Why is Donald Duck on the Radio? Math Behind Single Sideband Explained

AM, or amplitude modulation, was the earliest way of sending voice over radio waves. That makes sense because it is easy to modulate a signal and easy to demodulate it, as well. A carbon microphone is sufficient to crudely modulate an AM signal and diode — even a piece of natural crystal — will suffice to demodulate it. Outside of broadcast radio, most AM users migrated to single side band or SSB. On an AM receiver that sounds like Donald Duck, but with a little work, it will sound almost as good as AM, and in many cases better. If you want a better understanding of how SSB carries audio, have a look at [Radio Physics and Electronics] video on the subject.

The video covers the math of what you probably already know: AM has a carrier and two identical side bands. SSB suppresses the carrier and one redundant side band. But the math behind it is elegant, although you probably ought to know some trigonometry. Don’t worry though. At the end of the video, there’s a practical demonstration that will help even if you are math challenged.

Dealing with signals as math equations are a staple if you seriously study electrical engineering. For example, you can model a pure sine wave with the equation: S(t) = A sin(2*pi*F*t) where A is amplitude, F is the frequency in Hertz, and t is time. Try it. In that example, the frequency is 1 kHz and the amplitude is 20, but you can tweak the values and see what happens. If you add something to the argument to the sin function, you’ll change the phase of the wave. If you followed that, you should have no problem with the video.

This math isn’t just good for working homework problems. It is the same math you need to do synthesis with a computer or digital signal processor. If you want to dig in, we talked about phase angle mathematics, earlier.

But wait, what’s with that screwy title? Have you ever heard an SSB signal on an AM radio? Most people describe it as sounding like Donald Duck. Listen at about the 20 second mark of this mp3 file and just after that, too where it is Upper Side Band (the other kind of USB) but off frequency and you’ll hear that famous voice. If all of this is unfamiliar, you need to explore the speech transmission origins of AM.

 

18 thoughts on “Why is Donald Duck on the Radio? Math Behind Single Sideband Explained

        1. “All that technical stuff” is why it’s apparently the best thing for RC toys.
          In particular, it’s the immunity from interference which the frequency hopping spread spectrum gives you (not really immunity; you get interference sometimes, but it’s for very brief periods of time, rather than two people trying to shout over each other as they both keep on using the same frequency)

          1. By the way. Did you know that FHSS was invented (and patented in 1942) by Hedy Lamarr, the famous actress? Bluetooth uses adaptive frequency hopping spread spectrum. The adaptive part is that it avoids frequencies that have interference, making it more compatible with Wi-Fi which uses the same frequencies. Most FHSS radios used in models are really using Bluetooth standards.

    1. SSB is usually sent as very tight bandwidth, such as 2khz. If you demod an AM signal in a wider bandwidth filter, 6khz or more, it will be broadcast-quality, with the advantage of choosing which sideband gives better adjacent-channel rejection.

    2. The problem with SSB is that there’s no way to get the carrier reinserted in the right place. So you can’t really get the exact “tone”. It never bothered me, but I realize that when I tune a voice, I’m tuning till it sounds “right” to me, that may not be how the person sounds in real life.

      SSB is AM. The sidebands are identical, only one is needed. The carrier is only needed to demodulate the signal, though with “AM’ the carrier is in exactly the right place.

      Bandwidth of a sideband is irrelevant. Voice has limited bandwidth to begin with. AM broadcast was traditionally wide, because of music, but voice is limited. Any communication system will limit bandwidth, be it FM, AM, or SSB. The main difference is that the others will see the bandwidth limited in audio, sometimes just some RC, while crystal or mechanical filters used for most traditional SSB can have quite sharp cut-off which perhaps does impact “the sound”.

      For years I kept looking for SSB CB sets at garage sales, thinking them a source of SSB filters. I finally found one a few years go, but there was only one filter for AM and SSB, a 4KHz wide filter.

      It was a controlled situation since CB is channelized. The demands are less. But it made me think about early SSB in amateur radio. The filters were often primitive, but the bandwidth could be controlled by sculpting the audio chain. The IF filter was needed to get rid of the unwanted sideband, a sharp cut off needed at the lower end.

      A lot of that early work was in transmitters, the existing receiver being used. A transmitter is a controlled environment, the audio bandwidth limited and stray signals not coming from all over. It’s the receiver that needs a good IF filter, since signals could be coming in from the antenna anywhere.

      Michael

      1. I agree the main problem with SSB is that, unless you’re using VERY accurate timing on both ends, you have no possible way to reconstruct the audio in an identical fashion. The same problem is famous in NTSC aka Never The Same Color.

        1. NTSC sends a “pilot” burst during the horizontal synch/retrace interval. The pilot called the, color burst, is used to supply a carrier with the correct phase and frequency to demodulate the color subcarrier. Any color issues are due to deficiencies in the receiver or the signal source. Most modern receivers did very well on broadcast television, before they were superseded by ATSC. Some, VCRs and DVD players did not provide accurate or stable color signals.

    3. SSB **WILL** sound better than AM. Simple. You get four times the actual usable power in SSB that you do for AM, so a strong SSB signal will always sound better than AM buried is the noise.

      The typical textbook example: You have a AM transmitter running at 4 watts. Two watts goes into the carrier and one watt into the upper side band, and one watt into the lower side band. So, your voice gets one watt of power, and takes up bandwidth equal to twice your highest frequency.

      SSB: remove the carrier and one side band, so then your voice can get the full 4 watts (four times as much power), and it takes up half of the bandwidth. The only down side is that you have to tune carefully to get the sound to the right pitch, and the circuitry gets more complicated.

      1. Uh, no. You’re confusing received signal power to everything else causing SSB to sound the way it does. An easy example is that I put an AM broadcast kit next to 2 radios, a cheap transistor AM radio and an expensive SSB (trans/re)ceiver. Without some tuning effort, the SSB radio is guaranteed to sound worse because the cheap radio will lock onto the carrier, and the proximity means they both are receiving plenty of power.

        Again, if we talk received power versus bandwidth, then we go to why CW and low-bandwidth digital modes “sound better”, to use your words. At the other end, FM sounds (for real this time) better than any of these, but the large bandwidth means you need far more power than even full DSB-AM to achieve it.

    4. The stereo subcarrier on FM broadcast radio is double sideband suppressed carrier at 38 KHz. The signal also contains a pilot tone at 19 kHz. The pilot tone is used by the receiver to regenerate a signal with the exact correct frequency and phase to demodulate re left-minus-right signal in the subcarrier. By the way, I know that the most common circuits that are used in stereo demodulators do not appear to do this. They are mathematically and practically equivalent, however.

    5. Some hams still occasionally use the old AM. It is still perfectly legal. It is fun for some to use vintage equipment and modes. Most modern transceivers will generate AM with carrier, although at a rather low power output. I occasionally listen in. It sounds worse than SSB.

  1. Happy new year to all. My experience with an SSB signal received with a normal (i.e. not capable of SSB demodulation) radio, is like hearing a heavily distorted signal (as if rectified). I register the “Donald Duck” effect from a proper SSB radio demodulating the wrong sideband, where the audio spectrum is reversed. But maybe that’s just me.

  2. The video is quite good, but it contains a common error. Suppressing the carrier in an AM signal does improve signal-to-noise ratio for the same transmitter power. Suppressing one of the sidebands does not. Single sideband and double sideband suppressed carrier have the same signal-to-noise ration given the same power and noise density. SSB has other advantages such as improved spectrum utilization and the ability to avoid interfering (non-noise) signals.

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