.  Thirdly, the design allows for a ‘host download’ connection to the modem.  What this means, is that at start up, the host controller downloads the firmware code into the DSP.

Only the firmware required by the application is downloaded into the DSP memory at run time. 

The original firmware currently implements FSK communications. The firmware is based on the standard sine wave generation techniques used with CODEC’s but uses a unique demodulation process of statistically comparing the incoming signal with both the possible FSK signals representing 1 and 0. 

The modem is currently available as an OEM card without firmware and customized firmware is available on request.

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