Max guidesA technician must decide whether camera alias at 30 fps is safe before changing led pwm frequency on the real device. The result is unresolved until the rule and units are checked. Predict the direction first.
See the relationship before changing it
The figure reads from left to right. The blue card is led pwm frequency. The middle card applies this page's rule. The green card is camera alias at 30 fps. Walk the arrows once: set the input, apply the rule, then read the result with its unit.
The retained audit below checks several chapter fixtures. This model keeps those stated values fixed and changes only led pwm frequency, so the numeric fixture does not switch without explanation.
Derive the baseline in four named moves
- 1
Name the input. The chapter baseline is 5000 Hz.
- 2
Name the relationship. alias = |PWM frequency - 30 x round(PWM frequency / 30)|
- 3
Substitute with units. |5,000 - 30 x round(5,000 / 30)| = 10 Hz
- 4
Read the result. Keep the unit beside the value. Use it only inside the technical boundary on this page.
Predict, then change led pwm frequency
Try Predict the direction of alias = |PWM frequency - 30 x round(PWM frequency / 30)|. Test another led pwm frequency, then compare camera alias at 30 fps.
Observe A small PWM-frequency change can move the slow beat seen by a fixed camera. Reset led pwm frequency to 5000 and compare camera alias at 30 fps.
Explain A small PWM-frequency change can move the slow beat seen by a fixed camera.
Check yourself
What should you do before trusting a moved-control result?
What does this small model leave out?
1. The eye and camera are different sensors
The naked eye can average fast PWM into steady brightness. A camera samples in time and can fold the same switching into a slow beat.
2. Quantise brightness
Count intervalsThe chapter uses 255 full-scale code intervals.
Divide the 5 V spanq=5/255=19.6 mV.
Compare perception0.392% is about 2.5× finer than a 1% illustrative Weber fraction.
3. Fold the switching frequency
Subtract the nearest whole number of camera samples. The leftover is the slow beat that can appear as visible banding.
4. Try the camera frame rate
TryMove the nominal camera frame rate while the LED remains at the chapter's 5 kHz PWM.
ObserveThe chapter's 5 kHz PWM aliases to 8 Hz at 24 fps, 0 Hz at 25 fps, 10 Hz at 30 fps, and 20 Hz at 60 fps.
ExplainThe same nearest-sample subtraction drives every beat result. Brightness resolution stays separate because its formula uses voltage and code count, not frame rate.
A frame-rate model does not reproduce rolling-shutter line timing, exposure duration, PWM edge shape, LED/driver dynamics, camera auto-exposure, display processing, or human flicker sensitivity.
- Test the target camera and lighting geometry
- Needs separate evidence
Use field evidence or a deeper model before release.
5. Interpret zero carefully
At an exact 25 fps, 5,000/25 is an integer and the frame-rate beat is zero. That does not guarantee a rolling-shutter image is band-free.
6. Design for the observer
Record PWM frequency, duty, camera frame and line timing, exposure, brightness, geometry, and whether banding appears. Change frequency or add filtering against the actual target.
7. Check yourself
Why does 24 fps produce 8 Hz?
Does 8-bit PWM guarantee a smooth-looking fade?
Does a zero frame-rate alias prove no banding?
These are the chapter inputs, worked results, and named teaching assumptions.
- 8-bit
- Digital resolution or converter setting
- 5 V
- Voltage or voltage-step value
- 19.6 mV
- Voltage or voltage-step value
- 5.66 mV
- Voltage or voltage-step value
- 49.9 dB
- Gain, loss, margin, or level ratio
- 0.392%
- Percentage, ratio, or gain
- 1%
- Percentage, ratio, or gain
- 5 kHz
- Frequency, sample rate, or event rate
- 24/25/30/60 fps
- Frequency, sample rate, or event rate
They do not certify a particular camera or viewer; Under the Hood keeps those limits.