Spread Spectrum or PCM for Gassers
05-05-2007, 06:12 AM
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RE: Spread Spectrum or PCM for Gassers
Hi,
Has anybody actually looked into how the software works on the DX spread spectrum system? As a casual reader of the discussion, it appears to me that the system is similar to our 72 PCM in operation with dual receivers and a receive frequency of 2.4 gig. Frequency is negotiated on power up based on possible interference. Once the plane takes off, the frequency is set for the flight.
This discussion reminds me of when PCM was introduced as the salvation of all interference problems. Most of us realize today that PCM masks glitches and goes on command hold when a garbled command is received. Often the last command is held right into the ground. A few individuals still swear that PCM solves interference problems or is more tolerant to interference.
The other thing that I have observed over the years in RC is that new equipment has less problems than seasoned equipment. Perhaps the glowing reports reflect the newness of the equipment.
It is an interesting new technology and I will probably purchase one set of equipment and see if the receivers are tolerant to the onboard telemetry which I carry. I have found that few receivers can tolerate a 1.5 watt tx onboard the airplane without an additional layer of protection.
Elson
Has anybody actually looked into how the software works on the DX spread spectrum system? As a casual reader of the discussion, it appears to me that the system is similar to our 72 PCM in operation with dual receivers and a receive frequency of 2.4 gig. Frequency is negotiated on power up based on possible interference. Once the plane takes off, the frequency is set for the flight.
This discussion reminds me of when PCM was introduced as the salvation of all interference problems. Most of us realize today that PCM masks glitches and goes on command hold when a garbled command is received. Often the last command is held right into the ground. A few individuals still swear that PCM solves interference problems or is more tolerant to interference.
The other thing that I have observed over the years in RC is that new equipment has less problems than seasoned equipment. Perhaps the glowing reports reflect the newness of the equipment.
It is an interesting new technology and I will probably purchase one set of equipment and see if the receivers are tolerant to the onboard telemetry which I carry. I have found that few receivers can tolerate a 1.5 watt tx onboard the airplane without an additional layer of protection.
Elson
05-05-2007, 06:13 AM
#52
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RE: Spread Spectrum or PCM for Gassers
ORIGINAL: dick Hanson
some guys are really hungup on the "PPM testing"--
I can't really see any benifit -
I once had a ratty old ignition engine - ditched it - but for years - the modern ignition engines I have use never caused a glitch
PCMand even the new 2.4 have been referred to as masking problems----
so what - if it works it works
ditto for the 2.4 stuff - I use 2.4 DX2 exclusively now - -why would I want to mess around with essentially an obsolete radio?
Call a spade a spade
the systems up till now can be knocked out of the air by bumping metal parts or a kid with $99.95 RTF trainer
some guys are really hungup on the "PPM testing"--
I can't really see any benifit -
I once had a ratty old ignition engine - ditched it - but for years - the modern ignition engines I have use never caused a glitch
PCMand even the new 2.4 have been referred to as masking problems----
so what - if it works it works
ditto for the 2.4 stuff - I use 2.4 DX2 exclusively now - -why would I want to mess around with essentially an obsolete radio?
Call a spade a spade
the systems up till now can be knocked out of the air by bumping metal parts or a kid with $99.95 RTF trainer
------------
Wrong. The receiver section of our 72 MHz receivers for both PPM and PCM are the same. If something is interfering with a PPM receiver's RF section, it is also interfering with a PCM receiver's RF section and should be remedied. Why? Because the link between the pilot and the model of an interfered PCM rig has been compromised to the point that only a few packets of useful information are getting through to the model's control surfaces. PCM ignores packets that are not "up to snuff". At close distances this may not be much of a problem because of higher field strength, but as distance increases from the Tx, more and more control packets are ignored, leading to very crude control of the model and eventually no control if the interference worsens or the distance increases.
Yes, I am a qualified RF technician, so I'm not just spreading B.S. around the group.
I don't think I would go so far as to run a 72 MHz PPM flight test on a model that is going to be flown with a 2.4 GHZ system, though it certainly would not hurt a thing to do so. As someone else stated, all RF interference generating problems on the model should be remedied just out of good engineering practice anyway. Most RF interference generated by spark engines is in the 3 to 5 KHz range, which is approximately the same bandwidth and frequency utilized by narrow band Citizen's Band modulation encoders (27, 50-53, 72 MHz) to control our models. The 2.4 GHz band is far enough removed and the modulation envelope so wide (theoretically) that noise created in that amount of bandwidth with have little effect upon the 2.4GHz.
Ed Cregger
05-05-2007, 07:01 AM
#53
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RE: Spread Spectrum or PCM for Gassers
Has anybody actually looked into how the software works on the DX spread spectrum system? As a casual reader of the discussion, it appears to me that the system is similar to our 72 PCM in operation with dual receivers and a receive frequency of 2.4 gig. Frequency is negotiated on power up based on possible interference. Once the plane takes off, the frequency is set for the flight.
Bill
05-05-2007, 08:43 AM
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RE: Spread Spectrum or PCM for Gassers
ORIGINAL: Ed Cregger
------------
Wrong. The receiver section of our 72 MHz receivers for both PPM and PCM are the same. If something is interfering with a PPM receiver's RF section, it is also interfering with a PCM receiver's RF section and should be remedied. Why? Because the link between the pilot and the model of an interfered PCM rig has been compromised to the point that only a few packets of useful information are getting through to the model's control surfaces. PCM ignores packets that are not "up to snuff". At close distances this may not be much of a problem because of higher field strength, but as distance increases from the Tx, more and more control packets are ignored, leading to very crude control of the model and eventually no control if the interference worsens or the distance increases.
Yes, I am a qualified RF technician, so I'm not just spreading B.S. around the group.
I don't think I would go so far as to run a 72 MHz PPM flight test on a model that is going to be flown with a 2.4 GHZ system, though it certainly would not hurt a thing to do so. As someone else stated, all RF interference generating problems on the model should be remedied just out of good engineering practice anyway. Most RF interference generated by spark engines is in the 3 to 5 KHz range, which is approximately the same bandwidth and frequency utilized by narrow band Citizen's Band modulation encoders (27, 50-53, 72 MHz) to control our models. The 2.4 GHz band is far enough removed and the modulation envelope so wide (theoretically) that noise created in that amount of bandwidth with have little effect upon the 2.4GHz.
Ed Cregger
ORIGINAL: dick Hanson
some guys are really hungup on the "PPM testing"--
I can't really see any benifit -
I once had a ratty old ignition engine - ditched it - but for years - the modern ignition engines I have use never caused a glitch
PCMand even the new 2.4 have been referred to as masking problems----
so what - if it works it works
ditto for the 2.4 stuff - I use 2.4 DX2 exclusively now - -why would I want to mess around with essentially an obsolete radio?
Call a spade a spade
the systems up till now can be knocked out of the air by bumping metal parts or a kid with $99.95 RTF trainer
some guys are really hungup on the "PPM testing"--
I can't really see any benifit -
I once had a ratty old ignition engine - ditched it - but for years - the modern ignition engines I have use never caused a glitch
PCMand even the new 2.4 have been referred to as masking problems----
so what - if it works it works
ditto for the 2.4 stuff - I use 2.4 DX2 exclusively now - -why would I want to mess around with essentially an obsolete radio?
Call a spade a spade
the systems up till now can be knocked out of the air by bumping metal parts or a kid with $99.95 RTF trainer
------------
Wrong. The receiver section of our 72 MHz receivers for both PPM and PCM are the same. If something is interfering with a PPM receiver's RF section, it is also interfering with a PCM receiver's RF section and should be remedied. Why? Because the link between the pilot and the model of an interfered PCM rig has been compromised to the point that only a few packets of useful information are getting through to the model's control surfaces. PCM ignores packets that are not "up to snuff". At close distances this may not be much of a problem because of higher field strength, but as distance increases from the Tx, more and more control packets are ignored, leading to very crude control of the model and eventually no control if the interference worsens or the distance increases.
Yes, I am a qualified RF technician, so I'm not just spreading B.S. around the group.
I don't think I would go so far as to run a 72 MHz PPM flight test on a model that is going to be flown with a 2.4 GHZ system, though it certainly would not hurt a thing to do so. As someone else stated, all RF interference generating problems on the model should be remedied just out of good engineering practice anyway. Most RF interference generated by spark engines is in the 3 to 5 KHz range, which is approximately the same bandwidth and frequency utilized by narrow band Citizen's Band modulation encoders (27, 50-53, 72 MHz) to control our models. The 2.4 GHz band is far enough removed and the modulation envelope so wide (theoretically) that noise created in that amount of bandwidth with have little effect upon the 2.4GHz.
Ed Cregger
AMEN to that
right on Ed.
Roger
05-05-2007, 10:27 AM
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RE: Spread Spectrum or PCM for Gassers
Bill, then how can multiple pilots fly without dedicated freqs?
Answer: Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver.
A spread-spectrum transmission offers three main advantages over a fixed-frequency transmission:
Spread-spectrum signals are highly resistant to noise and interference. The process of re-collecting a spread signal spreads out noise and interference, causing them to recede into the background.
Spread-spectrum signals are difficult to intercept. A frequency-hop spread-spectrum signal simply sounds like an increase in the background noise to a narrowband receiver.
Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be utilized more efficiently.
And yes, I am an RF engineer.
Answer: Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver.
A spread-spectrum transmission offers three main advantages over a fixed-frequency transmission:
Spread-spectrum signals are highly resistant to noise and interference. The process of re-collecting a spread signal spreads out noise and interference, causing them to recede into the background.
Spread-spectrum signals are difficult to intercept. A frequency-hop spread-spectrum signal simply sounds like an increase in the background noise to a narrowband receiver.
Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be utilized more efficiently.
And yes, I am an RF engineer.
05-05-2007, 12:23 PM
#57
RE: Spread Spectrum or PCM for Gassers
-if you wanna test em -on PPM then go for it - I really see no need for it on any current equipment .
No one I fly with has a ppm rx -so I guess that is also part of the reason I see no ppm testing.
to be pointy about it - most of the guys here abandoned ppm long ago . except forfoamies -and now thankfully, I have 2.4 for those .
The only interferrence -from IC spark ignition engines I have seen --in a looong time --is headers rubbing/bumping on pipes . and that only affected some radios and was easily fixed.
No one I fly with has a ppm rx -so I guess that is also part of the reason I see no ppm testing.
to be pointy about it - most of the guys here abandoned ppm long ago . except forfoamies -and now thankfully, I have 2.4 for those .
The only interferrence -from IC spark ignition engines I have seen --in a looong time --is headers rubbing/bumping on pipes . and that only affected some radios and was easily fixed.
05-05-2007, 02:46 PM
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RE: Spread Spectrum or PCM for Gassers
ORIGINAL: dick Hanson
-if you wanna test em -on PPM then go for it - I really see no need for it on any current equipment .
No one I fly with has a ppm rx -so I guess that is also part of the reason I see no ppm testing.
to be pointy about it - most of the guys here abandoned ppm long ago . except forfoamies -and now thankfully, I have 2.4 for those .
The only interferrence -from IC spark ignition engines I have seen --in a looong time --is headers rubbing/bumping on pipes . and that only affected some radios and was easily fixed.
-if you wanna test em -on PPM then go for it - I really see no need for it on any current equipment .
No one I fly with has a ppm rx -so I guess that is also part of the reason I see no ppm testing.
to be pointy about it - most of the guys here abandoned ppm long ago . except forfoamies -and now thankfully, I have 2.4 for those .
The only interferrence -from IC spark ignition engines I have seen --in a looong time --is headers rubbing/bumping on pipes . and that only affected some radios and was easily fixed.
You mean no one fly's on regular FM PPM, wow, I would never fly on PCM but SSP sounds interesting
Roger
05-05-2007, 03:44 PM
#59
RE: Spread Spectrum or PCM for Gassers
Nope - no one --nada - zip- nyet-
Most of the stuff we use is simply equipment used at TOC and other such performance events .
The PCM's worked extremely well -and I kept mine --til the new Spektrums hit the market - then - switched "en masse" to 2.4 DX2 Spektrum - not a hitch -
I fought the ole 27 band crap -built my own radios etc.. been there -- these new radios - no coils - no tuning - no crystals -
the advance in tech and the reduction of parts - just too good to pass up.
Most of the stuff we use is simply equipment used at TOC and other such performance events .
The PCM's worked extremely well -and I kept mine --til the new Spektrums hit the market - then - switched "en masse" to 2.4 DX2 Spektrum - not a hitch -
I fought the ole 27 band crap -built my own radios etc.. been there -- these new radios - no coils - no tuning - no crystals -
the advance in tech and the reduction of parts - just too good to pass up.
05-05-2007, 04:23 PM
#60
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RE: Spread Spectrum or PCM for Gassers
Pontaic,
I am not convinced that the Spectrum/JR approach is truly a spread spectrum approach but might be more properly labeled “redundant receivers on two different 2.4 gig frequencies.” Although it works the technology is not considered earth shattering but maybe old technology reapplied with error checking.
I’ll pass on the generic descriptions of how spread spectrum systems are supposed to work, which are already known. Are you privy to the general logic of the algorithms used? It’s a serious question.
Bill
Most engineers are skeptics when the details are unknown or obfuscated by marketing types.
I am not convinced that the Spectrum/JR approach is truly a spread spectrum approach but might be more properly labeled “redundant receivers on two different 2.4 gig frequencies.” Although it works the technology is not considered earth shattering but maybe old technology reapplied with error checking.
I’ll pass on the generic descriptions of how spread spectrum systems are supposed to work, which are already known. Are you privy to the general logic of the algorithms used? It’s a serious question.
Bill
Most engineers are skeptics when the details are unknown or obfuscated by marketing types.
05-06-2007, 04:13 AM
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RE: Spread Spectrum or PCM for Gassers
This Spektrum versus Futaba thing reminds me of the Motorola versus E F Johnson thing in trunked business two-way radios some years back.
I'm with you in that I'm not really impressed with Spektrum's technology. It works, but it lacks the robustness of Futaba's system, at least at a glance. However, both will work in less challenging environments.
Ed Cregger
I'm with you in that I'm not really impressed with Spektrum's technology. It works, but it lacks the robustness of Futaba's system, at least at a glance. However, both will work in less challenging environments.
Ed Cregger
05-06-2007, 06:31 AM
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RE: Spread Spectrum or PCM for Gassers
My wife and Me took my G38 powered 100 inch Rearwin Speedster to Savannah for the Skyblazers fly-in Yesterday/Sat... This is the 4th site where I have flown my DX7. There were two other flyers there on SS, and I ask them to bring their transmitters to the flight line with me and turn on while I was flying.. Not a glitch or hitch.. I was able to fly with no signs of any problems.. I didn't expect any either. So I'm still very pleased with my DX7s.. Oh yeah.. I have a clean set up and range tested before flying if that makes a couple of the folks on here any happier.. 
. Brian Smith
. Brian Smith
05-06-2007, 08:43 AM
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RE: Spread Spectrum or PCM for Gassers
I will be willing to bet that all except the diehards will be using SS in two years. If you want to get a decent price out of your 72MHZ radios you had better do it now.
05-06-2007, 09:55 AM
#64
RE: Spread Spectrum or PCM for Gassers
In that developmental change, is an exponential thing- in two years , the naysayers may be totally out of the loop.
When the SPEKTRUM products came on line - I grabbed one and flew it in small cheap model- no real risk-
then compared results with the then current stuff.
Since then - I simply switched all of my equipment.
Those who do not fly the new technology, fit into the same group which declare with confidance that flat wings can not possibly work as well as a wind tunnel developed curves.
(was not true, of course)
Actual use of a product in it's intended application, is the ONLY true test of how it performs.
When the SPEKTRUM products came on line - I grabbed one and flew it in small cheap model- no real risk-
then compared results with the then current stuff.
Since then - I simply switched all of my equipment.
Those who do not fly the new technology, fit into the same group which declare with confidance that flat wings can not possibly work as well as a wind tunnel developed curves.
(was not true, of course)
Actual use of a product in it's intended application, is the ONLY true test of how it performs.
05-06-2007, 02:24 PM
#66
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RE: Spread Spectrum or PCM for Gassers
ORIGINAL: Pontiac_40
Bill, are you asking specifically about the JR/SS algorithms?
Bill, are you asking specifically about the JR/SS algorithms?
Yes I am very curious about how the JR/SS algorithms could possibly work as spread spectrum.
Bill
05-08-2007, 06:51 AM
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RE: Spread Spectrum or PCM for Gassers
I don't specifically know JR's algorithm for SS. If you do, maybe you can share the part you don't agree with and we can discuss.
05-08-2007, 08:05 AM
#68
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RE: Spread Spectrum or PCM for Gassers
ORIGINAL: Pontiac_40
I don't specifically know JR's algorithm for SS. If you do, maybe you can share the part you don't agree with and we can discuss.
I don't specifically know JR's algorithm for SS. If you do, maybe you can share the part you don't agree with and we can discuss.
Most of the conversation leads one to believe that JR/SS picks two channels from a table of 80 at the time of binding and passes the information to the receiver. Apparently the only transmit back is at binding but maybe not at all. Spread Spectrum should normally transmit back when an acceptable missed packet rate is exceeded requesting the transmitter change the hopping algorithm or tables to frequencies less busy. Today’s binding is probably inappropriate for tomorrows fly in.
The latency conversation makes no logical sense. Noticeable latency should be a function of missed packets and several orders of magnitude above hop or processing time.
Can anyone envision what a hop table should look like with only two frequencies in use? But maybe there is considerable misunderstanding about the two-frequency concept.
For starters.
Bill
05-08-2007, 08:20 AM
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RE: Spread Spectrum or PCM for Gassers
I dont suppose anyone here happned to see the video or read about QQ's plane going in using SS? Of course Horizon and related are blaming it on something else 
05-08-2007, 08:27 AM
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RE: Spread Spectrum or PCM for Gassers
ORIGINAL: Brian Smith
My wife and Me took my G38 powered 100 inch Rearwin Speedster to Savannah for the Skyblazers fly-in Yesterday/Sat... This is the 4th site where I have flown my DX7. There were two other flyers there on SS, and I ask them to bring their transmitters to the flight line with me and turn on while I was flying.. Not a glitch or hitch.. I was able to fly with no signs of any problems.. I didn't expect any either. So I'm still very pleased with my DX7s.. Oh yeah.. I have a clean set up and range tested before flying if that makes a couple of the folks on here any happier... Brian Smith
My wife and Me took my G38 powered 100 inch Rearwin Speedster to Savannah for the Skyblazers fly-in Yesterday/Sat... This is the 4th site where I have flown my DX7. There were two other flyers there on SS, and I ask them to bring their transmitters to the flight line with me and turn on while I was flying.. Not a glitch or hitch.. I was able to fly with no signs of any problems.. I didn't expect any either. So I'm still very pleased with my DX7s.. Oh yeah.. I have a clean set up and range tested before flying if that makes a couple of the folks on here any happier..
. Brian Smith
---------------
I own both types of systems (Spektrum and Futaba), but haven't been able to log much flying time with either. So far, both have performed satisfactorily.
I do not feel that either system is at a disadvantage when on/over the flying field. My only concern for the Spektrum system, and this may be unjustified, is when we fly our models at a distance and near residences or business offices.
The Futaba, while using a string of frequencies (I don't know how many as yet) would statistically have a better chance of remaining significantly interference free than the Spektrum system, should the model fly close to a home or business using gear on the 2.4 GHz band. The combination of less Tx signal (because of distance from the TX) combined with greater unintended residential or business 2.4 GHz signal field strength "could" present a problem in the minds of the uneducated (me - lacking empirical data). I'll have to study the 2.4 GHz band plan and see which specific frequencies are being utilized and then look at the specs of the receivers being used.
It is probable that all of this work has already been done by the systems' designers and manufacturers well in advance to the release of their products. It is also likely NOT to be a problem at all. I'm just thinking out loud.
There is a sort of precedent of interference between one 2.4 GHz device interfering with another. My 2.4 GHz microwave oven knocks out my 2.4 GHz WiFi wireless network when it operates.
Ed Cregger
05-08-2007, 08:46 AM
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RE: Spread Spectrum or PCM for Gassers
ORIGINAL: Ed Cregger
---------------
I own both types of systems (Spektrum and Futaba), but haven't been able to log much flying time with either. So far, both have performed satisfactorily.
I do not feel that either system is at a disadvantage when on/over the flying field. My only concern for the Spektrum system, and this may be unjustified, is when we fly our models at a distance and near residences or business offices.
The Futaba, while using a string of frequencies (I don't know how many as yet) would statistically have a better chance of remaining significantly interference free than the Spektrum system, should the model fly close to a home or business using gear on the 2.4 GHz band. The combination of less Tx signal combined with greater unintended residential or business signal field strength "could" present a problem in the minds of the uneducated (me). I'll have to study the 2.4 GHz band plan and see which specific frequencies are being utilized and then look at the specs of the receivers being used.
It is very probable that all of this work has already been done by the systems' designers and manufacturers well in advance to the release of their products. It is also likely NOT to be a problem at all. I'm just thinking out loud.
There is a sort of precedent of interference between one device interfering with another. My 2.4 GHz microwave oven knocks out my 2.4 GHz WiFi wireless network when it operates.
Ed Cregger
ORIGINAL: Brian Smith
My wife and Me took my G38 powered 100 inch Rearwin Speedster to Savannah for the Skyblazers fly-in Yesterday/Sat... This is the 4th site where I have flown my DX7. There were two other flyers there on SS, and I ask them to bring their transmitters to the flight line with me and turn on while I was flying.. Not a glitch or hitch.. I was able to fly with no signs of any problems.. I didn't expect any either. So I'm still very pleased with my DX7s.. Oh yeah.. I have a clean set up and range tested before flying if that makes a couple of the folks on here any happier... Brian Smith
My wife and Me took my G38 powered 100 inch Rearwin Speedster to Savannah for the Skyblazers fly-in Yesterday/Sat... This is the 4th site where I have flown my DX7. There were two other flyers there on SS, and I ask them to bring their transmitters to the flight line with me and turn on while I was flying.. Not a glitch or hitch.. I was able to fly with no signs of any problems.. I didn't expect any either. So I'm still very pleased with my DX7s.. Oh yeah.. I have a clean set up and range tested before flying if that makes a couple of the folks on here any happier..
. Brian Smith
---------------
I own both types of systems (Spektrum and Futaba), but haven't been able to log much flying time with either. So far, both have performed satisfactorily.
I do not feel that either system is at a disadvantage when on/over the flying field. My only concern for the Spektrum system, and this may be unjustified, is when we fly our models at a distance and near residences or business offices.
The Futaba, while using a string of frequencies (I don't know how many as yet) would statistically have a better chance of remaining significantly interference free than the Spektrum system, should the model fly close to a home or business using gear on the 2.4 GHz band. The combination of less Tx signal combined with greater unintended residential or business signal field strength "could" present a problem in the minds of the uneducated (me). I'll have to study the 2.4 GHz band plan and see which specific frequencies are being utilized and then look at the specs of the receivers being used.
It is very probable that all of this work has already been done by the systems' designers and manufacturers well in advance to the release of their products. It is also likely NOT to be a problem at all. I'm just thinking out loud.
There is a sort of precedent of interference between one device interfering with another. My 2.4 GHz microwave oven knocks out my 2.4 GHz WiFi wireless network when it operates.
Ed Cregger
If you check Futaba's web site in Japan, they had developped the SS long ago for industrial use like mining machines, and such, so its not new to them either
Roger
05-08-2007, 09:10 AM
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RE: Spread Spectrum or PCM for Gassers
There is a sort of precedent of interference between one device interfering with another. My 2.4 GHz microwave oven knocks out my 2.4 GHz WiFi wireless network when it operates.
The Futaba, while using a string of frequencies (I don't know how many as yet) would statistically have a better chance
In mid May the Joe Noll giant scale fly in will have about 5,000 in attendance and maybe provide new information.
Bill
05-08-2007, 07:34 PM
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RE: Spread Spectrum or PCM for Gassers
Bill, I don't know the specifics of JR's SS algorithm either. I do know that SS can be divided into two distinct categories:
1) Frequency hopping Spread Spectrum
2) DSSS (Direct Sequence Spread Spectrum)
I believe JR (correct me if I’m wrong, is using the DSSS)
A direct sequence system uses a locally generated pseudo noise code to encode digital data to be transmitted. The local code runs at much higher rate than the data rate. Data for transmission is simply logically modulo-2 added (an EXOR operation) with the faster pseudo noise code. The composite pseudo noise and data can be passed through a data scrambler to randomize the output spectrum (and thereby remove discrete spectral lines). A direct sequence modulator is then used to double sideband suppressed carrier modulate the carrier frequency to be transmitted.
An SS receiver uses a locally generated replica pseudo noise code and a receiver correlator to separate only the desired coded information from all possible signals. A SS correlator can be thought of as a very special matched filter -- it responds only to signals that are encoded with a pseudo noise code that matches its own code. Thus, an SS correlator can be "tuned" to different codes simply by changing its local code. This correlator does not respond to man made, natural or artificial noise or interference. It responds only to SS signals with identical matched signal characteristics and encoded with the identical pseudo noise code.
I don’t know how long of a PN (pseudo random noise) code JR uses to encode there data transmitted but this PN code is used to modulate the same frequency as say your buddy standing next to you is using without any interference. At the time of the hand shake between the Tx and the Rx it’s not the frequency that is being determined to be used but the PN sequence or in layman’s terms you could say the “secret password” is being determined between the Tx and the Rx modules. Once the code is determined the information transmitted by the the Tx can only be understood or demodulated using the correct PN sequence or “secret code” by the corresponding Rx.
I think what you are referring to in your post is freq hopping SS, I don’t know if JR uses that technique or not. One of the major issues with freq hopping SS systems was acquisition time of the new freq.
There can definitely be problems with these systems but I doubt that they would be due to interference related issues, I would guess more on the lines of programming and firmware type issues.---(my opinion, I could be wrong)
Also in the SS systems Signal/Noise ratio is key. You can think of S/N as a bucket with some water at the bottom. Water being the noise and the empty part of the bucket being the signal, as more and more users are on the system the water level rises in the bucket and requires more effort to keep above the noise floor. That’s when you get a lot of packets of information in error between the Tx and Rx.
I don’t know what kind of error correction JR systems employ. Getting the noise floor up to those levels would require a heck of a lot of users at one time.
Hope this helps and does not confuse folks reading this.
1) Frequency hopping Spread Spectrum
2) DSSS (Direct Sequence Spread Spectrum)
I believe JR (correct me if I’m wrong, is using the DSSS)
A direct sequence system uses a locally generated pseudo noise code to encode digital data to be transmitted. The local code runs at much higher rate than the data rate. Data for transmission is simply logically modulo-2 added (an EXOR operation) with the faster pseudo noise code. The composite pseudo noise and data can be passed through a data scrambler to randomize the output spectrum (and thereby remove discrete spectral lines). A direct sequence modulator is then used to double sideband suppressed carrier modulate the carrier frequency to be transmitted.
An SS receiver uses a locally generated replica pseudo noise code and a receiver correlator to separate only the desired coded information from all possible signals. A SS correlator can be thought of as a very special matched filter -- it responds only to signals that are encoded with a pseudo noise code that matches its own code. Thus, an SS correlator can be "tuned" to different codes simply by changing its local code. This correlator does not respond to man made, natural or artificial noise or interference. It responds only to SS signals with identical matched signal characteristics and encoded with the identical pseudo noise code.
I don’t know how long of a PN (pseudo random noise) code JR uses to encode there data transmitted but this PN code is used to modulate the same frequency as say your buddy standing next to you is using without any interference. At the time of the hand shake between the Tx and the Rx it’s not the frequency that is being determined to be used but the PN sequence or in layman’s terms you could say the “secret password” is being determined between the Tx and the Rx modules. Once the code is determined the information transmitted by the the Tx can only be understood or demodulated using the correct PN sequence or “secret code” by the corresponding Rx.
I think what you are referring to in your post is freq hopping SS, I don’t know if JR uses that technique or not. One of the major issues with freq hopping SS systems was acquisition time of the new freq.
There can definitely be problems with these systems but I doubt that they would be due to interference related issues, I would guess more on the lines of programming and firmware type issues.---(my opinion, I could be wrong)
Also in the SS systems Signal/Noise ratio is key. You can think of S/N as a bucket with some water at the bottom. Water being the noise and the empty part of the bucket being the signal, as more and more users are on the system the water level rises in the bucket and requires more effort to keep above the noise floor. That’s when you get a lot of packets of information in error between the Tx and Rx.
I don’t know what kind of error correction JR systems employ. Getting the noise floor up to those levels would require a heck of a lot of users at one time.
Hope this helps and does not confuse folks reading this.
05-08-2007, 07:45 PM
#74
Senior Member
My Feedback: (14)
RE: Spread Spectrum or PCM for Gassers
ORIGINAL: dick Hanson
In that developmental change, is an exponential thing- in two years , the naysayers may be totally out of the loop.
When the SPEKTRUM products came on line - I grabbed one and flew it in small cheap model- no real risk-
then compared results with the then current stuff.
Since then - I simply switched all of my equipment.
Those who do not fly the new technology, fit into the same group which declare with confidance that flat wings can not possibly work as well as a wind tunnel developed curves.
(was not true, of course)
Actual use of a product in it's intended application, is the ONLY true test of how it performs.
In that developmental change, is an exponential thing- in two years , the naysayers may be totally out of the loop.
When the SPEKTRUM products came on line - I grabbed one and flew it in small cheap model- no real risk-
then compared results with the then current stuff.
Since then - I simply switched all of my equipment.
Those who do not fly the new technology, fit into the same group which declare with confidance that flat wings can not possibly work as well as a wind tunnel developed curves.
(was not true, of course)
Actual use of a product in it's intended application, is the ONLY true test of how it performs.
---------------
You will get no argument from me in this regard, Dick.
Ed Cregger
05-08-2007, 08:08 PM
#75
Senior Member
My Feedback: (14)
RE: Spread Spectrum or PCM for Gassers
ORIGINAL: Pontiac_40
Bill, I don't know the specifics of JR's SS algorithm either. I do know that SS can be divided into two distinct categories:
1) Frequency hopping Spread Spectrum
2) DSSS (Direct Sequence Spread Spectrum)
I believe JR (correct me if I’m wrong, is using the DSSS)
A direct sequence system uses a locally generated pseudo noise code to encode digital data to be transmitted. The local code runs at much higher rate than the data rate. Data for transmission is simply logically modulo-2 added (an EXOR operation) with the faster pseudo noise code. The composite pseudo noise and data can be passed through a data scrambler to randomize the output spectrum (and thereby remove discrete spectral lines). A direct sequence modulator is then used to double sideband suppressed carrier modulate the carrier frequency to be transmitted.
An SS receiver uses a locally generated replica pseudo noise code and a receiver correlator to separate only the desired coded information from all possible signals. A SS correlator can be thought of as a very special matched filter -- it responds only to signals that are encoded with a pseudo noise code that matches its own code. Thus, an SS correlator can be "tuned" to different codes simply by changing its local code. This correlator does not respond to man made, natural or artificial noise or interference. It responds only to SS signals with identical matched signal characteristics and encoded with the identical pseudo noise code.
I don’t know how long of a PN (pseudo random noise) code JR uses to encode there data transmitted but this PN code is used to modulate the same frequency as say your buddy standing next to you is using without any interference. At the time of the hand shake between the Tx and the Rx it’s not the frequency that is being determined to be used but the PN sequence or in layman’s terms you could say the “secret password” is being determined between the Tx and the Rx modules. Once the code is determined the information transmitted by the the Tx can only be understood or demodulated using the correct PN sequence or “secret code” by the corresponding Rx.
I think what you are referring to in your post is freq hopping SS, I don’t know if JR uses that technique or not. One of the major issues with freq hopping SS systems was acquisition time of the new freq.
There can definitely be problems with these systems but I doubt that they would be due to interference related issues, I would guess more on the lines of programming and firmware type issues.---(my opinion, I could be wrong)
Also in the SS systems Signal/Noise ratio is key. You can think of S/N as a bucket with some water at the bottom. Water being the noise and the empty part of the bucket being the signal, as more and more users are on the system the water level rises in the bucket and requires more effort to keep above the noise floor. That’s when you get a lot of packets of information in error between the Tx and Rx.
I don’t know what kind of error correction JR systems employ. Getting the noise floor up to those levels would require a heck of a lot of users at one time.
Hope this helps and does not confuse folks reading this.
Bill, I don't know the specifics of JR's SS algorithm either. I do know that SS can be divided into two distinct categories:
1) Frequency hopping Spread Spectrum
2) DSSS (Direct Sequence Spread Spectrum)
I believe JR (correct me if I’m wrong, is using the DSSS)
A direct sequence system uses a locally generated pseudo noise code to encode digital data to be transmitted. The local code runs at much higher rate than the data rate. Data for transmission is simply logically modulo-2 added (an EXOR operation) with the faster pseudo noise code. The composite pseudo noise and data can be passed through a data scrambler to randomize the output spectrum (and thereby remove discrete spectral lines). A direct sequence modulator is then used to double sideband suppressed carrier modulate the carrier frequency to be transmitted.
An SS receiver uses a locally generated replica pseudo noise code and a receiver correlator to separate only the desired coded information from all possible signals. A SS correlator can be thought of as a very special matched filter -- it responds only to signals that are encoded with a pseudo noise code that matches its own code. Thus, an SS correlator can be "tuned" to different codes simply by changing its local code. This correlator does not respond to man made, natural or artificial noise or interference. It responds only to SS signals with identical matched signal characteristics and encoded with the identical pseudo noise code.
I don’t know how long of a PN (pseudo random noise) code JR uses to encode there data transmitted but this PN code is used to modulate the same frequency as say your buddy standing next to you is using without any interference. At the time of the hand shake between the Tx and the Rx it’s not the frequency that is being determined to be used but the PN sequence or in layman’s terms you could say the “secret password” is being determined between the Tx and the Rx modules. Once the code is determined the information transmitted by the the Tx can only be understood or demodulated using the correct PN sequence or “secret code” by the corresponding Rx.
I think what you are referring to in your post is freq hopping SS, I don’t know if JR uses that technique or not. One of the major issues with freq hopping SS systems was acquisition time of the new freq.
There can definitely be problems with these systems but I doubt that they would be due to interference related issues, I would guess more on the lines of programming and firmware type issues.---(my opinion, I could be wrong)
Also in the SS systems Signal/Noise ratio is key. You can think of S/N as a bucket with some water at the bottom. Water being the noise and the empty part of the bucket being the signal, as more and more users are on the system the water level rises in the bucket and requires more effort to keep above the noise floor. That’s when you get a lot of packets of information in error between the Tx and Rx.
I don’t know what kind of error correction JR systems employ. Getting the noise floor up to those levels would require a heck of a lot of users at one time.
Hope this helps and does not confuse folks reading this.
------------------
The Spektrum's RF implementation is simply to find a vacant frequency pair and then "lock" on to those two frequencies. A "voter" circuit in the receiver is then used to decide which frequency to listen to/best S/N ratio. This technique has been used for decades in business and public safety two-way radio communications systems, with the exception that their frequency pair is fixed and assigned by the FCC to a single set.
Since our model receivers lack the ability to communicate with our transmitters, once the frequency pair has been selected at boot-up, that is where they stay, regardless of the amount of information lost due to interference. This is where the Spektrum system is at a disadvantage. Granted, it would take very unusual circumstances for this condition to manifest. For all practical purposes, the possibility of this happening is remote - but theoretically possible.
Software is of no use at all unless there is a usable signal reaching the decoder section of the receiver. The S/N ratio, from an RF Tech's perspective, all occurs in the analog section of the receiver. I'm sure that the software types will feel differently about this issue. Certainly, the software aspect is of key importance too.
Ed Cregger