Extended parallel pulse code modulation of LEDs

We introduce an alternative to pulse width modulation (PWM) for LED intensity control called "Extended Parallel Pulse Code Modulation." Whereas PWM typically requires a microcontroller with a dedicated hardware PWM controller for each channel, we can easily implement pulse code modulation (PCM) in firmware. We show that the spectral content of PWM and PCM signals is equivalent, and so there is no disadvantage from an EMI perspective for circuit or cabling design. We next introduce a PCM-based algorithm that enables a single microcontroller to drive up to one hundred LED channels in real-time with 8- to 12-bit resolution. This parallel PCM technique is suitable, for example, for LED backlighting of video displays and LED-based theatrical lighting systems. Depending on the application, we can implement the algorithm in firmware or in hardware with a field-programmable gate array. A modification of the algorithm takes advantage of the characteristics of the Ferry-Porter law for visual flicker to reduce the modulation frequency. This extended parallel PCM technique relies on the principle of temporal dithering (adapted from digital audio techniques) to reduce quantization errors in the LED intensity signals.