Mixed-Mode Operation on Voice RepeatersAuthors Note: This article is obsolete as it stands, and is in the queue for a re-write/update. - N5PVL
by Charles Brabham N5PVL
Mixed-mode operation, where voice and digital transmissions are used in turn on the same frequency, has never seen much use by amateurs, though we have been able to do this for a quarter of a century or so. Recently mixed-mode operation on VHF/UHF has appeared in many discussions concerning e-comms, but there still hasn't been a lot of actual mixed-mode operation going on... The purpose of this article is to show how amateurs can either experiment with mixed modes right now, or go ahead and put them directly to work on VHF or UHF repeater systems.
Probably the most well-known mixed-mode operation occurs in police vehicles, where data is squirted across to the patrol car on voice channels. The data is usually information about a person, a vehicle, or a location and is displayed in a small screen in the cruiser.
Amateurs can think up many fun and useful things to do with mixed modes, so I will leave that aspect of mixed-mode operation up to other amateurs, and just talk about the "how to get on the air" part of it here.Operating Environment
Voice repeaters are optimized for voice operation, which generally introduces delays and limitations upon any kind of digital operation that you try there. Speaking in Packet terms, you have to use very long TXDELAY values, up to one full second or more instead of 300 ms or less, which is standard on VHF Packet. If you don't use enough TXDELAY the first part of your data stream won't make it through, which means no commo.
You will have to experiment with the amount of delay needed for a particular repeater... Keep bumping it up in small increments until you get good communications, then back it off until it messes up, then go back up *just a hair* over the amount of delay you absolutely have to have.
Another consideration is that voice repeaters can only handle so much speed, or "baud rate". The top speed limit appears to be around 4800 baud, but this of course will vary from one repeater to another.
One way to regain some of the efficiency lost by the long TXDELAY is by utilizing very long PACLEN and high MAXFRAME values.
Last, it should be mentioned that short, back and forth digital transmissions should be avoided if at all possible, due to the way the overhead stacks up under those conditions. It becomes very inefficient in short order. The most efficient use of the digital capability will be in transferring files.Flex32, Soundmodem and Paxon
There are a number of software/hardware packages that can be used for mixed-mode operation. The system I will describe here utilizes Flex32 with it's Soundmodem driver, along with the excellent Paxon terminal program with it's nice interface for file transfers.
Flex32 with it's Soundmodem soundcard driver is very flexible for mixed-mode use, as a number of different digital modes are available with this package. Modes especially useful for mixed-mode operation over a voice repeater include:
1.2kb AFSK, 2.4kb AFSK, 4.8kb AFSK and Newqpsk, which is actually Q15x25 mode.
The tutorial on setting up Flex32 with the Soundmodem driver for 1.2kb Packet can be found here, and it can be used to set things up. When it comes time to set the mode to 1.2kb AFSK, just scroll down the drop-down menu to look over the other choices there.
For ease of use and fairly universal reliability, I recommend Newqpsk mode instead of AFSK Packet. It is reasonably fast at 2.5kb, and naturally tends to the long PACLENS and high MAXFRAME values needed over a voice repeater. I've had good luck with it over the repeater, with TXDELAY being the only parameter I had to mess with. ( 800 ms in my case )
Performance freaks may find that they prefer to try 4.8kb Packet.
Flex32 goes well with the terminal program Paxon, which directly accesses the Flex32 kernal. A tutorial on setting up Paxon can be found here. I have found Paxon to be especially nice for file transfers, and it has special sound alert features that could be very useful for e-comms.The Bonus Plan
In the past, I have used Flex32's simple digi feature to create a Packet / Q15x25 gateway. I used the gateway for experiments on HF with Newqpsk (Q15x25) and didn't think of it at the time - but - the same gateway system could be used to tie your digital voice repeater link in to a conventional packet radio network.
You could use a voice repeater as an emergency stand-in for a packet node that is down, or use a voice repeater to extend the coverage of a packet network into a disaster area. Conversely, you can use the packet network to speed data to and from the e-comms front line area, where a mixed-mode setup might be employed. - All this besides the more conventional uses for mixed-mode operation that I wasn't going to talk about.
It is hoped that this article will inspire a few more hams to get out there and develop the use of mixed-mode operation on our VHF/UHF systems.
Hey, don't forget to clear what you are doing with the repeater's control operator or owner, eh?
Good luck! Charles Brabham, N5PVL