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    Post  geofrancis Mon Sep 27, 2021 6:34 pm

    I have been reading your forum and you have very obscure Radio requirements that I found interesting. I have some ideas that might help some of you out.

    the main issue you have is signal penetration, the lower frequency the better so i was looking at modern 27mhz modules and came across these, I think these could be fed with a ppm signal from a handset and the output could be split into multiple channels using a basic ppm > pwm decoder giving up to 8 channels, I done something similar with a cheap 315mhz transmitter to make a head tracking system. there is a chance there might not be enough bandwidth but I suspect 8ch will be possible.
    even low power levels at 27mhz should have far more range than any uhf system.
    radiometrix.com /our-products/Transmitters/Single-channel


    Another idea I had was something that was used for long range drone work was using a relay. Basically, get a rc boat and set it up as a radio relay to follow the submarine or sit in the centre of the pond. that way the rf from the submarine only has to penetrate to the surface and not through the whole pond to get to you. so your using regular 2.4ghx to the boat relay and then uhf to the sub. its simple to set up since your just taking ppm out from the 2.4ghz receiver and plugging it into the ppm in of a uhf TX modue. Technically it wouldn't have to be a boat, even a small mast near the pond would work to minimize the distance the signal has to go through water.

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    Post  david f Tue Sep 28, 2021 8:15 am

    That is a very interesting idea!

    You are right about model submarine requirements being obscure! I think I would also say unique and niche.
    And you are right, signal penetration is THE issue.
    Could I refer you to an article by John Elin about antenna. (Best article I've ever seen about underwater antennae. Conclusions are TX height important, water conductivity important, shortened aerials may be better in water.)"

    Go to the AMS Facebook site "files" section here:
    https://www.facebook.com/groups/AMSmembers/files
    and download this magazine copy:
    AMS 1986 6OCR

    Your idea of using 27Mhz in modern equipment could be a real winner not least because it would be a global market which could get commercial manufacturers interested.

    I wonder if the modules could cope with the bandwidth? They would need to work for about 6 simultaneous users. Maybe frequency hopping would work with this and also reduce the problem of interference from CB radio and toys? The latency or servo response need only be very slow compared with aircraft use. Maybe their would be enough bandwidth for some telemetry? Only 2 or 3 channels needed.

    Lots of questions! Any comments Simon, Tim, Jonathan or Cheapsub?

    I will have a look for some Radiometrix data sheets.

    (Later edit: Could this also be a contender the AX5243 ?: https://www.onsemi.com/products/wireless-connectivity/wireless-rf-transceivers/ax5243)

    David

    The relay idea is also a nice one. A bank side mast may be best to get the height requirement.
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    Post  geofrancis Tue Sep 28, 2021 9:37 pm

    these were the modules i used for my head tracker, they are about as simple as it gets, all i done was connect a 8 channel ppm signal directly to the transmitter and it gave me a good enough signal out of the receiver so i think those 27mhz Radiometrix modules should work in a very similar manner.
    ebay.co.uk/itm/221297956541

    the main problem is the lack of bandwidth on 27mhz for any kind of encoding or packet headers, those modules run at a fixed frequency so you would have to replace the module to use another 27mhz channel but i dont see more than 10 being on at one time, they should still work fine with the classic coloured peg system. CB radios are pretty rare these days, even childrens 27mhz toys have went away so 27 is a lot cleaner than it used to be so i dont think it should be too much of an issue.


    the issue I had with your openlrs receivers was the lack of a ground on the antenna, monopole antennas always gave me awful performance on openlrs especially with telemetry activated.
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    Post  Cheapsub Wed Sep 29, 2021 8:04 pm

    In US, is very hard to find any 27MHz receiver, that is more then 3 channels.
    Most receiver is in AM and the RF module is in FM. ( I have 3 RF modules but can't find any matching receiver; there are parkzone 27MHz RX at Ebay but it only works with parkzone transmitter.) 20 yrs too late.

    I gave up, after I builded the Frankenstein transmitter [ER9X](cheap) with a used Novak 75MHz synthesized module and be done with. ( mix and match almost any RF module 27, 35, 40, 72, 433, 2.4 )


    May be in EU, is easier to find a old used 27MHz or 40MHz set.

    There is still some as its 40MHz synthesized receiver from China, but CQ is so so.( I got 3 out of 4 are good)

    Off the shelf from China 433MHz stuff, is replaced with the 900MHz stuff. So that's a dead end. Look for wolfbox.
    All 433MHz system has Some learning curve to use.
    Like hobbyking module, out of the box needs reprogram.

    If you want all the bells and whistles that flyboy's has in 2.4GHz. Run a long receiver antenna extension to the top. In snorkel mode.

    Toy's RF. No, the channels are on, off only.

    DIY, not everyone's cup of tea.

    Last thing good luck looking for crystal.


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    Post  SimonH Wed Oct 06, 2021 10:06 pm

    Thinking about the 27MHz & 40MHz bands, to be compatible with existing models we would probably have to frequency hop within the 10kHz channels or at least be able to not use channels already being used by other models. that would be quite challenging as the frequency accuracy required becommes so tight. That in turn would reduce the data rate, the old data rate vs bandwith problem.
    The thing with the GHz bands is that all the RF components (coils, caps etc) can be made as part of the chip, for the lower frequencies the have to be real springs and things, so more expensive to buy, fit and crucially tune. Ever tried tuning a superhet radio!
    Again, given the niche product and small numbers (compared with toys and white goods) thats probably why they are so difficult to get commercially.

    The alternative would be some open source design avaiable to all, and proven, with all compnents easily availble or at least someone prepared to hold a stock of tricky items (e.g. PCB). Even then I suspect that it would need the ability to buy ready built/tested units for those that don't want to assemble their own. Would that then required the design to be licensed or at least verified to support the contentions that it does comply with the regulations, that of course are different in different contries?

    The various forums (is that correct latin?) do show that systems at 458MHz (UK) work, but being able to produce consistently in larger numbers by less experienced constructors is different.  It may be that DIY electronic sklls becomme as important as machining skills when modelling subs.

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    Post  tsenecal Thu Oct 07, 2021 8:55 pm

    SimonH wrote:Thinking about the 27MHz & 40MHz bands, to be compatible with existing models we would probably have to frequency hop within the 10kHz channels or at least be able to not use channels already being used by other models. that would be quite challenging as the frequency accuracy required becommes so tight. That in turn would reduce the data rate, the old data rate vs bandwith problem.

    all available frequencies are covered (there are 30 discrete crystals in the 75mhz range), so any frequency hopping functionality would hit a used frequency if even one person at an event were using an "older" non hopping device.

    SimonH wrote:
    The alternative would be some open source design available to all, and proven, with all components easily available or at least someone prepared to hold a stock of tricky items (e.g. PCB). Even then I suspect that it would need the ability to buy ready built/tested units for those that don't want to assemble their own. Would that then required the design to be licensed or at least verified to support the contentions that it does comply with the regulations, that of course are different in different countries?

    The various forums (is that correct latin?) do show that systems at 458MHz (UK) work, but being able to produce consistently in larger numbers by less experienced constructors is different.  It may be that DIY electronic skills become as important as machining skills when modelling subs.

    that is exactly how OpenLRSng started.  a set of documented PCBs and parts that anyone with basic soldering skills should be able to make for themselves.  only after it became moderately popular did third party manufacturing show up.  after it died back down, the third party manufacturing dried up and left.  same thing is currently happening in the US with 915mhz equipment.  Jumper and FrSky are making commercial devices in that frequency at this time, but i am absolutely certain that  once the hype dies down, one or both will leave.

    our biggest problem is that there simply aren't enough people in the world interested in these niche areas of the hobby to warrant a big company making anything for an extended period of time. a single production run for them would saturate the market, and they would never come back.

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    Post  geofrancis Mon Oct 11, 2021 6:19 pm

    I think your over thinking everything. forget digital systems, to build an analogue 27mhz RC system you need is a transmitter and receiver pair that can send and receive an analogue PPM signal that's about as sophisticated as morse code. I suspect it would probably work over a walkie talkie if the signal was clear enough. that's why I linked the 27mhz modules in the first post, they should have everything there all they should need is a ppm signal from a handset and a ppm decoder connected to the receiver its its now an RC system. as far as i can tell they work on regular 27mhz channels and can be ordered on any frequency.



    If you want all the bells and whistles that flyboy's has in 2.4GHz. Run a long receiver antenna extension to the top. In snorkel mode.

    thats how i can live stream video my cheap Gopro camera clone over wifi, i extended the wifi antenna out of the water proof case to the surface so it has line of sight to my phone.

    I first tested my idea on these cheap modules, you can get them in 433 and 315mhz but they have no options for channels so they wouldnt play well with more than one person but its still enough to make a basic RC system using the method i describe. they are normally used for controlling things like home automation, garages etc

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    Post  david f Thu Oct 14, 2021 10:28 am

    Thanks for very useful discussions on here.
    It is clear from these that the overriding problem for us is lack of equipment. Commercial equpt.  Is probably never going to happen, as Tim indicates.
    Even RF modules seem to be heading ever upwards in frequency  terms.
    The ideas on use of 27 meg from Geo are very interesting.  This is a frequency which we know is very good for underwater use and is not much used now.
    The trouble is that I had a brief look for suitable RF modules at 27 mHz and there seems to be limited availability and at a high price $50 or so.
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    Post  geofrancis Thu Oct 14, 2021 12:43 pm

    david f wrote:Thanks for very useful discussions on here.
    It is clear from these that the overriding problem for us is lack of equipment. Commercial equpt.  Is probably never going to happen, as Tim indicates.
    Even RF modules seem to be heading ever upwards in frequency  terms.
    The ideas on use of 27 meg from Geo are very interesting.  This is a frequency which we know is very good for underwater use and is not much used now.
    The trouble is that I had a brief look for suitable RF modules at 27 mHz and there seems to be limited availability and at a high price $50 or so.

    yes those are pretty expensive but there really aren't many companies still making 27mhz equipment anymore, its almost a vestigial frequency at this point and has been surpassed in every way by newer systems apart from signal penetration.

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    Post  geofrancis Fri Oct 15, 2021 10:22 pm

    you should message the manufacturer of those 27mhz modules and ask them if they would support a ppm signal.

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    Post  david f Sat Oct 16, 2021 4:22 pm

    Good idea
    I  have just sent Radiometrix an enquiry on behalf of AMS members.
    Will let you know what they say and some idea of price. It could be a fairly simple retrofit for existing TXs. Their datasheet says they offer different 27mHz  bands, presumably without the nuisance of crystals.
    It is a longshot but worth asking.

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    Post  SimonH Sat Oct 16, 2021 8:12 pm

    Hi all, I had a quick look a at the Radiometrix 27MHz RF module datasheet and whilst not entierly clear (I would have to go through in detail with example frequencies) it looks as if it has 10kHz RF channels, so setting a specific RF channel within the model band shouldn't be a problem, but it looks as if they might only support a limited number for each s/ware version programmed in.
    Similarly, since the modulation input is  DC to 4kHz CMOS so a straight digital signal, PPM would be no problem as PPM is pulse position (time) not pulse amplitude. You might have to work on the antenna matching since no details of the RF port impedance are given as far as I can see.

    Assuming a standard 2ms servo at 50% gives a 500Hz signal (ignoring sync pulses etc.) so with an assumed channel bandwidth of 4kHz there might be some jitter since if the channel bandwith is too low it effectively slows the switching edges making them sloping, so any noise gets translatted into timming jitter.
    A 4kHz bandwith would convert a 4kHz square wave to a 4kHz triangle wave (sort of) with a 'rise time' of 125usec, so the max timing jitter could be +/-62.5usec due to noise
    Probably no worse than the old 'analogue' systems though, since they probably had a similar signal bandwidth.

    If noise or interference is an issue then going digital allows the use of digital message coding techniques that basically use multiple message bits to define each servo command bit, so you can use a majority decision system to allow for corrupted bits. If you had a max channel bandwidth of 4kHz, then that gives a max bit rate of 8kbits/sec, so spreading 8 servos of 10 bits each accross 4 bits (320bits per frame, say 500bits per frame) then that would allow 16 frames/sec (500 x 16 = 8,000).

    The 27MHz probably has the advantage of less volume, as the antenna is just a wire, as opposed to the rigid 'rubber duck' antennas that are probably not waterprooof, so have to the inside the WTC.

    Digressing slightly, I was interested in the commesnt about the origins of the openLRS, I did look at it but it seemd as if over time it has grownw to be more flexible and complex, but it seemed quite complex to configure and using an arduino mini-pro I had to go back to basics to fit the s/ware in the memory provided, hence my version of s/ware.
    Going to any commercial system, or even a new open source I think the trink is to be clear up front what is required, and to prevent the 'wouldn't it be nice if....' (professionaly known as WIBNI or creeping elegance!) for even more niche ideas.

    Even more of a digression, has anbody tried the new RaspberryPi pico instead of the Arduinos? This looks as if it has way more memory, 3 x anlogue inputs and 16 PWM channels .. and < £4
    That processing power might allow the use of a software radio module, I think been mentioned before here, such as
    https://thepihut.com/products/software-defined-radio-receiver-usb-stick-rtl2832-w-r820t?variant=27739418385&currency=GBP&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&gclid=EAIaIQobChMIjLrg583P8wIVogV7Ch1BTwsMEAQYAiABEgLi1_D_BwE
    That claims to cover 24MHz to 1.8GHz all for £22 or so, but it still needs a transmitter design. Once I've finished my current project .......!

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    Post  Cheapsub Sat Oct 16, 2021 9:28 pm

    Very interesting they still make that new!

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    Post  geofrancis Sun Oct 17, 2021 1:34 am

    SimonH wrote:Hi all, I had a quick look a at the Radiometrix 27MHz RF module datasheet and whilst not entierly clear (I would have to go through in detail with example frequencies) it looks as if it has 10kHz RF channels, so setting a specific RF channel within the model band shouldn't be a problem, but it looks as if they might only support a limited number for each s/ware version programmed in.
    Similarly, since the modulation input is  DC to 4kHz CMOS so a straight digital signal, PPM would be no problem as PPM is pulse position (time) not pulse amplitude. You might have to work on the antenna matching since no details of the RF port impedance are given as far as I can see.

    Assuming a standard 2ms servo at 50% gives a 500Hz signal (ignoring sync pulses etc.) so with an assumed channel bandwidth of 4kHz there might be some jitter since if the channel bandwith is too low it effectively slows the switching edges making them sloping, so any noise gets translatted into timming jitter.
    A 4kHz bandwith would convert a 4kHz square wave to a 4kHz triangle wave (sort of) with a 'rise time' of 125usec, so the max timing jitter could be +/-62.5usec due to noise
    Probably no worse than the old 'analogue' systems though, since they probably had a similar signal bandwidth.

    If noise or interference is an issue then going digital allows the use of digital message coding techniques that basically use multiple message bits to define each servo command bit, so you can use a majority decision system to allow for corrupted bits. If you had a max channel bandwidth of 4kHz, then that gives a max bit rate of 8kbits/sec, so spreading 8 servos of 10 bits each accross 4 bits (320bits per frame, say 500bits per frame) then that would allow 16 frames/sec (500 x 16 = 8,000).

    The 27MHz probably has the advantage of less volume, as the antenna is just a wire, as opposed to the rigid 'rubber duck' antennas that are probably not waterprooof, so have to the inside the WTC.

    Digressing slightly, I was interested in the commesnt about the origins of the openLRS, I did look at it but it seemd as if over time it has grownw to be more flexible and complex, but it seemed quite complex to configure and using an arduino mini-pro I had to go back to basics to fit the s/ware in the memory provided, hence my version of s/ware.
    Going to any commercial system, or even a new open source I think the trink is to be clear up front what is required, and to prevent the 'wouldn't it be nice if....' (professionaly known as WIBNI or creeping elegance!) for even more niche ideas.

    Even more of a digression, has anbody tried the new RaspberryPi pico instead of the Arduinos? This looks as if it has way more memory, 3 x anlogue inputs and 16 PWM channels .. and < £4
    That processing power might allow the use of a software radio module, I think been mentioned before here, such as
    https://thepihut.com/products/software-defined-radio-receiver-usb-stick-rtl2832-w-r820t?variant=27739418385&currency=GBP&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&gclid=EAIaIQobChMIjLrg583P8wIVogV7Ch1BTwsMEAQYAiABEgLi1_D_BwE
    That claims to cover 24MHz to 1.8GHz all for £22 or so, but it still needs a transmitter design. Once I've finished my current project .......!



    that is very cool, 8 channels at 16hz would be more than any 27mhz system I have ever seen. A pure digital system would be a far superior solution, it would open up options like channel bandwidth allocation, since you dont need high-resolution or high speed on anything other than the first 4 channels the rest could be sent much slower like 1234+5, 1234+6 etc or at a lower resolution like 1 or 2 bit opening up 16 channels on the same bandwidth.

    openlrs wasnt always an open source project, it was originally started by flytron who then released the source code version 1, then "theundeadmod" firmware was made to upgrade the capabilities of the original flytron firmware by adding binding and tweaking the radio settings for maximum range lowering servo resolution and dropping the radio rate down to 9600k. that firmware eventually became the basis of openlrs-ng that was wrote by Kha, there are also forks of openlrsng like gitsly and some Russian versions, someone then started making them with modules using arduino boards connected to hope RF rfm22b radio modules modules, there is also some other firmwares out there like ULRS, Chiplrs, oplink, that are independent of openlrs but mostly hardware compatible.

    I designed the guide for converting 9xr radios to mavlink telemetry over openlrsng for ardupilot flight control systems
    http://marc.merlins.org/mirrors/flyingforfun.weebly.com/open-lrs.html

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    Post  david f Mon Oct 18, 2021 10:45 am

    I'm liking this 27mHz idea more and more!

    This article makes some interesting points.

    https://lemosint.com/way-down-low-27mhz-an-unexpectedly-useful-radio-control-band-allocation/

    - 27 mHz is a globally available frequency. So even though model subs are a niche, developers / manufacturers would at least have a global market niche . (The USA is much the largest part of this market. )

    - fitting aerials into model subs is not easy (even at say 40/75 MHz. At least 27 meg antennae are developed and cheaply available.

    Thanks Simon for putting some design figures to all this and thanks to Geo for what originally sounded like an odd idea!

    In answer to Simon's comment on SDR , I now have a lot of this gear  as a result of Jonathan's (C3P0) original suggestion.

    They are very useful. I use them as TX monitors for all the different frequencies we have had a go at. Could an SDR rx be made small enough for a sub?

    David

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    Post  geofrancis Mon Oct 18, 2021 2:41 pm

    the problem with SDR is it takes a lot of processing power to decode signals because its all done in software, pi ADS-B SDR receivers need at least a pi3 to work so i suspect you will need something similar. you might even run into cooling issues since you cant just put air vents on the side of a sub.

    to make it behave like a modern radio system the essential things it would have to do is:

    Bind between radios, a 8 bit header set when bound so that is set so even if someone is on the same frequency the packets are rejected.

    Reject corrupted packets, a basic checksum would detect any bad packets and ignore them

    Failsafe, it has to be able to recognize that no valid packets are being received and set outputs to failsafe values.


    what would be nice to have:

    16ch SBUS digital input and output or better multimodule support so it can be configured from a opentx lua script.

    servo RSSI output or packet lost beep for FPV systems


    that would cover all use cases to the point i would feel safe to fly with it.


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    Post  geofrancis Mon Oct 18, 2021 3:10 pm

    you could do some interesting things using a receiver servo RSSI output, basically a servo channel output that goes 0-100 based on receiver signal strength. its normally used for indicating rssi to a flight control system but you could mix it into your depth control system either dive planes or ballast tanks to use your receiver signal to control your depth, so it would automatically start to surface if the signal got weak. rather than it having to failsafe, it would actively control depth so the signal doesn't drop to far by going closer to the surface when the signal is weak. most uhf systems have this built in and it can be done in software using telemetry on opentx.


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    Post  geofrancis Tue Oct 19, 2021 6:53 pm

    if 27mhz could be made into a modern digital system with fail-safes then there will be a lot of other parties that will be interested in it, the ability to transmit miles on just a few mw of power and the ability for the signal to penetrate will be good for long range or urban FPV with very low power levels compared to the 1w+ some UHF systems are putting out to do the same thing.
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    Post  david f Fri Oct 22, 2021 5:44 pm

    Still no reply from Radiometrix and it has been nearly a week.
    I have sent a repeat request on behalf of AMS members by a different route.
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    Post  SimonH Sun Oct 24, 2021 10:49 pm

    my code at work was taking ages to download so I did some thinking in work mode about using Rf modules with a few thoughts, applogies if you already know this.....

    I think most 'clever' thechiques for allowing multiple users in the same RF channel rely on being able to muck about with the Rf signals, but that wouldn't be possible if the RF module just allows a straight digital modulation, but say we want 8 channels x 10 bit reolution digital data plus a bit more for device ID (for binding) etc. so say 100 bits per frame, and say we have 4kbits/sec bandwidth (in theory 4kHz should support 8kbits/sec, but you'l see why in a bit) and say we can accept at least 5 frames per sec. that gives 500 bits/sec  out of a capacity of 4,000. That in theory that would allow 8 users to share if they could be synchronised in time. That could be done, but there would have to be a complex protocol that all users adopt to allow the designation of a master, and each transmitter would need a receive channel to monitor what is going on.

    A simpler alternative would be to drop the user count to say 3 or 4 and allow statistics to take over so that on average enough messages get through. Probably OK for slow models, probably rubbish for say aeroplonks. Ideally each user would transmit at a different frame interval so no 2 get synchronised together, or more easily, a random element to the frame rate is included, so that avergae frame rate is acceptable, but each frame is at a different time interval from the one before.

    Given a burst type of data frame/packet we need a coding system that supports that, and one ideal candidate is manchester encoding. This codes each bit such that there is a level transition in the mddle of each bit, and with a peramble/header of a number of '1' or a number of '0' generates a squarewave at the start. The down side is that it needs more bandwidth, hence dropping down to 4kbits/sec.
    A good description is on the Atmel website
    https://ww1.microchip.com/downloads/en/AppNotes/Atmel-9164-Manchester-Coding-Basics_Application-Note.pdf
    Also, there is an arduino compatible library that implements this system on GitHub
    mchr3k/arduino-libs-manchester
    This is not quite useable 'as is' as it is based on 8 bit data and we want say 127 bits (again you'll see why 127 in a bit), but it looks as if the code could be modified.
    The down side is that the Tx & Rx must have stable clocks so that the drift is not too great but if based on an xtal clock this should be do-able.

    The next problem is data integrity to detect errors and prevent dodgy values getting to servos.
    This could be addressed by using Hamming codes, that basically add extra parity bits and if enough are added in the correct places that they are capable of not only detecting errors but actually correcting them. See the web site
    https://en.wikipedia.org/wiki/Hamming_code
    In particular note the SECDED system common in computers and Hamming(127,120) that expands 120 data bits to 127 bits by adding 7 parity bits, SECDED standing for 'single error correction, double error detection'.

    I haven't found any adruino code that directly does this encoding/deoding but there are various algorthms to look at.

    One issue with hamming codes is thay have to have frame/data blocks of 15, 31, 63, 127, 255 etc, bits so whilst 127 may be considered a bit OTT the next one down at 63 bits (57 bit user data) is probably too small, by the time you allow for say a 8-bit device ID, but is perfectly feasible for say 4 channel x 8-bit data.
    These techniques should be able to cope with ocassionaly message corruption, but I doubt any system could cope with a continuous interferrer in an RF channel.

    I must admit I have no idea what the actual error rate would be, but if Hamming does not give enough then other codes have more error correction, but at the expense of complexity and/or bigger overhead.

    Binding could be done automaticaaly by sending the Rx to 'binding mode' on power up that waits for a defined message containing the device ID sent by the Tx when a button is pressed, but I would go for a simple retary hex switch(es) in the Rx & Tx, though it does require Rx access to change it, and extra space on the Rx PCB.

    I've not used digital code servos, but changing from what is basically the analogue voltage on a joystick pot to whatever code shouldn't be too complex. I'm using a Adafruit PCA9685 16-Channel Servo Driver to offload the PWM generation, but I know some do it all in s/ware.

    A (much) more complex system could be based on transmitting the same message/frame data on multiple RF channels, either at the same time (needs multiple RF modules) and/or by repeating the data after changing the frequency (limited by the time required to re-tune the Tx & Rx). You then sample the Rx data on all channels and do a time based comparison, and assume that all data points that agree are the wanted data and the rest is noise. Hopefully you have enough data to re-create the wanted message, but you would have to be able store and compare a lot of data points.
    Either way you would be spreading across more than 1 RF channel so possibly not acceptable regulatory wise (any commercial system would have to meet any regulations in full in each territory). You could repeat the data on the same RF channel multiple times, but you'd need an accurate time base to allow the time based comparison to work, and of course lots of data storage, probably multiple data points per data bit per repetition. Of course if you repeat the data you reduce the number of users per RF channel (or reduced data rate), but if you limit to 1 user than you might not need the error correction!

    Enjoy trying it!

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    Post  david f Mon Oct 25, 2021 5:15 pm

    Wow! That was one heck of a productive coffee break!
    Thanks for laying out an approach which will take (me!) a lot of thinking about.
    David
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    Post  david f Wed Oct 27, 2021 12:59 pm

    I now have some information from Radiometrix.  (I phoned them in the end - they are all working from home.)

    They were helpful but confirmed that the NTX0/NRX0 modules are only available for a single, fixed frequency.

    They recommended the LMT0/LMRO  multi channel modules (£55 and £65 respectively for orders up to 10 units).

    Looks a bit expensive to me. (Simon and I have both been using the LoRa modules successfully in model subs for the last few months. These are still available (https://www.adafruit.com/product/3073 ) for about $20 each and offer other benefits such as telemetry)

    Without a reliable supply of modules at a lowish price the use of 27Mhz will be a non-starter , in my view.

    What do you think?

    The full emails are copied below:


    27MHz modules for r/c model submarines
    Inbox

    R D Forrest <rdforrest1@googlemail.com>
    Oct 22, 2021, 5:40 PM (5 days ago)
    to info

    Hi,

    Our members are having problems sourcing radio control equpt. which has a low enough frequency to penetrate water. (2.4 GHz does not work.)

    We would like to know if your NTX0 and NRX0 Modules could be used with the trainer socket output (a ppm output) on existing r/c transmitters?

    Can you provide modules for different frequencies within the 27MHz band?

    Please advise of price and ordering information for a tx/rx pair with output power of 100mW.

    Many thanks,
    David Forrest
    Association  of model submariners


    R D Forrest
    12:48 PM (0 minutes ago)
    to sales

    Hi Jim,

    Many thanks for the quote for the  multi channel modules LMT0 and LMRO (£55 and £65 respectively.)

    Quite expensive for our intended application, unfortunately.

    The background to our requirements are on here:

    https://www.theassociationofmodelsubmariners.com/t1972-radio-ideas-27mhz#11751

    Many thanks for your help and do let me know about any developments.

    David Forrest
    Treasurer - Association of Model Submariners

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    Post  SimonH Wed Nov 03, 2021 9:59 pm

    It does look as if the use of 27MHz is becomming un-economic since the 485MHz band seems to better provided with devices with a lower cost/better performance.
    I had a feeling I had seen 27MHz modules, with channel selection but can't find them now.

    Going back to the original post about using a standard PPM/digital data stream and 'any' RF module, the LoRa modules I use I think do have basic digital modulation input/outputs, but I'm not sure what the caveats are.

    Before I started on 485MHz I did have a look to see what chips are available that could be used to build a 27MHz system, but it seems to fall in between the AM radio chips (sub 2MHz) and the FM radio chips (>70 MHz) so I dropped that approach.

    I'm sure a viable 27MHz system could be created using modern chips, but it would take some dev work and test equipment, and probably not be trivial to make.

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    Post  geofrancis Thu Nov 04, 2021 12:22 am

    SimonH wrote:It does look as if the use of 27MHz is becomming un-economic since the 485MHz band seems to better provided with devices with a lower cost/better performance.
    I had a feeling I had seen 27MHz modules, with channel selection but can't find them now.

    Going back to the original post about using a standard PPM/digital data stream and 'any' RF module, the LoRa modules I use I think do have basic digital modulation input/outputs, but I'm not sure what the caveats are.

    Before I started on 485MHz I did have a look to see what chips are available that could be used to build a 27MHz system, but it seems to fall in between the AM radio chips (sub 2MHz) and the FM radio chips (>70 MHz) so I dropped that approach.

    I'm sure a viable 27MHz system could be created using modern chips, but it would take some dev work and test equipment, and probably not be trivial to make.

    yes, I thought some cheaper modules similar to those would have turned up eventually but i haven't found anything unfortunately, I suspect you're just not going to be going deep enough where your going to need something lower than 458mhz, even if you run into issues there are other methods of getting better range and depth like using a relay or directional antennas.

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    Post  david f Mon Nov 15, 2021 9:50 am

    For information :
    Copy of last email to Radiometrix answering some marketing information. No reply to date so I think that is probably the end of it.
    Oct 29, 2021, 1:10 PM

    Hi,

    An interesting question about the potential market!

    I would say that there are about :
    10000 r/c model submariners WORLDWIDE*
    1000 in the US
    100 in the UK

    A fairly conservative group so you may get 1 or 2% converting to 27Mhz each year, assuming that equipment is commercially available more or less "out of the box".

    You ask about target price?  Well, a multi channel transmitter and receiver combination at 2.4Ghz costs about £200, (This is the type used by model aircraft users who are much more numerous than model submarine users.)

    We wouldn't want the radio modules to be too big a proportion of this - 25% say (£50).

    On the principle of "putting my money where my mouth is" I would be happy to order an LMT0 / LMR0 pair NOW for £50. If you could extend this offer to 4 other people who have already shown their interest on the AMS Forum it could get some software development going.

    Just my thoughts.

    David

    *Reply by "Quartermaster" (https://subcommittee.com/forum/showthread.php?38271-2-4GHz-for-the-RC-Submariner%85-the-next-step/page2&highlight=2.4Ghz)

      Current date/time is Thu Dec 09, 2021 1:55 am