Modem Pass Through (G.711)

Started by Cloudschatze, March 19, 2013, 09:31:17 PM

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Cloudschatze

Greetings,

I'm interested in interfacing an OBi202 unit with an analog, dial-up modem. (Yes, I realize it's 2013 and all, but hey, PPP Battle Chess and stuff...)

The product data-sheet lists "Modem Pass Through (G.711)" support, and I'm just wondering if this is enabled by default, with modem activity being automatically detected, or if certain settings need to be changed in order to emulate this functionality.

The basic guidelines for modem pass-through support, as best I can tell, include the following:

- Use of the G.711 CODEC
- Disabling any high-pass filtration
- Disabling voice activity detection (corresponding to "Silence Suppression" in the OBi units?)
- Use of jitter buffer algorithms
- Disabling echo cancellation features

Most of these requirements can be addressed manually, from what I've been able to gather from the OBi admin guide, with the exception of disabling the echo cancelling, and any reference to high-pass filtering, neither of which are mentioned as configuration items. If these things are all taken care of automatically, per the stated pass-through support, then all-the-better.

Any help/information/ridicule is appreciated.

giqcass

#1
I hate to discourage you but I really doubt this is going to work.  I give you about one chance in ten.  If it does work you will probably have to turn the speed down to the lowest possible settings.  I am interested in understanding what you are trying to accomplish with this setup.  Perhaps there is a better solution to your problem.

EDIT: I'm thinking about trying this myself just to see if it's possible.  I believe I'm the only person to ever hook a floppy disk drive to an OBi202.
Long live our new ObiLords!

Cloudschatze

#2
It should work though, and without having to resort to < 9600 baud speeds. Modem passthrough support for VoIP, with its respective requirements, is pretty well defined at this point, and is listed as a feature of the Obihai devices - it's more just a matter of figuring out what, if anything, needs to be done to enable this support. Some devices feature modem tone detection, while others may require the use of a specific star-code (*99, in the case of Cisco).

The intent is to simply, and cheaply, add a secondary "line" to be used just for modem activity.

giqcass

#3
The reason I suspect it won't work is because faxing is so hit or miss.  In addition many devices that report to other computer systems with a modem tend to not work.  I agree in theory it should work.

Assuming I understand the setting correctly the Obi uses tone detection.  There are a couple of options to change the type of tone detection.  You should be able to force it as well.

You can force the following settings individually.
- Use of the G.711 CODEC
- Disabling Silence Suppression
- Disabling echo cancellation features

High pass filtering should be off when you use the G.711 codec.

I think the problems lie in sampling rates and high pass/low pass filtering.  There is no way to disable sampling in a digital system.  You can only increase the sample rate.


I was more interested in what types of services required the use of a modem.  There may be better alternatives.

You may find some useful info by searching for threads here about faxing, alarms systems, and check out this link.  http://www.obitalk.com/forum/index.php?topic=5234.0
Long live our new ObiLords!