Issue #26: Guest Article - The RFBitBanger Transceiver and SCAMP Digital Mode

Won't speak for the developer(s), but I see that, at least within the domain of the RfBitBanger, the mode is built-in and needs only a corded PC keyboard to conveniently use. I would think that most other modes of this type might need a more robust and expensive processor/memory/etc. We'll have to see if this mode can be fitted into other very basic radios suited for such a basic task. I can easily envision radios with small keyboards suited to just this one task, and VERY cheap, from China or whatnot.

 

That's what I was thinking. The processor needed to run SCAMP is included in the radio -- no laptop required. I can see a big advantage to this in a SHTF scenario. Dave

 

Coding an existing asynchronous conversation mode on a single processor w/built in memory has been done in a small portable package.
TAPR did a white paper on the NU-PSK31 modem and went into a lot of detail including the software base of PSK31.
West Mountain Radio made a small, affordable PSK31/RTTY/CW Modem and an accompanying mini keyboard.
IIRC they have since discontinued production, but IIRC the schematic is on the ARRL website. All done on a single chip processor.
Readout was a LCD display. The Chinese didn't pick up on it that I am aware of.

Also other manufactures haven't had an issue putting encoders/decoders in their rigs (Don't forget about APRS and it's messaging capability in some 2m rigs, or RTTY Decode on some HF rigs), this isn't a revolution except for the specialized digital mode.

So it's cool that he had fun making a nice QRP radio with built in encode/decode.
But I don't see the reasoning on making yet another digital mode.
To my mind, more digital modes are like more analog repeaters, digital voice modes, and VOIP "repeaters"; it all serves to divide us.

 

I'm at a loss for words as to how to try to reconcile this concept with that of amateur radio's ideals and practices.

 

I think it's valuable to have a digital mode that includes strong forward error correction, and yet is implementable on a relatively slow but widely sourced $2 microprocessor. Most of the other digital modes seem to require a faster, more power hungry, or more expensive processor, or even a whole computer or other box external to the radio to operate over low SNR channels (as low or lower than would support CW, and with zero Morse code expertise required).

 

The NUE-PSK modem was made for many years. This was the unit resold by West Mt Radios.
NUE-PSK Digital Modem
My son gifted me one of these very nice units. Very nice indeed and a perfect fit for my FT-817. Now with CW, so a great memory keyer as well.
***

since the TAPR product was 'open source' - it didn't take long for a Chinese rip-off of the NUE-PSK to show up

A BD5ROJ copycat using the NUEPSK modem software.
The BD5ROJ stayed on the market just long enough to kill the US product, then it all but disappeared as well.

This site (India) is still flogging the BD5ROJ unit
Digital DIGI PSK Modem BPSK31/63 RTTY QPSK for YAESU FT-817 – UNIQKART
indicating a few in stock at Rupee ₹ 25,782.00 (~310USD plus shipping). I know nothing about this vendor, listed for the curiosity value only.
****

For use with your pocket PC / PDA (remember those?)
PocketDigi - PSK, RTTY and CW operation from your PDA (n0hr.com)
***

Some folks have tried to roll their own
KK5JY PSK Modem
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Side note on the BD5ROJ group - also behind the DCW1 rig

this rig, I am assuming, was aimed to be a replacement for the Elecraft KX1 - even had an optional internal tuner. Made it to the production prototype stage, then vaporized as they could not successfully steal the software. Dave, @W7UUU has the two prototypes to make it out of China. I had no luck with salvaging the software.
H/T to Wes, @W3KW, who initially sold the radios to me.

BD5ROJ is nowhere to be found on the web last I looked.....
It is/was a well-made radio with high production value/good components - I have to believe the profit margin was too thin - even for a CCP knockoff.
*****

If nothing else, this sad tale set shows just how niche a portable digi modem or digi-radio is today. Too much competition and too small a market. I would also note that the KX-2 has a native PSK-31 and RTTY mode baked in, used with the paddles, no keyboard is required.

 

Hi Don,

Appreciate the sad tale. As an aside, Don here did a great technical report for another small Chinese radio, and the Chinese guy who is part of the design team (Dale) credits him for helping improve the unit. I reviewed that little radio on this site -- nice value. Dave, W7DGJ

 

Thanks for the info Don!
I figured someone had done more research than I did
Good on the KX3; I knew there was someone who had included PSK31 encode/decode in their radio, but couldn't remember exactly who.

 

I decided not to use PSK31 deliberately because especially over long distances the phase of the RF wave can become scrambled, and since DPSK compares the phases of adjacent bits, if one does not detect a phase transition, this results in cumulative errors in the decoding. This is a major reason why most other modes use MFSK rather than PSK. PSK is only usable with a connection with a relatively stable phase, and often without differential phase signaling and with a PLL locked local oscillator to lock onto the sender phase, and ionospheric reflections, especially with fading, can strongly modulate the phase which is a problem for DPSK.

MFSK often uses many closely spaced frequencies, necessitating a stable carrier and significant decoding power (usually using a FFT waterfall). One can separate the frequencies by more than the minimum required for orthogonal signaling, but then that uses a lot of bandwidth, at least double the required bandwidth.

So in order to minimize the processing power needed to decode and bandwidth, I used two well separated frequencies in orthogonal signaling (RTTY does not use orthogonal or synchronous signaling). Also, I used synchronous signaling, so that every single bit and frame is nominally the exact same duration. Therefore if the receiver can not decode a frame, it still knows when the next frame should start, as each bit is a known duration, and each frame is 30 bits. So it can pick up decoding the next frame with a high probability as long as the interference ends before the next frame. There are also sync bits within each frame so that if the receiver does lag or lead the transmitter by a bit, it can slow down or catch up.

Many methods use good error correcting codes, but when the timing is lost, multiple bit errors occur that can't be corrected, and so the error correcting code is not helpful. Therefore it is extremely important to put cues in the signal to help timing. This is one of the major innovations of the JT/FT modes, so that the decoding probability is not limited by timing errors. Because I could not rely on global timing cues (a clock), instead I use synchronous signaling like the JT/FT modes, and I use sync frames, like the Costas arrays uses in JT/FT. Furthermore, I make frequent resynchronization an option in transmission, so that if the connection is really bad, one can still obtain a resynchronization signal frequently enough so be synchronized with the bit stream.

I think that SCAMP learns from existing modes, especially JT/FT, but tries to slim it down into something more amenable to portable and emergency use.

73,
Dan
KW4TI

 

Just curious, is this mode, the same as the mode as discussed in this 2004 TAPR white paper on SCAMP?

https://www.tapr.org/pdf/DCC2004-SCAMP-KN6KB.pdf

 

No it isn't. I wasn't aware that the name was already used when I named it SCAMP, and it doesn't appear than any other document than this references it. The SCAMP proposal eventually became WINMOR.

 

Hyped for the FLDigi support.

I put my RFBitBanger kit on the air yesterday but so far only used CW and RTTY.

 

How did the little rig do!

 

@KO4CEE -PES post a video on it working and I will WATCH it to see how this gadget works. Or at least post a longer bit on your build/experiences.

 

Eh, I'm not much for making videos, but here's a longer post.

How'd it do?
I called CQ on 20m RTTY last night for a while, but got no answers. I know it was getting out because a few RBN spotters picked it up as far as Arizona and New York. Not bad for ~4W RTTY, which is not exactly a weak signal mode. I was listening on my other HF rig and it seems to stay pretty much dead on frequency and DroidRTTY had no trouble decoding it. I used CW too, but got no spots and no answers, which might be my fault for being sloppy.

I haven't tried any other bands yet since I've only built the 20m bandpass filter so far.

Assembling the kit is pretty easy, especially since its mostly done by machine already. The AVR came preinstalled with the Arduino bootloader and some version of the software from January when it was sent out. I immediately built the latest version from Github and installed that instead, but it probably would have worked out of the box.

For most of the parts I just stuck them in with the order given in the build notes, but for the 2N7000 transistors, I checked all 4 of them on one of those cheepo "M-Tester" boards and picked the 3 closest measured ones for the finals (Q3, Q6, Q7) and the oddball for the driver (Q9). This is probably hair splitting since they are all so close together, but it wasn't a lot of extra work and I'd rather they be sort of matched if possible.

Calibrating the XTAL frequency was fairly easy, but I kind of made up my own way to do it since I didn't have a reasonable way to measure the CW tone frequency like it says in the manual. I tuned to WWV on 15mhz and set the radio to SSB mode (I forget which), then tuned around until the carrier was no longer audible and had the slowest beating in the RX audio tones. I hope that makes sense? It's closer on frequency than my TS-570D is...

For tuning the bandpass filter, I soldered some PCB mount SMA jacks to a couple pairs of header pins. Plugged the thing in to a NanoVNA and squeezed the inductor turns until the S21 logmag dip was around the bottom of the 20m band. Pretty straightforward. On the bench the crappy wattmeter I have shows around 4W into a dummy load through the filter as well as with it bypassed, and the built-in current meter LED is very bright, so I'm guessing the filter works OK. That was at 12.4V off a 3S Li-Ion pack. Might be able to get a bit more power out of it with higher voltage and adjusting the final drive but I'm not in a hurry to burn anything out.

The UI menus take a bit of getting used to, but it's not hard to operate once you find your way around. A printed menu map would help, but it's less bad than some commercial rigs in this regard. I might fork the code and customize mine. Using a PS/2 keyboard is surprisingly comfy and makes things like frequency entry and moving around the menus easier.

Things that might catch people off-guard are the filtering, audio, and gain control.
The speaker/headphone output is super crisp and clear and very wide. Signals a few kHz away sound like sparkling high tones but they don't really interfere if you have the gain set right and they aren't right on top of the one you're listening to. There's some DSP filtering being done on the AVR but it's only for the decoder. What you hear is the strictly analog audio of both sidebands. That's made it a little tricky to get it lined up on the right frequency for CW signal. Some kind of tuning indicator on the display would make it easier to hone in on a signal. I've seen that on packet TNCs and RTTY stuff before.
There is no AGC. Gain control is totally manual. On one hand this means a strong signal with too much gain will clip and distort and sound awful, but on the other hand, it makes the RX audio way less stressful with headphones on since you don't have the AGC "pumping" constantly and a loud nearby signal doesn't immediately cause the volume of everything else to plummet. It's making me wish I could turn off the AGC on some of my other radios. I might bodge a switchable attenuator on to mine via JP7 since I found a few signals that were so strong they were clipping even with the RF gain all the way down.

Going to make another filter module and put it on 40m tonight. Hope it works.