原创 Nikon CLS Advanced Wireless Lighting, Part I

Nikon Creative Lighting System (CLS) is a complete lighting solution with the latest Nikon Speedlight flashes. The system consists of the following core functions: i-TTL Balanced Fill-Flash, Flash Value Lock, Auto FP High-Speed Sync, Advanced Wireless Lighting, Multi-Area AF-Assist Illuminator, and Flash Color Information Communication. For a quick descriptions of each functions and CLS basic capabilities, please check out this Nikon support link.

Advanced Wireless Lighting allows photographers to control their Nikon Speedlight flashes from a distance without wires. Compared to other wireless trigger or optical slaves, the Nikon system make it very easy to control up to three remote flash groups that can have unlimited number of flashes per group. For each group, you can remotely setup the flash in TTL, Auto Aperture (AA), Manual modes, or turn it off completely without the need of walking up to the flash or running any wires.

Nikon makes great camera systems but often failed to explain the technologies behind them to the public. For the Advanced Wireless Lighting system, Nikon offers some vague description as usual. The following was from the Nikon support link.

(Advanced Wireless Lighting is the) method of controlling multiple wireless flash units. The Master sends out a very small light pulses to provide the Remotes and they communicate back subject exposure readings before the main exposure.

Are you satisfied with this? I was not. So I tried to find out more about Nikon CLS Advanced Wireless Lighting. While I was doing searches on Google, I found quite a few interesting posts in which people have tried to capture the sequences of wireless interactions between the master flash unit and remote flash unit. In this Nikonian post (free registration required), Hal Byron Becker tried to capture the flash sequences by turning the time domain signal into space domain signal. In this article, Alson van der Meulen tried to record the communication sequence of Nikon SB-800 using an oscilloscope.

Thanks for a good tip here (see the comment section), I was able to setup a cheap but effective optical signal capturing system using a $0.87 photodiode, a RadioShack cable with 3.5mm stereo audio plug, a desktop computer with a on-board Realtek sound chip (other sound cards should work), and a free audio editor and recorder software Audacity. The following is the picture of the photodiode soldered to one end of the audio cable. If you want to try this yourself, you need to make sure the polarity is correct.

The plug goes into the microphone jack on the computer. Using sampling rate of 96000Hz (44100Hz works OK), I was able to record optical pulse communication initiated by the master flash unit. The following screen capture shows some example recordings I made using a Nikon D200 and its on-board flash in Commander mode. I set the master to off (“–”), Group A to Manual 1/128, and varied the Channel from 1 to 4. There was no actual remote unit.

You can clearly see three distinct groups of pulses. This is the zoomed-in view of the first group of pulses.

This is the zoomed-in view of the 2nd group of pulses. I aligned the pulses on the left so it is easier to see the differences.

In both pulses groups, there are differences only for the first three pulses. For each test, the first three pulses for the 1st and 2nd pulse groups are the same. Since all I changed was the channel number, it makes sense to assume what the first three pulses are used to indicate channel number so only remote flashes in the same channel would respond to the information sent in the subsequent pulses.

To find out more about the coding scheme, I did some more tests. In the first test, I set Master to off, Group A to M: 1/64. In the second test, I set Master to off, Group A to M: 1/128, Group B to M: 1/128. In the third test, I set Master to off, Group A to M: 1/128, Group B to 1/64. All cases used Ch. 4. The following shows the comparison of these tests and the previous single group test case (Master to off, Group A to M: 1/128, Group B to off, Ch. 4).

From the first pulse group, you can see 3 more new pulses that are simply duplicated when Group B is added in manual mode. Changing the flash output level does not cause change to the pulses in the first pulse group.

From the second pulse group, you can see more pulses when Group B is added with the same output of 1/128 as Group A. Changing output level to 1/64 caused the change to the last a few pulses in the 2nd pulse group.

In all the tests above, the 3rd pulse groups at the far end is the same: just a single pulse that is used to trigger the final flash emission from the remote unit. When there is an actual remote unit, the trigger pulse is indistinguishable from the final flash emission from the remote flash.

Without getting too much into the detailed schemes of communication, we can be pretty certain about how this simplest case of Advanced Wireless Lighting works. The first bunch of pulses has the channel indicator and some information about remote groups but does not have anything about the amount of flash output. The second bunch of pulses has the same channel indicator and carries the information for the amount of flash output for all remote groups. The third pulse group (just a single pulse) is the trigger pulse.