7 ms·
The 60 Hz grid is why we have HDDs spinning at 7200 RPM. Because of 60 Hz, it was simpler to design AC electrical motors to spin at 3600 RPM or 7200 RPM (1x or
by mrb 2y ago
The 60 Hz grid is why we have HDDs spinning at 7200 RPM. Because of 60 Hz, it was simpler to design AC electrical motors to spin at 3600 RPM or 7200 RPM (1x or 2x factor). When DC motors were made, for compatibility with AC motors, they often spun at the same speed (3600 or 7200 RPM) even though there was no electrical reason to do so. When HDD manufacturers selected DC motors for their drives, they picked 3600 RPM models as they were very common. Then, for performance reasons, in the 1990s, 7200 became more common than 3600 eventually.
- hcarvalhoalves 2y agoAlso 5400rpm HDDs were common – that would be 1,5x factor.
- emmelaich 2y agoThere was a brief stop off at 5400 at long the way. Actually, still available. Good to have something quieter when performance requirements are not as high.
- kees99 2y ago5400 is (was?) very common RPM for laptop HDDs.
- Tempest1981 2y agoAnd apparently why we have LED car taillights that flicker annoyingly. If anyone here works at a car manufacturer, please try to get this fixed. Apparently not everyone notices.
- arcanemachiner 2y agoI see it, but only in my peripheral vision. Drives me nuts.
- spockz 2y agoOr for some reason in the rear view and side mirrors!
- Tempest1981 2y agoFor me, it's also noticeable when my eyes are sweeping from one object to another. Like you should be doing when driving. I guess that's somewhat peripheral, but not entirely. Find a Cadillac Escalade with the vertical LED lights, and give it a try. What's a good way to capture this on video? Or to measure the frequency?
- taneq 2y agoThat's a different thing. Your eyes automatically filter out the blurry signal during a saccade (ie. while they're doing a fast pan from one location to another.) Rapidly strobing lights (eg. LEDs being PWM'd) mess with this process and cause you to perceive the intermediate images as your eye moves, leading to some weird visual effects. This was a big thing in the early days of DMD projectors, where they projected sequential red/green/blue frames, and while it looked great when you stared at it, any time your eyes moved the whole image strobed with overlaid RGB fields. (more info: https://www.prodigitalweb.com/rainbow-effect/ https://www.prodigitalweb.com/rainbow-effect/)
- taneq 2y agoThis is because your cones (colour-specific 'pixels' in your retina) cluster around your fovea (high-resolution area around the center of focus) while the rods (black-and-white, bigger, dumber, but much more sensitive and therefore faster-responding 'pixels') fill the rest of your visual field. Rods respond fast enough to see the flicker but cones don't. You might also notice in light that's almost too dim to see, you can see things better if you look a bit to the side of them, for the same reason.
- hunter2_ 2y agoWhat's the cause? Alternators run at very high frequencies with good rectifiers, so I'm guessing the flicker is introduced by PWM dimming, but why would that be a low enough frequency to bother people? I'm sensitive to flicker myself, but only on the more extreme half of the spectrum. For example, half rectified LED drivers on 60 Hz AC drive me nuts, but full rectified (120 Hz) I very rarely notice. I don't notice any problem with car tail lights, except in the case of a video camera where the flicker and the frame rate are beating. The beating tends to be on the order of 10 Hz (just shooting from the hip here) so if frame rates are 30/60/120 then I guess the PWM frequency is something like 110 or 130 Hz?
- brandmeyer 2y ago> introduced by PWM dimming, but why would that be a low enough frequency to bother people? The human fovea has a much lower effective refresh rate than your peripheral vision. So you might notice the flickering of tail lights (and daytime running lights) seen out of the corner of your eye even though you can't notice when looking directly at them.
- hunter2_ 2y agoFor sure. I summarized my particular sensitivity too aggressively earlier, but I tend to see flicker straight on up to 65 Hz and peripherally up to 120 Hz if it's particularly egregious (i.e., long valleys) but usually up to something less. In any case, I've never noticed car tail lights flickering even peripherally, despite video artifacts revealing that they do flicker.
- SV_BubbleTime 2y agoAutomotive engineer here! I don’t do lights, so I have no idea! BUT… I kinda do. You want the lowest PWM frequency you can get away with. In this case, at the back of the car, furthest from the battery, you really don’t want a 8kHz PWM nor do you need it. It costs money to isolate the supply demand, so you don’t want a noisy field for no reason. The “good enough” frame rate is 60Hz static, not moving, no other flashing lights, not using a camera, etc. 60Hz or 60fps has issues though. If you expose a PWM LED to another flashing light or movement you get really bad imaging. Imagine you took an LED in your hand and shook it, at 60 Hz you will see snapshots of where the LED was as you’re shaking it. At 240Hz you will see a blur. Guess which is better for a vehicle? I figure most car LED taillights internal to their housing would be 200-1000Hz depending on factors but I haven’t ever measured. 200Hz PWM is a really common value. No need for Samaritan base-12 here. For halogen and incandescent, we use PWM, fun fact. Low Hz though! About 88Hz, depending on voltage. You might wonder why. We can get 98% of the light output with 85% of the required wire. It’s all about weight and cost. Although not many vehicles use this anymore.
- sathackr 2y agoIt's PWM dimming. I see it most on Cadillac SUVs. Drives me nuts too. They could design it not to be that low of a frequency but apparently someone thought that 40-50hz was imperceptible to humans and went with it
- lutorm 2y agoI find it most annoying when moving your eyes. Anyone will notice the flicker if their eyes aren't stationary since then each "flick" will end up on a different part of the retina. So, instead of a normal motion blur you see a dashed line which is hard to ignore.
- geor9e 2y agoMaybe this was obvious, but I will spell it out for slow people like myself: 60 Hz = 60 cycles per second = 3600 cycles per minute A simple 2 pole AC motor spins 1 rotation per cycle, so 3600 RPM. AC is a sine wave cycle of current. Current flows one way to attract it to one pole, the current flows the other way to attract it to the other pole. For a big mainframe disc drive, it sounds like the obvious choice. Why they stuck with it after switching to DC, who knows. Maybe they didn't want to redesign the controller circuits.
- SoftTalker 2y ago10k RPM also not too uncommon.
- deleted 2y ago[deleted]
- lutorm 2y agoDid European hard drives run at 3000 rpm then?
- jhoechtl 2y agoNo