Forensic analysis of mains hum can be used to timestamp recordings!
...well, I found it intriguing!
...well, I found it intriguing!
Who knew?! A legitimate application of mains hum!
- Thread starter RandomGrinch
- Start date
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It was proposed during investigations of President Nixon's White House tapes, when his secretary had erased certain parts.
However, at the time, the tape recorders were driven by synchronous motors, geared to the mains frequency, so it didn't work.
However, at the time, the tape recorders were driven by synchronous motors, geared to the mains frequency, so it didn't work.
There are other uses, some early television receivers used it as a reference signal
One that I devised for the audio video system in a companys main board room, they wanted a 'Private' button on the touch screen preventing any sound or video leaving the room, sadly there was an AFILS which leaked into the corridor, I installed another loop on that wall powered by an ELV transformer and halfwave rectified at a level just a little higher than the AFILS.
One that I devised for the audio video system in a companys main board room, they wanted a 'Private' button on the touch screen preventing any sound or video leaving the room, sadly there was an AFILS which leaked into the corridor, I installed another loop on that wall powered by an ELV transformer and halfwave rectified at a level just a little higher than the AFILS.
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Forensic analysis of mains hum can be used to timestamp recordings!
...well, I found it intriguing!
Thanks and yes very interesting indeed. A few years ago I experimented with mains hum, just curious to know how far away from a mains supply - the hum could be detected. I wasn't able to find any distance where it couldn't be detected. Yes, I am very aware that the actual frequency wobbles a little depending upon the load on the generators. It goes down under load, rises under lighter loads, all you can be sure of is that it doesn't reduce or increase by very much and the average over a day is 50 cycles per second.
The average used to be more important than perhaps it is now, because many people had mostly mains synchronised clocks. We only have two at the moment, the microwave and the gas oven relying upon the mains frequency - everything else is MSF, Internet, broadcast, or satellite timing.
Yes, indeed
I built a Nixie Tube clock for my dad from some redundant lab equipment - there was lots of mains voltage on that little PCB!
Last edited:
Yes, indeed
I built a Nixie Tube clock for my dad from some redundant lab equipment - there was lots of mains voltage on that little PCB!
I built one of those, I ended up having to nip down to London for the day, just to buy the components. My first attempt to build digital, pre-digital components being available - used a clock motor, geared and driving rotary 10 and 12 way switches - it was a total complete, miserable failure
I love the design on that case.
Thank you, I was quite happy with it.
The wood was an offcut that was left lying around.
The lid was something similar to a Ferrero Rocher case!
...and it was securely glued on, when I was sure it worked!
clock
Thanks! That reminds me, I've been meaning to have a go at this for a while! :
...I also once wrote a very simple Python script to count up the voltage and current output of a programmable PSU - it was quite an OTT digital clock!
Thank you, I was quite happy with it.
The wood was an offcut that was left lying around.
The lid was something similar to a Ferrero Rocher case!
...and it was securely glued on, when I was sure it worked!
Nixie and similar display tubes have quite a retro following.
D'oh!
Find that a little strange as I'd have thought the inertial 'drag' of the motors would have had little influence on the voice recording variation
One of us has misunderstood...
The motor/recording runs at a consistent speed, what varies is the precise frequency of the hum from the mains supply. The hum is picked out of the recording by filters, to isolate it, then fine variations in speed can be compared against already known and filed variations, rather like comparing a fingerprint.
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