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When recording old analog synthesizers (different brands and models) I'm getting interference in the audio. This started suddenly about two years ago (I thought the synth I was using was malfunctioning, had it serviced, and even bought another identical one, only to find the problem persisting.)

I'm used to old synths picking up a lot of 50Hz (and multiples) mains hum, but this signal is exactly 1000 Hz and all multiples, up to above 20kHz. It's about 60dB below the signal level and 12dB above the noise floor (see image). It's not there if I unplug the synth, so it's not being picked up by the cable or my audio interface.

I'm guessing some other appliance is creating this interference, but when it started I hadn't bought any new audio or computer equipment or household appliance. It could be something my neighbours are using, but I'd like to have some idea of what to look for.

So, what kind of equipment or phenomenon could be causing interference at exactly 1000 Hz and multiples? Is this a typical problem? It seems like a strange frequency for any kind of wifi or other communications signal.

440Hz note with 1000Hz interference indicated in yellow

440Hz note with 1000Hz interference indicated in yellow

(In case this is relevant, the synths used are two different Korg MS-20's, a Korg MS-10 and a Yamaha CS-15, all between 35 and 40 years old; one of the MS-20's was recently serviced and had parts like capacitors replaced. Service manuals with circuit diagrams and part lists for the Korg MS-20 and MS-10 and the Yamaha CS-15 can be found online.)

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  • \$\begingroup\$ Is your mobile phone anywhere near it? Or a device with a Wi-Fi card? \$\endgroup\$ – Tom Carpenter Jul 4 '17 at 7:07
  • \$\begingroup\$ One more question, are you using a USB recording device? \$\endgroup\$ – Tom Carpenter Jul 4 '17 at 7:10
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    \$\begingroup\$ @TomCarpenter the interference seems to be present whatever appliances I have near or not, switched on or not, whether my fridge or heater are switched on or not, in the summer winter, day, night... The only thing that's always on is my computer, screen and external usb audio interface. But I was using those before the interference started. \$\endgroup\$ – Your Uncle Bob Jul 4 '17 at 7:10
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    \$\begingroup\$ "external USB audio interface" --> quite likely a culprit \$\endgroup\$ – Tom Carpenter Jul 4 '17 at 7:11
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    \$\begingroup\$ @analogsystemsrf I think Tom identified the source, though. It's too close of a fit. USB uses a precise 1 ms timer for its work. And I think Sam identified the reason why the right direction about why it might not have been present earlier but shows up later. \$\endgroup\$ – jonk Jul 4 '17 at 18:09

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