On Wed, Jul 11, 2001 at 11:50:32AM -0400, Derek Glidden wrote:
> edoc wrote:
> >
> > Anyone know if a Motorola Modemsurfer 56K External Modem
> > is OK under Linux?
> >
> > I checked the usual sites and found no mention of it positive or
> > negative.
>
> All external modems I've ever seen are real hardware modems and should
> work just fine with Linux.
Let's qualify that as "all external rs-232 serial modems". And even
then, remember the silliness of RPI (which really isn't a Winmodem
by definition - the compression/framing is merely moved off to the
host):
http://www.zoomtel.com/techsprt/rpi/rpi_faq.html
The RS232 "bus" is simply too slow to handle oversampled digital
audio capture of the raw analog signal for DSP processing on the host
CPU (the true definition of a "Winmodem").
Think of a Winmodem as a soundcard that hooks up to your phoneline.
Instead of a hardware Digital Signal Processor on the modem, the
Winmodem uses software on your host CPU to encode/decode the signal.
The soft-modem takes the raw digital audio samples of the analog
information from your phoneline and processes it (modulates/demodulates
it from fsk, psk, or v.90 symbol lookup) into the raw digital framed
data that is being sent over the wire.
Now, if the RS-232 serial linespeed isn't enough, what about USB?
Don't we already have USB soundcards? What's the big difference
between a soundcard and a Winmodem? It hooks up to a phoneline?
With an 11Mbit bus, and working digital<>audio convertors already
available cheaply (any "USB speaker" chipset, really), what's so
hard to believe that a USB Winmodem wouldn't be possible or even
highly feasible?
Some USB modems use the Abstract Control Modem (ACM) definition of
modem abstraction to present a standard serial device to the host.
Those modems have a built-in DSP, and work well with Linux.
Sadly, most cheap USB modems *DO* appear to be soft-modems (thus
fitting the basic description of Winmodems). Many of them simply
won't work with Linux.
> I don't know if anyone has ever, or even *can* make an external
> Winmodem, but I think it would very very difficult and probably would
> outwiegh the advantages of not having to build the hardware modem to
> begin with.
An RS-232 external winmodem? Nope.
A USB external winmodem? Of course.
- Ian
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