The REAL Difference Between FM AM and SSB

[@stargazer7644]

There's no heterodyne (squeal) on SSB when multiple people talk because there's not multiple carriers mixing. SSB is also usually preferred over AM because it concentrates all your power into the single sideband carrying your voice, where AM splits it up into the carrier and two sidebands. So SSB can be heard farther, and uses less bandwidth allowing more signals to be packed into the same space.

[@DXCommanderHQ]

Nicely said!!

[@COASTALWAVESWIRES]

I’m a structural engineer, we were taught it this way: AM = Almost Magic FM = F’n Magic There, see how easy that was! 😂 Great video Callum! 73 my friend!

[@spaceflight1019]

Shades of electronics school 53 years ago!

[@DXCommanderHQ]

HAHA Walt!

[@pcramil]

Callum, I’m still new with AM/SSB with ham radio, but I’ve been licensed to fly for the past 18 years, working professionally for the past 8 years. Aircraft radios most definitely don’t allow two pilots to be heard at the same time. On a busy frequency, it’s very common to hear loud squeals when two key up at the same time, and usually either transmission can’t be deciphered (happened a few times today). Sometimes, if one is much closer than the other, but usually not. It’s common for another person also on frequency to call out “Blocked!” any time there’s a “pileup” on an aviation frequency, sometimes causing another squeal when multiple pilots call it out. I still appreciate AM over FM because I’d rather not get the message but know one was trying to be sent rather than have a closer aircraft transmit over ATC and I miss their message entirely due to capture effect.

[@DXCommanderHQ]

Fascinating..

[@cfsyoutube5799]

replying to @pcramil's comment: I appreciate he's speaking from practical experience at the pilot end - I've worked on the ground end with aviation communications "a long time" (RAF then Frequentis since 1990). In reality if 2 aircraft transmit exactly on the same frequency, with similar power/modulation etc., in the tower you will get a "heterodine" reception. You occasionally can make out one aircraft over the other if one has stronger signal, deeper modulation, or a higher pitched modulation (female pilots win 🙂), but you definitely always know that 2 (or more) aircraft were transmitting at the same time. In the opposite direction (ground to air) often the ground station is more legible to the aircraft, due in part to more power and more efficient antennas. We also use something called "climax operation" where 2 or more AM ground transmitters, geographically separated, and with a very small offset on the carrier frequency, can deliberately transmit the same thing at the same time, and be fully legible to the aircraft (as long as we set it up properly 🙂).

[@paulsengupta971]

@@cfsyoutube5799With Climax, you do get a bit of heterodyning if you're flying in a position to receive both transmitters at the same time.

[@IZ0MTW]

@@cfsyoutube5799hello, I’ve read few comments here and I see there’s someone from the aeronautical world. I’m a commercial pilot other than a ham and in fact I’ve noticed many times ground stations have multiple carrier frequencies with an offset from each other and from the center frequency. Sometimes I can hear the beating tone from the aircraft. I’ve always wondered why is that done. Is it done on purpose or is just a frequency misalignment from the multiple ground stations transmitting a the same time? Also why they use geographically different transmitters simultaneously? perhaps for radio coverage? this is interesting. Another thing I’ve noticed is that with average ham receivers the rx filters aren’t wide enough to copy the aircraft and the ground station without moving the tuning knob to center on the station. From the aircraft instead never had this type of issue. Filter width and audio quality is good enough. Last but not least I can confirm that the radios are not perfectly centered but enough to work. And when two transmission happens at the same time, ( which I can tell it happens all the times) you can definitely hear them both, and sometimes, depending on relative strengths etc you can read one station over the other. And yes, somebody will say blocked to suggest two transmissions crossed each other.

[@cfsyoutube5799]

@@IZ0MTW - when it's deliberate it's referred to as "climax", have a google around and you'll find quite a bit of information. It's mainly used where an ATC sector is large, and has a relatively small amount of traffic. For example in mountain areas. Historically when ATC frequencies were channelised with 25khz spacing, ground stations could be offset by 5khz+ and still be within the bandwidth of aircraft receivers. In the last few years we have moved to 8.33khz spacing, and thing need to be much tighter. We are increasingly using BSS (best signal selection) to decide which ground radio site has the best link with a particular aircraft, then selecting the best Tx'r based in the best Rx. All the processing needs to take place within a few 10s of m sec, even allowing for the delays in getting signals to/from remote radio sites.

[@JoshColletta]

Just to put a name to it, what we're talking about on FM is called the "capture effect." It's not so much a function of the waveform in the RF realm as it is the way the FM decoder works in your radio. All FM receivers, by necessity, contain an RF limiter to prevent overmodulation. This limiter attenuates everything but the strongest signal at the antenna. So, when you're traveling down the highway, listening to the radio, and you hear two stations fluttering back and forth on the same frequency because you're on the fringes of both signals, that's the FM capture effect rapidly switching back and forth between whichever is the strongest at that moment in time. By contrast, AM (and, by extension, SSB) does not exhibit the capture effect, because in AM, the strength of the signal is what being modulated to produce audio, so you get the additive effect instead.

[@problemwithauthority]

Just a few days ago I had a visual of how SSB works. I knew how it workded, but I tuned into one of the time broadcasts from WWV. I was set to USB and in the waterfall I could see the signal to one side, I switched to LSB and it moved to the other side of my frequency marker, then switched to AM and watched the same signal take up twice the space in the waterfall on both sided of my frequency marker. It added a visual to what already knew was happening.

[@DXCommanderHQ]

Yes!

[@DonzLockz]

Thanks Callum. Isabella smashed up that pile up!😊💪

[@DXCommanderHQ]

Great job!

[@jeffmerlin2580]

Good explanation Cal. SSB is "kind of" AM, or half AM without a carrier. That's why we hear everything and are able to pick YL's easy. 🙂 Same with AM, when everyone have the same carrier frequency.

[@richiet3841]

A simpler way of thinking about it is where is the information stored. You need two parts for RF to convey information. A carrier wave and a way to modulate that carrier with the information (speech). In Amplitude modulation, the transmitter supplies a carrier at a fixed frequency, and the tx modulates that Amplitude according to that speech. In Frequency modulation, the tx modulates the frequency of the wave proportionally to the speech. In SSB, which was adopted due to its bandwidth efficiency for hams, it was found that the AM wave had 50% of the power going to the carrier, and the speech info we care about was in the TWo sidebands of the modulation. So we suppress the carrier energy, and have available both sidebands to listen to. The bandwidth of these sidebands is 3 kilohertz, we choose either upper or lower bands by convention there is no reason we can't hear 7125 on upper side and, we chose to follow a plan. Now the fun part... In order for the Rx to demodulate the SSB signal it needs to inject back that carrier, via a beat frequency oscillator (BFO) . That's were all the interference arises. Your Rx is trying to march a specific frequency and is bombarded with others slightly off that frequency (the Donald duck voice) or on the same frequency only stronger giving your ears pain trying to hear YOUR station. Filtering and processing magic has helped greatly. Why aircraft use AM over FM is due to a phenomenon known as capture effect, the stronger signal wins and you only hear it, not the weaker station. Am precisely does not have this effect and you can hear both strong and weak signals. Hope this gives a clear picture of the magic happening inside the radio.

[@DrWorth-ez5es]

I haven't seen FT8 mentioned in this thread. Love it or hate it, it makes full use of the benefits of SSB that are being discussed here. In particular, on one dial frequency, in one 15-second period theoretically there could be a 'pile up' of >50 simultaneous transmissions, each being individually identified and decoded at -20dB thanks to the magic of SSB (and some cool software).

[@DXCommanderHQ]

Yes! I recorded a video yesterday of that!

[@timdbl7804]

Hi Callum, thanks for a valiant attempt to explain this very interesting subject! Just to mention what I think was a slip of the tongue: The conventional frequency-deviation, on (narrow) FM communications is about 2.5 kHz I think; 12.5 kHz is the channel-spacing. (This is because the RF bandwidth, of FM, can be much greater than the deviation of the carrier). I wish I could find an on-line version of the great book, about modulation formats, which my dad gave me as I started my Degree in Communication Engineering! Hopefully, those, who would like a better explanation, can find a suitable professional publication on-line; maybe the Institute of Engineering and Technology, in the UK, have something to offer non-members. 73 all!

[@DXCommanderHQ]

Well spotted!!

[@paulsengupta971]

Airband in Europe now uses 8.33kHz channel spacing.

[@timdbl7804]

@@paulsengupta971 Yep. Narrow-band FM (not airband AM) uses 12.5 kHz spacing (in the UK at least)

[@julianrobertson3303]

On the install of your Genius 8X2 do try to ground the chassis , you can tap an earth electrode into the ground & connect it with a simple earth cable .

[@DXCommanderHQ]

Actually, the Genius "cabinet" is still in infancy. Currently, only the coax lines are grounded directly to a massive copper ron next to the cabinet. But I have a ton of work to to in that department...! Thanks for tip.

[@maartenc6099]

When you mix two frequencies you get three products. Like when you mix 3KHz with 27MHz, you get 27MHz minus 3KHz you get the 27MHz and you get 27Mhz plus 3Khz. This is what a normal AM signal looks like. The minus is the lower sideband, the middle is in this example the carrier and the plus is the upper side band. So when you talk in your microphone you get the carries plus two sidebands. Now the carrier does not have any information. The information is in the two sidebands. And since the two sidebands contain the same information, you only need to transmit one sideband, the lower our the uppere sideband. So what you do with a balanced modulator is surpress the carrier, since the carrier does not contain information. So you left with only two sidebands. Now what you need to do is surpress one sideband. This is done with a crystal filter. You either surpress the lower sideband our you surpress the upper side band. Leaving you with one sideband to transimt and you can put all your HF power in the one sideband. This is single sideband. The carrier is only needed to demodulate the signal and can be provided in your receiver and this is done with a circuit called BFO (Beat Frequency Osaclillator) and should be placed at the exact spot as you should expect the carrier to be. This is why you have your clarifier on your cb-rig. to put the BFO at the exact spot as it should be. Now to make this story more interesting, there is a reason why radio-amateurs use LSB on the bands lower then 10MHz, and USB on the bands higher then 10MHz. Now to use an example we build a radio with a MF (middle frequency) of 9MHz. We mix this with a mixing frequency if 5MHz. This will give us 5+9=14MHz and this will give us 9-5 is 4MHz. If we make this 5MHz variable from 5-5.5 Mhz this will guve is 9+5-5.5= 14 to 14.5 MHz the 20m band. And it will give us 9-5 to 5.5MHz = 4 to 3.5MHz this is the 80m band Now if we mix our LF our voice with the 5MHz signal you will see that if will produce the lower side band on 80m, and the upper side band on 20m. And if we choose to surpress the 14MHz our the 4 MHz you will see that with a few components we can easy build a dual bander. Only the sideband will change because of up, our down mixing. Now you know why we use Lower sideband our Upper side band on some bands. The professionals do not do this.

[@DXCommanderHQ]

Brilliant comment!!

[@andylinton2798]

FM has a thing called the 'capture effect' which as Cal explains well, means a receiver will only demodulate (i.e. convert to audio) the strongest signal. AM doesn't, so two transmissions can be heard at once. If, however two AM transmitters are going at the same time but are not quite on the same frequency, the carriers will 'beat' with each other causing a 'heterodyne' - which is a squeal or whistle the audio frequency of which is actually the difference in frequency between the two transmissions. At night, on the AM band, sometimes American AM stations' signals make it across the Atlantic. But they're on multiples of 10kHz, whereas European AM stations are on multiples of 9kHz. So if you listen to, say, 909kHz in the UK you'll hear Radio 5 Live. However, if a US AM station on 910kHz is making its way across, you'll hear a 1kHz whistle under the probably stronger Radio 5 Live. AM like this is properly called Double-Sideband Full Carrier. Other versions exist, such as DSB reduced or suppressed carrier, and of course good old single sideband which doesn't transmit the carrier at all, so it's more efficient at the transmitter and doesn't cause heterodynes if more than one signal is received at once. However its audio quality isn't as good as DSB AM, which is why most international HF broadcast stations still use DSB full-carrier AM. But perfectly good enough quality for voice. I did some experiments a few years ago transmitting music on SSB - I used to have an experimental licence that allowed me to do this. Here's a vid of music being transmitted on SSB - it's possible to make it sound pretty good, but the quality depends on really well set up transmitters and properly tuned receivers. kzbin.info/www/bejne/ZoS1hpeqmJ6CjKc By the way, I'm not for one second disagreeing or countering Cal's excellent explanation, just expanding upon it a bit for those interested. If you're not interested, don't read this!

[@DXCommanderHQ]

Beautiful.

[@StreakyP]

Here's a good one.. what is the difference between a "linear" & an "amplifier"?.. linear means straight line & in a linear if you put 2w in & get 20w out if you put 1w in you will get 10w out & this is needed for all the modulation modes where the magnitude/shape of the envelope is important.. that is AM & SSB. This is clean but not very efficient. If your amplifier goes non-linear you will start transmitting spurii & harmonics & you cannot get rid of them so am & SSB amps MUST be "linears". On FM what really matters is the number of zero crossings & the amplitude shape doesn't really matter. You can put FM through a totally non-linear amplifier (that is more efficient) then slug it through a big filter to get rid of all the sproggies & you are still ok. Based on this if you have a 2m FM amp it won't be a linear so don't put SSB through it.... & If you have a 2m linear that is obviously good for ssb & although FM could go through it it probably isn't spec'ed for a 100% duty cycle mode like FM (or am or some data modes) so you would only be able to run it at a fraction of its max rating to stop it melting down. Be wary not all amplifiers are linears (& if you are trying to boost a hotspot watch what type of amp eBay sends you & what data mode needs what.. it isn't always obvious).😮

[@DXCommanderHQ]

Fascinating!

[@EvolvingDaily]

Do you have any videos about elevated verticals? I have went back through and watched a bunch of your videos trying to.find info but can't seem to find any. Thanks

[@DXCommanderHQ]

Nothing so far.. But effectively the same BUT ideally, you need them a bit higher.. Rudy Severns did a paper about this.

[@JReed305]

I do AM on 75m an 40m, every once in awhile I will be on 10m calling CQ around 29.050. It's a fun mode when conditions are good.

[@DXCommanderHQ]

Yes!

[@julianrobertson3303]

Thank you for sharing as always . I wanted to mention that I've recently been researching the huge benefits of coax cable through copper tubing , amazing noise reduction & very low RF loss , the copper tubing has to be grounded to prevent electrolysis , but coax in copper tubing under ground will be ideal within your location .

[@DXCommanderHQ]

I'm sure but it's a bit late now, the trench, duct and coax is already in.. However, I agree which is why I used EcoFlex 15.

[@julianrobertson3303]

That's a shame , but well done with the EcofFex15 , it's indeed a good solution . The copper tubing would of shielded absolutely everything & maintained RF line transmission , if you should create another RF install copper tubing will be a must have , the copper tubing will need to be grounded to earth to prevent electrolysis, but in meanwhile you can ground the antenna switch unit with an earth stake , perhaps at some point you can carry out a separate line transmission in copper tubing ?, believe it or not RG213 or even RG 58 will perform very well hence it's enclosed within the copper tubing & it will provide remarkable results .@@DXCommanderHQ

[@julianrobertson3303]

another plus point with copper tubing conduit is that providing the tubing is earthed any RF feedback from the coax will be slugged straight to ground helping to protect the transceiver .@@DXCommanderHQ

[@Giger001]

Is it safe to say that AM mode is still being used (despite its decrease in popularity), and can still be relied on for emergency situations? Thank you in advance.

[@DXCommanderHQ]

They would all work in an emergency.. I'd say you get more "omph" from FM.

[@TheScaleModelShack]

I’m guessing this is why they have the concept of ‘channels’ on FM radios so that 2 signals don’t collide as it were ?

[@DustInTheWindAZ]

Imagine my surprise this morning, when I was having my coffee, that I hear you call out my "handle"! Now, I understand the difference between Amplitude Modulation (AM) and Frequency Modulation (FM), and that sideband is just that: either the upper or lower "side" or half of the wave. Now that just made me wonder: Can you "sideband" on both AM and FM? My experience is with CB (so far), which until recently, has been strictly AM. What I"m still not understanding is why the "squeal" occurs when two operators key-up at the same time. I understand that proximity is a factor, but Is it the squeal caused by the two transmitted frequencies being in-sync or out-of-sync with each other? Here's a musical analogy: I play a 12-string guitar, and when I'm tuning a pair of strings to the same note, they will clash at various rates until they are perfectly in-sync. So then, is the radio squeal (heard by someone beside the two transmitting parties) created by two nearby transmitters that are just a bit out of sync? Thanks for answering my question in a video Cal! 73!

[@DXCommanderHQ]

Out of sync Dusty!

[@spaceflight1019]

Here's the deal with the "squeal" or "heterodyne". It's caused by the difference between the two AM carriers' frequencies manifesting itself as an audio tone. The frequency tolerance for CB is +/-.005%, or about 1300 Hz. Back in the 23 channel era, frequency control was by crystals, and it wasn't unusual to have carriers up to 2600 Hz or more apart.

[@JoshColletta]

Answering the question about sidebanding with FM... not in the same sense as SSB is sidebanding AM, no. Since FM modulates the frequency rather than the amplitude, every bit of modulation in FM is taking place in the sidebands to either side of the center frequency, so if you cut off half of the signal, it's simply going to clip and reproduce only the upper or lower audio frequencies, depending on which sideband you choose. Basically, if you think of FM as an audio waveform in a scope turned on its side, you can see what I'm getting at. You wouldn't be suppressing extraneous audio information, you would just be cutting off either all of the bass or all of the treble. In fact, if you look at the NRSC-5 "HD Radio" standard here in North America, you'll see that the digital portions of those signals operate in the "sidebands" of an FM broadcast station, but those sideband portions are literally the digital signal split into two, equidistant from the carrier. The entire digital system will fail to function if one of those sidebands is not there. What we CAN do is limit FM signals to narrower bandwidths, which is what aircraft radios do. If you look at SDR software and see "NFM" or "Narrow FM" in the tuner section -- and I've even seen it on some older analog receivers -- that's the "spiritual" equivalent of SSB on FM, if not the direct technical equivalent. EDIT: I should add that subcarriers are used on FM, and that could be considered similar to SSB in terms of the waveform layout within the channel. Basically, that's taking the higher audio frequencies that are out of the audio range (of either the receiver or the human ear, or both; so usually in the 30 kHz and up range) and modulating them to produce a separate audio signal. This is how "Muzak" and reading services for the vision impaired used to be transmitted up through the 1980's or so. It's not quite sidebanding, as it still requires the primary carrier to function, but it's another function that might be thought of when sidebands are brought up in relation to FM.

[@BrandonLeeBrown]

FM receivers also don't pick up or respond to atmospheric static or electrical interference, such as lightning, that AM and single sideband receivers pick up. To produce a transmitted modulated AM signal with two sidebands, only one sideband is required to modulate the carrier. I have a couple of old vacuum tube single sideband radios that have a manual carrier control knob on the front panel. In AM, only the upper sideband is produced to modulate the carrier, but you can hear the lower sideband that is also produced when the carrier is modulated by only one sideband. The carrier has to be powerful enough to handle the modulation being produced by the one sideband though. With those old radios, when I reduce the carrier below a certain power level, only the upper sideband is transmitted.As I introduce more carrier power, both upper and lower sidebands are transmitted with the modulated carrier.

[@DXCommanderHQ]

Nice!

[@Professzore]

Technically (at @2:00 and around). Both AM and FM are "de facto" modulated carriers. The difference is the way of modulation. FM modulates the FREQUENCY of the carrier, AM modulates the AMPLITUDE of the carrier. The real reason why many AM (and SSB with suppressed carrier) stations can be heard at the same time without an issue, is rather simple: If there is no carrier (or there are many carriers on the very same frequency), you can DEMODULATE all (added) AM modulated sinusoidal waveforms without a problem. The key is the ability of demodulation. When general AM or not-suppressed carrier SSB sinusoidal waves are added in a crowd, the problem is: that you'll not be able to demodulate it correctly, because the carrier of the off-frequency transmission will corrupt the "good"/useful part of the other transmission. On FM, the situation is even more complicated. FM "silence" is a clean single-frequency carrier with a pure sinus waveform. The receiver will _always_ lock to the signal with the bigger amplitude (the stronger one) and will try to demodulate the speech (or data, or whatever) relative to the stronger signal's carrier frequency. If there is any, the stronger signal will win and the receiver will clearly demodulate the information, even when the other signal is slightly off-frequency and may corrupt one side of the FM modulation. This is called the FM Capture effect. That's why FM has way better sound quality and is used on commercial radio broadcasts. BTW, there is a very good video explanation of it somewhere here from Tall Paul.

[@DXCommanderHQ]

Very fine. Tnx.

[@Professzore]

@@DXCommanderHQ Oh, yes.. Just a footnote. That's EXACTLY why FT8 can't work on FM.

[@NebukedNezzer]

an ice storm just kinked my old cushcraft 40-10 meter vertical so I have a dx commander all band (40 up) on order to come to me friday. opportunity to make an improvement.

[@DXCommanderHQ]

Good choice!

[@stuartmccloud307]

Nice simple explanation Callum, makes perfect sense. I wonder why UK CB went FM in that case?

[@DXCommanderHQ]

Good question!

[@14KiloWhisky]

Maybe for avoid TV qrm...

[@maartenc6099]

The reason for choosing fm is really simpele and is called LFI (low frequency interference). Fm is a constant signal and AM is not.

[@souta95]

AFAIK, AM is not allowed on 60 meters in the US... There's an AM net on Saturday mornings on 80 meters near me, though... The Midwest Classic Radio net. I've listened in a couple times, but never checked in.

[@DXCommanderHQ]

Ah. Interesting.

[@RicksHamShack]

One of these days i want to pit a 12 watt SSB cb against a 50 watt GMRS FM radio. Should make an interesting video.

[@DXCommanderHQ]

It'll all depend on the antenna, height above ground etc because they are quite different wavelengths.

[@paulsengupta971]

"They've got very high quality pieces of kit, everybody's locked on the right frequency because it's all calibrated right" Not the radio in my aeroplane! :-) Admittedly these days I use a handheld for the 8.33kHz spacing which is a lot better than my old radio in the aeroplane.

[@miker8379]

No am or fm in usa on 60m. Love me some am especially on 29.000

[@DXCommanderHQ]

Yeah, we did AM on 29MHz last year.. Fun times.

[@spaceflight1019]

Let's go back in time to the late 1950s. Amateur radio was just seeing the then-new SSB mode. Although AM was still widely used it became relegated to rag-chewing because SSB was proving itself superior for DX. VHF hams were still using AM by a wide margin. FM wouldn't become popular until the early 1970s, when repeater operation became legal, and a flood of abandoned commercial radios hit the markets. Speaking of commercial radio... While the AM vs. FM debate was largely settled, there was still a few users of AM in the 25-30 MHz region. FM was winning because of its ease of use and its simplicity. Truck drivers turned the radio on, set the squelch, and didn't have to fiddle with it again. When skip became an issue CTCSS systems were implemented to deal with the problem.

[@DXCommanderHQ]

Fascinating. Thank you.

[@tsclly2377]

So what s this Lora stuff? Digital mayhem?

[@DXCommanderHQ]

No idea.

[@RobSolid]

Still can't understand SSB

[@DXCommanderHQ]

In "English", SSB is like AM but without the carrier - and it's only have the waveform (effectively).

[@mattikankaanpera1979]

am I in the forest 😱, Am modulates the carrier wave, Fm modulates the frequency. 🤔 (lucky there's GPS, so you can find your way back to your radio friends, the rag bitten 😂 )

[@DXCommanderHQ]

.. and that

[@bitemykrank1970]

And here I was thinking it was the spelling.......

[@DominicMazoch]

AM is ancient modulation!

[@DrWorth-ez5es]

AM lives on! Without QAM we wouldn't have wifi or digital television🙂.

[@Bob-1802]

@@DrWorth-ez5es Cellular networks also use QAM 😁