duplex, PTT activation

[Roman Belyakovsky]

Hello! I'm looking for software for a link over radio and can't figure out if this one suites, so let me ask a few questions. First of all, does it operate in a half-duplex mode?

Second question is a bit more complicated. The common scenario is to set up a VOX function on a radio so it could automatically start and stop TX when it detects sound input. It doesn't work smooth and leads to delays in activation and deactivation. The second option is to use one channel for data and another one for PTT activation signal, it requires some simple electronics but should work much smoother. So I wonder if I could use both of these scenarios with this software.

[Samúel Úlfr Þór Guðjónsson]

APRS operates in (recorded) simplex mode.
IE. Packet is received, decoded, stored, encoded and then transmitted on the same frequency as received.

What you are looking for is more likely a simplex audio repeater?

I'm not quite sure I understand what you are looking for tho'.

You need to transmit both voice and data at once?

73 de TF2SUT - Samúel

On 23 January 2014 21:10, Roman Belyakovsky <ihry...@gmail.com> wrote:

Hello! I'm looking for software for a link over radio and can't figure out if this one suites, so let me ask a few questions. First of all, does it operate in a half-duplex mode?

Second question is a bit more complicated. The common scenario is to set up a VOX function on a radio so it could automatically start and stop TX when it detects sound input. It doesn't work smooth and leads to delays in activation and deactivation. The second option is to use one channel for data and another one for PTT activation signal, it requires some simple electronics but should work much smoother. So I wonder if I could use both of these scenarios with this software.

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[Roman Belyakovsky]

Nope, I'm looking for a radio link between 2 or 2+ computers.

[[PC _sound card] radio] <====> [radio [sound card_ PC]]

But radio cannot transmit and receive at the same time, and it also requires transmission activation.

[Samúel Úlfr Þór Guðjónsson]

If the encode/decode is done by the PC, I'd assume that using an MCU for controlling the PTT line is a bit of an overengineering.
Might as well just use a parallell port from the computer (if i has one) to trigger a simple PTT circuit.

(Basically few resistors and a transistor)

What programs will you be using for the encode/decode process on the PCs?

73 de TF2SUT - Samúel

[Roman Belyakovsky]

Well, I was going to try extmodem for decode/encode as a starting point. 

Just checked it's help, there's a tx delay option, is it for a VOX activation?

Sent from my phone

[Samúel Úlfr Þór Guðjónsson]

If you already have a PC with soundcard, you'd be better off using some software on the computer for encode/decode proccess. Could use vox if the radio supports that for keying the transmitter.

If you are gonna use the avr as the modem chip (for encode/decode) you'd want to use serial to the computer, and then just use ptt from the avr or vox if the radio supports that.

TX delay is used to make sure the transmitter is already keyed, ie in the air so to speak, before the burst of the AX.25 (aprs) package is sent to the radio.

73 de TF2SUT - Samúel

[Roman Belyakovsky]

Ah I see, I've forgotten this list is also AVR related. I'm going to use sound card only, VOX is fine for a starting point. But this software provides only point to point connection, right?

Sent from my phone

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[Samúel Úlfr Þór Guðjónsson]

haha... no wonder I wasn't sure what you meant. I didn't realize you were referring to extmodem.

My head has been focused on AVRs for the last weeks so no wonder I make weird assumptions.

It's point to point, but there isn't any handshaking or identity checks going on so all the computers running the program with radios should be able to decode the packages that they get.

In other words, if you set up three computers or more, with radios, and PC A transmits, then PC B and PC C would be able to receive the packages. As long as they are all working on the same frequency (for aprs in europe that'd be 144.8MHz).

I haven't really tested Santos' extmodem but I assume it works in a similar manner as most soundmodems for APRS or ax.25 (Note, I assume, as it's aprs compatible that it's not fully an ax.25 modem, as the APRS drops alot of functions that were in the old AX.25 protocol.)

Note: I haven't really tested this program, but I'll hook it up later today and see how it works if I have the spare time.

But you'd need another application to deal with the messaging and to create APRS packages, location reports and telemetry.

Like AGWPE or SarTrack or some other TNC capable APRS programs.

extmodem (afaik) only acts as a software TNC I think, (preparing the packages and encoding/decoding, setting the TX delay and such)

if the VOX on the radios you're using is a quality one, then I don't see any reason for it not to work. Might have to bring the TX delay a bit up. (TX Delay basically just adds "empty" packages in front of the real packages to make sure the transmitter is already sending, when the package goes to it up)

What kind of data are you trying to send out? normal APRS location packages? Messages or something else?

I hope this helps something, and sorry for the misunderstanding. :)

73 de TF2SUT - Samúel

[Roman Belyakovsky]

The most interesting thing is a TCP connection on top of radio links, looks like it was implemented more than 30 years ago.

http://en.wikipedia.org/wiki/Packet_radio#Data_link_layer:_AX.25

But in a real life scenario only a simple chat could be wanted. So I was googling for an AX.25 implementation and found extmodem. I’m going to use license-free 430mhz frequencies. I’ll try VOX in a few days but I think that “pushing” a PTT button with a special signal on one of two output channels could really speed up the link and I do like this idea. Hope to get all the required components next week and build this button-pusher.

—

Roman

[Samúel Úlfr Þór Guðjónsson]

Ah, I get ya.

You should use the "message" feature of APRS then.

Simply hook up radios connected to extmodem and agwp or sartrack or any other message capable software and you should be able to transfer messages back and forward.

Messages in APRS do have some level of ACK, they retry for 5 or 10 times ( differs between software I suppose) and the receiver sends some kind of ACK confirming they got the message.

AX.25 isn't really TCP, but kind of. APRS is AX.25 migrated, because, well AX.25 was too complicated for normal radio use where  you'd want a single burst of transmission without knowing the "receivers" address.

73 de TF2SUT - Samúel

[Roman Belyakovsky]

Thank you, I’ll give APRS a try once I’ll get radios connected to PCs. Looks like AMPRNet does the TCP but googling for AMPRNet software is a bit frustrating.

—

Roman

[Samúel Úlfr Þór Guðjónsson]

AMPRnet is outdated, actually. There are quite a few better options available, along with APRS.

Try to take a look in GMSK.

73 de TF2SUT - Samúel