Audio feedback loops on HF

(often referred to as "XM", "Whales" or "The Backward Music Station")


Stations sending audio feedback loops on HF seem to fascinate a number of people and are a regular topic on the various radio-related internet newsgroups and
chat-forums. Due to the acoustic nature of these signals, they seem to be a constant source for rather uneducated and speculative theories about their origin and their
purpose. Some hobbyists think the source of these signals is some advanced voice encryption system, music played backwards or some other esoteric background.

This is of course pure nonsense, since these audio feedback loops are simply caused by keyed up HF-transmitters picking up audio from separate HF-receivers
or receiver control units that are located in the same radio room and tuned to the frequency of the radio transmitter. Keep in mind that this effect will only appear
on circuits that have separate TX and RX units.

The following screenshot will illustrate how this effect is caused.

In this example, a remote station sends continuous feedback loops on 6940 kHz USB. This is caused by the fact that the transmitter is tuned to 6940 kHz USB
and a separate receiver, colocated with this transmitter, is tuned to 6940 kHz USB as well, causing the transmitter to pick up audio from the receiver, thus creating
feedback loops and simultaneously retransmitting incoming audio from the RX frequency.

So how do we know the station is retransmitting audio?

If you look at the below screenshot, you will see an ALE signal, originally being transmitted on 6944 kHz (ISB = USB + LSB) and at the same time being retransmitted
by the feedback source on 6940 kHz USB. We know that the lower part of the ALE signal is a retransmission, because of the difference in signal strength between the
original USB signal and that of the retransmitted LSB-component of the ALE signal

The ALE station in the screenshot by the way, is a Saudi MIL circuit (callsign "JCI") that transmits on  6944 in ISB mode. In the below screenshot, the LSB component
of the original ALE signal is however covered by the much stronger retransmitted LSB component, coming in via the remote radio receiver tuned to 6940 kHz USB.
 

Another good example illustrating that the transmitter on 6940 kHz USB simply retransmits audio from a separate receiver, is the following
screenshot:

You can see that static crashes, hardly present at my location, are being amplified and retransmitted by the station on 6940 USB!
Click here to actually listen to the section of the audio file shown in the screenshot below


 

But how do we know that the remote receiver causing the feedback loop is tuned to 6940 kHz USB and not to 6944 kHz USB?

If you look at the following screenshot, you will see a STANAG 4285 signal being transmitted on 6943 kHz USB.
If the receiver was tuned to 6944 kHz USB, it would retransmit parts of the STANAG 4285 signal on 6940 kHz USB - but
as you can see below, this is not the case!

If you want to learn more about feedback loop effects, simply look up "Larsen effect" in your favourite search engine!

The Larsen effect can also be easily simulated at home - all you need is a simple two way radio and an additional receiver tuned to the
TX frequency of the radio. As soon as you press the PTT, you will get similar feedback loops (provided the receiver audio is set
correctly and the receiver is close enough to the microphone of the transmitter).

And last but not least - some additional feedback related audio samples can be found under the following link:
http://www.hfunderground.com/wiki/XM

Happy monitoring!

//Leif