Help Brian identify this 1970s era interval signal

[Mystery solved! Click here to read the update.]

Many thanks to SWLing Post contributor, Brian (W9IND), who writes:

I want to thank you for stirring a memory that I never thought I’d relive – even though it still doesn’t solve the mystery of what the heck I was listening to in the first place!

Back in the early 1970s, I was a teenage SWLer with a curiosity for the worldwide signals that emanated from the speaker of my shortwave radio. Bitten by the SWLing bug after stumbling across Radio Nederland’s Bonaire relay station, I spent many a happy hour twirling the dial in search of fresh game to hear and QSL.

But on one such occasion (I’m going to guess it was in 1971), I was surprised and fascinated by what sounded like a snake-charming horn playing notes at random. Stranger still, the transmission would seemingly go off the air for a couple of seconds and then return to play the strange melody again. I chalked it up to one of the countless beeps, hums and other electronic noises that often appeared on utility frequencies in those days.

I never recorded it, I never had a clue what it was, and I never heard it again.

Until recently. On the Shortwave Radio Audio Archive.

A couple of weeks ago, while looking for old shortwave interval signals from the 1970s, I saw a recording marked “Unidentified interval signal 1” listed right after the interval signals of Deutsche Welle and Radio Nederland.

“OK,” I thought. “Sounds like a challenge. Maybe I can even help figure out what it was.”

Then it played … and I almost fell off my chair! I literally sat with my mouth open as the long-lost sounds of the “snake-charming horn” played again. Could it indeed have been an interval signal – and if so, for what station?

I wanted to contact the person who recorded it, but then I learned the sad news that Mr. Greg Shoom is no longer with us.

So I remain mystified, probably forever. But it sure was fun hearing that weird recording again! Thanks for the memories.

Let’s see if an SWLing Post reader can help, Brian!

I know of at least a dozen readers who are experts on all that is interval signals, so hopefully someone can listen and ID this one.

I’ve embedded audio from this SRAA recording below. Note that the unidentified interval signal can be heard between time marks 1:27 – 2:07 in the following recording:

Can you positively ID this interval signal?  If so, please comment!

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33 thoughts on “Help Brian identify this 1970s era interval signal

  1. Henryk Florczy?ski

    Oooh, there are more interval signals – ok, I will help you tomorrow to identify – today we have here 2:00 am and I am going to sleep. Good night 🙂

  2. Bill Dakin

    The interval signal of bells at 0:44 approximately truly made me reminisce of the 70’s listening.

  3. J.Losada

    The interval signal from the time mark 1:27 belongs to the Radiotelephone PTT service from Havana, Cuba. You can also listen to that ID on the sound archive at

    1. Brian, W9IND

      Thanks for the ID, regardless of who submitted the correct answer first.

      That strange sound had puzzled me for nearly 50 years!

  4. Brian, W9IND

    Many thanks for all the comments so far. Maybe my initial hunch was correct: This isn’t an interval signal at all, but a utility station.

    Bob LaRose W6ACU makes an interesting observation: “Most people don ‘t seem to have followed the instructions about where it is (time wise) on the recording.”

    I wonder if my citation of the mystery station’s place on the recording — “1:27 – 2:07” — doesn’t line up with other people’s. That is, maybe when their computer goes to the 1:27-2:07 mark of the recording, they hear the interval signal for Deutsche Welle and/or Radio Nederland.

    So let me express it a different way: This is the third segment on the recording. If you start at the beginning of the recording, you will hear Deutsche Welle, then Radio Nederland, then … the mystery station, which sounds like a snake-charming horn.

    Hopefully this will clear up any confusion that may have resulted from other people’s computers not being in sync with mine. Again, I really appreciate the help in clearing up my 50-year-old mystery!

  5. Laurin Cavender

    I can’t remember the name of the station or what it was called, however this was a place holder for a transmission that was about to begin or was for some reason not going to be broadcast that day. There were many of this type they were meant to say this Frequency is in use or is about to be and no one should assume that this Frequency is available to use. Also it let listeners know that they could here the transmitter even if the broadcast program material was not going to be transmitted as usual. Laurin Cavender WB4IVG Monitoring since 1959.

  6. Bob LaRose W6ACU

    Most people don ‘t seem to have followed the instructions about where it is (time wise) on the recording. I agree that it was a Radiotelephone station (Maritime or Point to Point), not a broadcast station. I vaguely remember hearing it but don’t remember who it was (maybe I never knew?) A good resource for finding it is at which has a lot of the old utility interval signals (sometimes called Voice Mirrors). They were often used to tell the distant station that the circuit was still active and ready for the next call. Playing any kind of music was certainly the exception to the rule!

  7. Paul

    I wouldn’t be surprised if this was a numbers station, the gaps in between the repeats sounded as if the transmitter went off air which is the effect you would get if the transmission was in single side band.

  8. Barry Felstein

    I remember it well, but I have no idea what it was. I started listening to SW about 1962, when I was six and discovered a family friend’s SW radios and was hooked instantly.

  9. NL12446

    Merck toch hoe sterck Dutch song from Bergen op Zoom
    also used for multiple radiotunes and from RNW
    Radio Nederland Wereldomroep

  10. David Crawford

    The “International Radiotelephone Station, Havana, Cuba”. The tune was apparently run off of an analog computer of sorts, and evolved over time.

    1. Brian, W9IND

      Thanks for taking the time to provide the correct ID.

      I never thought I’d find out which station that bizarre sound belonged to.

    1. Thomas Post author

      Yes, RNW’s is a classic interval signal for sure! Brings back so many memories. The mystery interval signal starts at 1:27 in the recording.

  11. Tom Kent

    I remember this well! As I recall it wasn’t an interval signal from a regular broadcaster, but a tuning signal from a radiotelephone station. It would stop when a person was talking and resume when the person paused. I did most of my SWLing in the 1960s.

    1. cph

      It may have been AT&T. I remember hearing one of these “tunes” in 1977, not too long after I received my first shortwave radio. About a year later, I heard an actual telephone call over one of these frequencies (somewhere around 13.400 MHz, but my memory is hazy)


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