3 ms·

I'm surprised when it says "the exact same buzz can be heard at the exact same moment nationwide from Land's End to John o' Groats", I didn't realise it would b

I'm surprised when it says "the exact same buzz can be heard at the exact same moment nationwide from Land's End to John o' Groats", I didn't realise it would be the same across the UK. I played a little with a transformer to take 230VAC -> 12VAC, and then using a potentiometer to feed the waveform into a soundcard. But I'm not sure the sample rate of the soundcard was high enough to accurately measure the times between 0 crossings. Photo of the very messy setup - https://www.anfractuosity.com/files/hum.jpg https://www.anfractuosity.com/files/hum.jpg

sgtnoodle 4y ago
Explicitly watching for zero crossing seems like a rather limiting approach to quantifying a sine wave. Even then, at 48Khz you should get 400 samples between zero crossings, so you should do better than 0.5% accuracy. There's a lot of information held within the time series data of an audio signal, and something like an FFT can tap into it. For a 60Hz signal the Nyquist frequency is 120Hz, which is well below any audio interface. It seems like accuracy of any processing would be limited by a crystal oscillator somewhere more than anything, and perhaps any unintentional signal aliasing if there's no low pass filter before the audio interface's ADC.
- anfractuosity 4y ago
  That's a good point re. the crystal oscillator. Maybe I'm wrong re. the sample rate then, I found this paper - http://www.forensic.to/ENF%20processed.pdf http://www.forensic.to/ENF%20processed.pdf , where they mentioned 8kHz as the sample rate. I think what I was thinking is commercial systems such as https://synectic.co.uk/product/sd037-frequency-monitor-html/ https://synectic.co.uk/product/sd037-frequency-monitor-html/ mention specs such as 'The frequency monitor units are supplied pre-calibrated to an accuracy of +/-0.0001Hz'
  - sgtnoodle 4y ago
    I was mainly responding to the hobby experiment of connecting a 12V transformer to an audio input on a computer. Even then, it seems like 8Khz sampling would be plenty to fingerprint a 60Hz signal. 8Khz sampling can capture information up to 4Khz, per the Nyquist frequency. That frequency monitor spec sheet mentions 90ms response and filtering, which would imply it averages information across 5 periods. Also, because it's a purpose built device, it's likely not periodically sampling the waveform with an ADC, but rather has a dedicated zero-crossing detection circuit that feeds into a capture input on a digital timer with at least microsecond resolution.
- im3w1l 4y ago
  Well 60/50Hz isn't the only relevant frequency here. Lets say you plug in ethernet-over-power (available off the shelf). You'd need a very fast interface to notice that.
  - sgtnoodle 4y ago
    I can't imagine folks' ethernet-over-power signals are propagating across the national power grid, and even if they were, they aren't getting picked up in any audio streams.
myself248 4y ago
> I didn't realise it would be the same across the UK. It has to be, that's the definition of a power grid. The relative phase can have minute variations and you get massive power flows as a result, which result in the frequency across the whole grid being the same. I believe the whole UK is a single grid. The US is divided into three: The eastern interconnect, the western interconnect, and Texas. (Insert meme here.) There are a few DC-links that allow power to flow between them, but due to the expense of rectifying and inverting all that power (necessary for the frequency and phase to vary), they are rare. It's much cheaper to connect directly at AC, even though this requires that everyone on the system run at the same frequency. Amusingly, this also means that if you could (theoretically arrange enough long shafts and bearings and neglect the curvature of the earth and) orient all the generators on the grid in a line, a single shaft could run through all of them, and the shaft would not twist -- they all spin in precise synchrony since they are, effectively, a single machine. When a new generator is brought on-line, it has to be phase-matched to the grid. These days there are probably fancy PLCs to handle it, but the simplest instrument is called a synchroscope, which is a motor driven by the phase relationship between two AC waves. You can find these on YouTube, they're pretty neat. Basically if the isolated generator is running slower than the grid, the synchroscope needle spins one way, and if it's running faster, the needle spins the other way. You adjust the throttle on the generator until it's running _ever so slightly_ faster, like 60.00Hz on the grid and 60.10Hz on the isolated generator, (50.xx in the UK) which will make the 'scope revolve once per ten seconds. Then you wait until the needle is pointing straight up which indicates zero phase shift, and slam the switches to connect it to the grid, and that's that! If you did it just right, there's a minimal shudder and the generator now begins sending power into the grid, its frequency constrained to 60.00Hz and the extra throttle showing up as positive power flow. If you did it wrong, there's a tremendous amount of torque exerted on the rotating machinery by the grid power, which may simply force it into compliance, or in more extreme cases, may shred millions of dollars of equipment in an instant. Bigger synch setups have lockouts that won't even allow the button to be pressed unless the phase is _pretty close_. The most colossal kind of cockup is when someone wired the synchroscope wrong, so the rotation still works (it has to, that's impossible to screw up) but in-phase would be represented by some needle angle other than straight up. Joining the grid 120 degrees out would be a career-limiting event for someone.
gwbas1c 4y ago
Basic Fourier theory from the 1800s explains that your sound card is good enough to record the changes in frequency. Chances are internally it uses a much higher sampling rate and digitally downsamples.

sgtnoodle 4y ago

Explicitly watching for zero crossing seems like a rather limiting approach to quantifying a sine wave. Even then, at 48Khz you should get 400 samples between zero crossings, so you should do better than 0.5% accuracy. There's a lot of information held within the time series data of an audio signal, and something like an FFT can tap into it. For a 60Hz signal the Nyquist frequency is 120Hz, which is well below any audio interface. It seems like accuracy of any processing would be limited by a crystal oscillator somewhere more than anything, and perhaps any unintentional signal aliasing if there's no low pass filter before the audio interface's ADC.

anfractuosity 4y ago
That's a good point re. the crystal oscillator. Maybe I'm wrong re. the sample rate then, I found this paper - http://www.forensic.to/ENF%20processed.pdf http://www.forensic.to/ENF%20processed.pdf , where they mentioned 8kHz as the sample rate. I think what I was thinking is commercial systems such as https://synectic.co.uk/product/sd037-frequency-monitor-html/ https://synectic.co.uk/product/sd037-frequency-monitor-html/ mention specs such as 'The frequency monitor units are supplied pre-calibrated to an accuracy of +/-0.0001Hz'
- sgtnoodle 4y ago
  I was mainly responding to the hobby experiment of connecting a 12V transformer to an audio input on a computer. Even then, it seems like 8Khz sampling would be plenty to fingerprint a 60Hz signal. 8Khz sampling can capture information up to 4Khz, per the Nyquist frequency. That frequency monitor spec sheet mentions 90ms response and filtering, which would imply it averages information across 5 periods. Also, because it's a purpose built device, it's likely not periodically sampling the waveform with an ADC, but rather has a dedicated zero-crossing detection circuit that feeds into a capture input on a digital timer with at least microsecond resolution.
im3w1l 4y ago
Well 60/50Hz isn't the only relevant frequency here. Lets say you plug in ethernet-over-power (available off the shelf). You'd need a very fast interface to notice that.
- sgtnoodle 4y ago
  I can't imagine folks' ethernet-over-power signals are propagating across the national power grid, and even if they were, they aren't getting picked up in any audio streams.

myself248 4y ago

> I didn't realise it would be the same across the UK. It has to be, that's the definition of a power grid. The relative phase can have minute variations and you get massive power flows as a result, which result in the frequency across the whole grid being the same. I believe the whole UK is a single grid. The US is divided into three: The eastern interconnect, the western interconnect, and Texas. (Insert meme here.) There are a few DC-links that allow power to flow between them, but due to the expense of rectifying and inverting all that power (necessary for the frequency and phase to vary), they are rare. It's much cheaper to connect directly at AC, even though this requires that everyone on the system run at the same frequency. Amusingly, this also means that if you could (theoretically arrange enough long shafts and bearings and neglect the curvature of the earth and) orient all the generators on the grid in a line, a single shaft could run through all of them, and the shaft would not twist -- they all spin in precise synchrony since they are, effectively, a single machine. When a new generator is brought on-line, it has to be phase-matched to the grid. These days there are probably fancy PLCs to handle it, but the simplest instrument is called a synchroscope, which is a motor driven by the phase relationship between two AC waves. You can find these on YouTube, they're pretty neat. Basically if the isolated generator is running slower than the grid, the synchroscope needle spins one way, and if it's running faster, the needle spins the other way. You adjust the throttle on the generator until it's running _ever so slightly_ faster, like 60.00Hz on the grid and 60.10Hz on the isolated generator, (50.xx in the UK) which will make the 'scope revolve once per ten seconds. Then you wait until the needle is pointing straight up which indicates zero phase shift, and slam the switches to connect it to the grid, and that's that! If you did it just right, there's a minimal shudder and the generator now begins sending power into the grid, its frequency constrained to 60.00Hz and the extra throttle showing up as positive power flow. If you did it wrong, there's a tremendous amount of torque exerted on the rotating machinery by the grid power, which may simply force it into compliance, or in more extreme cases, may shred millions of dollars of equipment in an instant. Bigger synch setups have lockouts that won't even allow the button to be pressed unless the phase is _pretty close_. The most colossal kind of cockup is when someone wired the synchroscope wrong, so the rotation still works (it has to, that's impossible to screw up) but in-phase would be represented by some needle angle other than straight up. Joining the grid 120 degrees out would be a career-limiting event for someone.

gwbas1c 4y ago

Basic Fourier theory from the 1800s explains that your sound card is good enough to record the changes in frequency. Chances are internally it uses a much higher sampling rate and digitally downsamples.