[asterisk-users] [Zaptel] Why no driver for PCI voice modems?
Steve Underwood
steveu at coppice.org
Wed Apr 1 12:47:00 CDT 2009
Tim Nelson wrote:
> ----- "Wilton Helm" <whelm at compuserve.com> wrote:
> >
> >If half-duplex audio is good enough for you, sure.
>
> You've lost me there. I am not aware of a modem that is for sale
> today that is half duplex. (OK some support a couple of minor half
> duplex modes). All state of the art modem protocols send and receive
> simultaneously using the full 300 - 3000 Hz bandwidth in both
> directions with adaptive equalization and echo cancellation to make it
> work, which is pretty much what a voice circuit need. There are two
> differences: 1) The response and quality of a current modem must be
> considerably higher than what is needed for voice use or it would
> never achieve the throughput expected of it, and 2) the adaptive
> equalization algorithm is designed around modem specific techniques.
> The latter is (especially for a softmodem) a software issue, not a
> hardware limitation.
The hardware modems are usually half-duplex. If you use the hardware of
winmodems directly, bypassing their normal drivers, you can get a good
quality bidirectional channel.
>
> >Only a fraction of the hardware available is actually capable of full
> duplex audio.
> >
>
> Absolutely not the case. Particularly the softmodems (the most
> inexpensive) contain little else than what is required for placing and
> answering full duplex audio calls. Everything else is in the driver.
> The OP is 100% correct, that they would be an excellent candidate for
> FXO use in low volume applications.
Its only really the winmodems that are of interest, so you are quite right.
>
>
> >What it really comes down to is a value proposition:
>
> Quite true. This is the real issue. As mentioned, these drivers
> require considerable skill and knowledge to write. While there is no
> doubt that the result would be very cost effective, the business model
> is lacking. The modem manufacturer is going to see the potential
> market for this as somewhere down in the noise compared to their
> normal modem sales, so isn't inclined to invest. A third party
> developer with the skills would have a difficult time recouping
> development costs (let alone any profit) because they don't control
> the hardware, and therefore have no leverage. A user with enough
> volume to justify paying for the development (or doing it if they had
> the skill) probably has enough volume to use T1s instead. If everyone
> that could benefit from using a modem card were to pitch in $10
> towards the development, it would probably be quite possible. But how
> to make that happen?
Its straightforward to achieve, but nobody bothers. The Linux drivers
for most winmodems have the DSP (which you don't need) as a binary blob,
and the kernel driver (which you'll need to modify) as source code. As a
modem they normally run at 9600 samples/second. Most of the chips can be
programmed for 8000 samples/second, though, so they'll do what you need.
Use them with OSLEC, and you could get great results. There are only a
few suppliers of these winmodem chips - some USB and some PCI. You
wouldn't need a lot of drivers to cover practically the whole market.
As I said. Its all possible. The necessary hardware info is mostly out
there in downloadable source code. Its just that nobody has bothered.
Regards,
Steve
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