DECT interference

[PeterMartinez]

Has anyone experienced interference, in the form of a loud 532Hz tone, heard on DECT cordless handsets during normal landline calls?  A traditional corded handset is clear.  Two different types of BT DECT phones show the problem at the subscriber's home (not mine) but not when used in his sister's home.  Someone suggested that a Redcare security system might be the culprit.  Does this use a piggy-back tone on the landline, and if so, at what frequency?  The subscriber doesn't have Redcare, but could there be an above-speech-band tone on the line (or an adjacent line) that could cause trouble to DECT handsets in this  way?

Peter M

[PeterMartinez]

I should have added that ADSL (or maybe VDSL) is also used on the landline in question.

Peter M

[Paul608085]

Out of interest, how do you know that the frequency of the tone is, (exactly), 532Hz?

[PeterMartinez]

Paul:

I got my pal to record the tone to a file and email that to me, and I measured the frequency of the tone on the file.  It may be +/- 1% or so.  I have a theory that something on the landline - perhaps a security system - is emitting a high frequency tone (near 8kHz) which is causing the DECT base unit to misbehave (ADC aliasing).  However, if I was right then I would have expected others would have experienced the same problem.

Peter M

[Paul608085]

@PeterMartinez 

re: "but could there be an above-speech-band tone on the line (or an adjacent line)"

532Hz is within the normal telephony speech band of approx 300 to 3400Hz.

You said in your OP that the tone was not audible on a corded phone - doesn't that therefore exclude the telephone line, (by induction from an adjacent one), as the source of the tone?

My next suggestion was going to be something generated in the switched-mode power supplies of the DECT phones, but then I saw that you said the problem does not occur when the same equipment is used at a different house.

My next suspicion would be something local on the mains supply at the house that is experiencing the problem, that is able to pass through the chokes in the power supply cables and in to the equipment. Proving that could prove difficult though - the only way that I can think of is if you, (they), could lash up to temporarily power the DECT base station from batteries for a few minutes to see if the tone is still there?

[PeterMartinez]

Paul:

My thought was that a tone around 8532 or 7468 Hz on the landine could be heard on the DECT handsets at 532Hz because the DECT base unit converts the landline audio to digital.  This effect is known to science!  A corded phone would not respond in this way, so that doesn't exclude the landline as a source.  I was wondering if this was a common problem and the source may have been identified by someone on this forum.  This doesn't seem to be the case so I will probably now abandon this line of enquiry.

For the moderators:  I ticked the "email me if someone replies" button, but no emails were received in respect of either reply.

Peter M