Light Emitting Diodes (LEDs) are inherently current driven, rather than voltage driven. A side effect of this difference is that linear light dimming requires square-law input current. All good LED dimmers use ISL (“I” means inverse, starting to rise slowly, getting faster as it progresses).

Almost all LED dimmers, including RC4, are “constant voltage” dimmers. This means that a conversion from voltage to current must happen in the LED device you are connecting to. LED tape and most products that use many discrete LEDs on a panel use small current limiting resistors for each individual LED color. This creates the best looking dimmer output, but it is not the most efficient — it works perfectly all the way down to zero. The downside is that resistors generate heat, which is wasted energy.

Larger sized LED fixtures, especially ones designed for maximum efficiency, use sophisticated active electronics built into the panel for current limiting. Those devices require a minimum amount of power for the electronics to turn on, so they have a drop-out margin at the bottom end.  If your LED panel doesn’t come on until dimming hits 10%, or 30%, or some other minimum level, you are dealing with active electronics in the panel. In many cases, these devices respond better to a Linear control curve. The electronics in the panel are doing the actual dimming, and they are sometimes a little steppier than a resistor-limited “direct drive” panel. That steppiness is not caused by the RC4 dimmer, it’s the driver electronics in the panel itself.

** More about the 16-bit Raw curves in the Smoothing section below. **

In Short: if there is no active electronics in the LED fixture, you should use the ISL curve. ISL is the RC4 LumenDim default curve. This usually produces the smoothest dimming results, with a linear relationship between the DMX control level and the light output (50% DMX level = approx. 50% of maximum light output). But if the panel has active electronics in it, try the Linear curve. The dimming quality is determined by the fixture, the RC4 device just tells it what light level is wanted.

Experiment to find the settings you prefer. There are no defacto right answers for everyone in every situation.

When using LED fixtures that do not use active electronics (ideally they will use small current-limiting resistors on every LED color throughout the panel or strip), RC4 dimmers deliver pulse-width-modulated (PWM) DC power for dimming. At slower PWM frequencies you get more steps, which results in tinier increments and a smoother look. But slower frequencies can “beat” against the shutter speed of a video camera, so it’s important that the PWM frequency is high enough to avoid that. The shutter angle also impacts this, because it limits how long the shutter is open and how many PWM pulses are visible during the open shutter time — ideally, you want several PWM pulses to occur during the open period of a single frame capture.

At 77Hz, which is 77 pulses (or “updates”) per second, RC4 dimmers are running 19-bit, which provides more than 524 millions steps of dimming. At 20kHz (20,000 pulses per second) you get 14-bit, which is only 16,000 steps. At that point, your human eye can see the steps. At 13-bit it’s half

that, only 8000 steps.

There is one more trade-off to consider: When there are more steps per second, the dimmer is do a lot more switching on and off to make the pulses. Each switching action reduces efficiency, which reduces overall power handling and increases heating in the dimmer electronics. This is another reason to use the lowest possible frequency. It’s also important to know that that maximum load you can connect to a dimmer is less when running higher PWM frequencies.

Use the lowest frequency that does not result in visible artifacts on camera. The only way to get this right is through trial-and-error. For normal frame rates in typical production lighting, a 5kHz PWM frequency is the usual selection, and that’s the RC4 LumenDim default.

A single channel of DMX control is 8-bit, which is only 256 steps of dimming. To make use of all the additional resolution in an RC4 dimmer, smoothing is used. This is not just an RC4 thing, pretty much every good dimmer does something of this nature. However, we’ve put a lot of extra effort into emulating what an incandescent lamp filament does — the thermal rise and fall, emulating what happens when the little filament wire starts cold, warms up to glow, ever brighter, until it’s bright white. All of that happens over time, and it’s not a strictly linear progression. The larger the filament (for higher wattage incandescent lamps), the slower the rise and fall becomes. We call this RC4 Digital Persistence.

When RC4 Digital Persistence (smoothing) is off, there is no bit-rate conversion happening, so you only get 256 steps when controlling from a single DMX channel. It’s very visibly steppy, because that’s what the console signal is, with nothing in between — no smoothing.

At Medium Digital Persistence, the timing is very similar to a 50W MR16 Halogen lamp. That’s generally considered a good look, and it’s the RC4 LumenDim default. At Very Slow Digital Persistence, it’s the appearance of a very very large filament lamp.

The smoothing process makes full use of the RC4 hardware dimmer’s real resolution. Thus, when you use an 8-bit DMX channel to control an LED using 5kHz Modulation Frequency, the Smoothing is ramping through 65,536 levels, which looks pretty good.

Most people use one of the middle settings for normal operation. But if you need the look of a fast photo flash, or a strobe light, faster smoothing (or off) will be preferred.

Again, experimentation is the only way to find the right setting for your application.

When using an LED fixture that uses internal electronics, it  handles the dimming. The maximum resolution is determined by it, not the RC4 settings. Sending RC4 super high-res PWM to a device like that will not improve the appearance, because it can only do whatever it is designed internally to do.

When using one of the 16-bit Raw dimmer curves, smoothing is disabled and response is linear. In that case, you are using two DMX channels ganged together for true 16-bit DMX control. That control is mapped to the RC4 dimmer hardware directly. In that case, you must do the curve processing and smoothing in your console — you have bypassed that later of processing in the RC4 dimmer. In general, using RC4 Digital Persistence in the RC4 dimmer will result in smoother dimming, because our internal dimmer resolution can be higher than 16-bit (depending on the Modulation Frequency being used), and it is optimized for dimming LEDs. Features in the console are generic math algorithms with no knowledge of what’s happening further up the line.

Recommended defaults for LED control to get started are:
ISL curve
5kHz PWM
Medium Smoothing

For an LED fixture with built-in electronics, you might want to start with:
Linear curve
77Hz PWM
Smoothing OFF or Medium

There are no perfect or ideal settings, every fixture and application is different. Experiment and fine-tune for the best result. Don’t just use your eyes, do test shots on camera to determine the thresholds for visible artifacts like banding and flickering, increasing the PWM frequency to find the lowest freq where there are no artifacts. That is the sweet spot that gives you the smoothing dimming with the highest power handling.