On Thursday 31 August 2006 17:20, Nick Rout wrote: > I am struggling to see how an external serial port (as opposed to usb) > modem can be a winmodem. How can the OS require a driver, other than a > serial driver? Windows drivers for serial modems are usually limited to > a file specifying the strings to send to the modem to initialise it into > a given state, and an interpretation of the return codes provided by the > modem.
It would appear that the modem folks have realized that a serial port on a normal, i.e. relatively modern, computer will perform at nearly 1Mbyte/sec - 921600 bytes/sec actually - look at the fastest rate supported by kppp. This is plenty fast enough to transmit an accurate digital representation of the analog tones coming off the 'phone line, and similarly convert the digital numbers out of the software modem into a voltage analogue for transmission out to the 'phone exchange. Thus the 'External WinModem' is really not much more than the line terminating capacitors and whatnot, an ADC [1], a DAC [2], and the circuitry to interface to the serial port. And the whole thing is probably just one of those $0.50 PIC chips suitably programmed. [1] Analogue Digital Converter [2] Digital Analogue Converter The problem of course is that the signal processing software needs CPU cycles just at the exact same moment as does the Web Browser, so the whole exercise bogs down to a crawl at the crucial moment, unless of course you have a 3GHz+ machine. Now Joe Six-Pack doesn't realize this and buys solely on the price. Naturally enough I suppose the modem manufacturers take the commercial hint. -- CS
