These modes are aimed at keyboard typing speeds, not high speed data transfer. Most hams chat on the air, and the design emphasis has been on modes that allow easy, natural conversation without a lot of delay for error checking and correction. They also tend to be aimed at overcoming the common propagation issues hams face: fading, interference from other stations, and multipath (where several versions of the transmitted message come over different propagation paths and reach the receiver almost simultaneously) are the big ones. This is keyboard chat, much like texting each other.
There are currently at least a couple of dozen modes that can be found with many software programs that support them. Any sound card based mode is going to emphasize signals in the audio bands the radios handle, usually that 3 kHz bandwidth that voice signals occupy.
What I’d like to think of as a best approach is a mode that sounds more like noise than like audio tones being played. The best I’ve heard is a mode called MT63. There’s a few bandwidths of MT63 supported in the cheap/free software, and MT63-1000 is 1000 Hz wide (audio tones from 500 to 1500 Hz) and it supports a typing speed of over 100 WPM. 100 WPM is pretty good, but everything you’ve read to this point takes just over 3 minutes to transmit (plus some time for overhead). There’s an MT63-2000 that goes 200 WPM, too. Here's a screen capture of my transmitter sending a short test ("testing testing de mycall mycall") in MT63-2000:
MT63 is a family of MFSK – Multiple Frequency Shift Keying – modulation modes. That means many audio tones are used, and while there are many MFSK modes, what’s novel about MT63 is that it’s OFDM with FEC. (FEC’ing what?) OFDM means orthogonal frequency domain multiplexing; it has 64 independent tones in that 2000 Hz bandwidth (500 Hz to 2500 Hz) spaced at 1/64 of the 2000 Hz BW, and each tone is differentially Bi-Phase Shift Keyed (BPSK – sometimes called DPSK when differentially encoded). It also includes FEC – Forward Error Correction – a method of reducing errors and increasing robustness. FEC depends on sending redundant information, so the price for FEC is forward speed, but 200 WPM is pretty decent sending speed. It will be slow for sending text files, but is good for tactical messages. To be honest, I’ve seen professional systems that aren’t this sophisticated.
There are other good MFSK or PSK modes, but they all have a tendency to sound musical to some degree. The most popular mode is PSK31, a BPSK mode with a distinctive audio sound, that runs at 31.25 Bits per second or roughly 20 WPM. Many of these PSK or FSK modes use tones that change in some sort of rhythm and can sound like some sort of music. An early mode was called piccolo, and the name was what it sounded like. DominoEx16 is a fast MFSK mode, too, allowing 140 WPM typing. It doesn’t incorporate FEC, but is said to be so good in its basic approach that it doesn't really need FEC. It's very tolerant of frequency drift, up to 200 Hz/minute, something that might be important with radios built from spare parts. It's also helpful with microwave frequency radios, since even relatively good crystals drift a lot when multiplied by thousands to around 10 GHz. The only drawback to the mode is that it sounds more like a quick riff on a flute than noise. It is a quick riff, though. Here's the same message as before, plotted from DominoEX16:
A mode that puts emphasis on reliability of communications rather than pure speed is called Olivia. Olivia is another MFSK mode, which uses two layers of FEC; one in the the number of tones and protocol for switching tones, with another FEC layer in the information coding. It is robust, very error resistant, but 39.1 WPM is the fastest protocol for Olivia. Most contacts occur at the lower 24.4 WPM. 39.1 is still faster than the vast majority of people can receive Morse code, though, so it's still a good mode for conversing. Here's a picture of the same text as the previous two transmissions using Olivia 1000/16
(source: Multipsk by Patrick F6CTE)
The application I think of is modulating a small HT for UHF or microwaves with this audio. If heard, it would probably be considered some sort of local noise source. Especially if it was only present intermittently (if you’re not used to it, it will surprise you just how much radio noise is present, especially at lower frequencies).
"But old man! I don't want to try to drag a radio and a laptop to communicate! Don't you have anything better for me??" ("There is no try! There is only do...") sorry. I think I will, but not tonight. I think the killer app here is a little box that plugs into any HT's microphone and speaker jacks. The box takes in microphone audio, digitizes it and turns it into these tones to transmit. On the receive side, it takes the tones and turns them into bits that then are converted to analog voice that you hear in your headphones. The box doesn't quite exist, but I think I can see how to get there.
Still more to come...
A few years ago, I was going to design, make, and sell a little box like you are talking about. But lots of caterwauling ensued about legality and "it just attracts the government", so I gave up on it. See the thread here.
Backwoods Engineer's Blog
Thanks - interesting link there, with the input from Gunny on why you shouldn't. I find it interesting he posts his call in his sig, something I consider no comsec at all. But I suppose if he lives by the motto "Nothing is Secure" then it's all the same to give everyone in the world your home address. Which leads instantly to Google Maps Street View pictures of your place.Delete
I'm sure I'm not the first to think about a little box like this, and I wouldn't be surprised if someone made some already. The vocoder and encryption you talk about would be a natural add-in - amateur laws against encryption not withstanding. In the case of Olivia, the Walsh codes are scrambled with a key that's in that Wikipedia article. To make a private version, "all you gotta do" is change that key for everyone in your group.
I'm a bit less ambitious about productizing it, though. I was thinking about building a few prototypes and then putting it up on the web for anyone who wants to copy.
Within the last year, digital voice for amateur use has come out, and it sounds pretty promising.
Alinco used to sell a digital voice module that went between the mic and the transmitter's mic input. It was pretty spiffy, but expensive, and it never really caught on.Delete
Some seed links for you: This is a project I'm going to pull together this summer. Maybe not with these exact components but heading in this direction I think.ReplyDelete
MT63-2000 available for android via googleplay.
Combine with something like a "Rikomagic MK802 IIIS Dual Core Android 4.1 Jelly Bean Mini PC"
Very cool! This is along the lines of what I was thinking of doing, too.Delete
I saw that MT63-2000 app while putting this together. I didn't try it because my only Android device right now is a Kindle Fire HD, and while some things from the Google Play store work, the war between Amazon and Google keeps them from playing nice with each other.
SG - Any suggestions on a series of textbooks that start at the basic level on amateur radio and proceed to, and past, the range you started into above? I need to learn a LOT more about this.ReplyDelete
I'm getting ready to take my technicians exam so I've really enjoyed these posts. I realize though, how much I really do NOT know, but learning is good.ReplyDelete
I think of this as Amateur Extra level. I haven't looked at Extra class study guides in a while, but it was always oriented to being a "specialized communications modes" test, with emphasis on teletype, fax, satellites and other modes besides plain old voice and CW.Delete
I think I learned most of the fundamentals of this while studying for my Extra, back in the stone age (1983 - back when you had to pass a 20 WPM test in front of an FCC examiner). While the computer sound card modes didn't exist, the ideas of Frequency Shift Keying were there, and more modes.
When I took my Extra Class exam some time ago, it had degenerated into almost an "administrative" level. It was more concerned about how to give examinations and be a VE than it was with technical stuff.Delete
The Advanced test was quite impressive, though. It covered all kinds of technical matters. I had friends that were crying they couldn't get their Extra so they had access to the Extra-only parts of the bands because they had to "learn a lot of stuff I'll never use".
And I had to struggle along to learn Morse code that I rarely use!
One of my pet peeves with writers is the non-introduction of acronyms. Where I used to work, acronyms were used extensively by people (supposed professionals) who had no idea what the root meaning was. It makesa me crazee.
I would find it helpful if you would spell things out for me; i.e. Digital Signal Processor (DSP) one time when you proceed with this fantastic series.
Hmmm I thought I was being pretty diligent about at least defining every acronym within a few sentences of the first time it was appeared, but I see I missed that.Delete
I'll try to do better.
By all means, let me know if there are others I missed.
"DSP" can be used two ways. One is "Digital Signal Processing", which is the theory and practice of converting analog signals to the digital domain, performing mathematical operations on them and converting them back to the analog domain, and the other is "Digital Signal Processor" which would be a dedicated piece(s) of hardware that carry out the processing.Delete
Can you say "Raspberry Pi"?ReplyDelete
Small as in just bigger than a cell phone fully capable Linux computer.
Just looking for a home in an Otter box with waterproof connections and a battery pack.
I read someone saying they had tried their Raspberry pi and it didn't have the horsepower. I realize that can vary with the coding, so I wouldn't rule it completely out, but I don't think it's a slam dunk it would work, either.Delete
You guys might want to look into Winmor peer-to-peer. Very fast with FEC and there is no way to just sit and monitor the traffic. APRN also uses Winmor for a BBS with Winlink. Can send text as well as binary files which can be encrypted before sending...Just sayin'...ReplyDelete