The YN968EX-RT uses the Canon RT radio protocol. The Yongnuo YN968EX-RT is not compatible with the YN622C protocol. It's also not compatible with the different YN560/RF603/RF605 protocol used by Yongnuo for manual only flashes.
Your confusion about the YN968EX-RT compatibility with the YN622C-TX probably stems from the fact that the Nikon version of the YN968, the YN968N, is compatible with the Nikon version of the YN622 transmitter, the YN622N-TX. Notice that the Nikon version of the YN968 has no "RT" in the name.
To control the YN968EX-RT wirelessly directly via the flash's built-in radio receiver from your Canon EOS 70D camera, you need an RT compatible radio transmitter attached to the camera's hot shoe. These would include:
- Yongnuo YN-E3-RT radio transmitter
- Canon ST-E3-RT radio transmitter
- Yongnuo YN968EX-RT flash
- Yongnuo YN600EX-RT flash
- Yongnuo YN600EX-RT II flash
- Canon 600EX-RT flash
- Canon 600EX II-RT flash
To control the YN968EX-RT flash wirelessly via Canon's optical wireless system you need to set the flash to wireless optical mode and use a wireless optical master on the camera. These would include:
- The 70D's popup flash
- Canon Speedlite Transmitter ST-E2
- Yongnuo ST-E2 transmitter (clone of Canon ST-E2)
- Canon flashes with optical Master capability (580EX, 580EX II, YN600EX-RT, YN600EX-RT II)
- Third party flashes with Canon optical Master capability (YN568EX II, YN600EX-RT, etc.)
If you want to use the YN968EX-RT flash with the YN622C-TX transmitter via radio, you need to use a YN622C or YN622C II receiver attached to the flash's hot foot.
The major differences between an optical communication system, such as that used by all Canon master flashes prior to the 600EX-RT, and a radio communication system such as that used by most wireless triggers:
- Radio tends to have a greater range than optical, particularly when using a relatively weak pop-up flash for optical control
- Radio doesn't require line-of-sight like optical does
- Radio can operate in very bright environments such as direct sunlight (that gives the optical system a tough time)
- Radio has the ability for more than one set of the same type to be used in proximity to one another without interfering with each other
When using a YN622C-TX transmitter with the only currently available flash having a YN622C compatible radio receiver (YN685) or with any Canon TTL capable flash attached to a YN622C or YN622C II receiver the following applies:
First off, the camera does not need to be in "wireless" mode. As far as the camera is concerned, the YN622C-TX attached to the camera's hot shoe is the master flash. The "wireless" setting in the camera's menu is for when you want to use the camera's popup flash or a compatible Master flash mounted on the hot shoe to wirelessly control off camera flashes using Canon's optical wireless system. The YN622 system uses radio to control off camera flashes.
That being the case, you can't set the channel from your camera's menu. Instead, you need to set the channel from the YN622C-TX menu. You then need to set the channel of the receiver in (or attached to) the flash, using the flash's (or receiver's) control panel, to the same channel number as the one selected for the YN622C-TX. If it has a YN622C compatible radio receiver built-in, the flash needs to be set to "YN622" receiver mode. If you are using a YN622C/YN622C II receiver attached to the flash's hot foot, the flash should not be set to a wireless mode as that will cause it to ignore any signals from the hot shoe of the attached receiver. Instead, set the flash as if it were directly attached to a camera's hot shoe. As far as the flash in concerned, the YN622C/YN622C II receiver is the camera.
You also need to be sure the group the flash is set to is the same as the group you are making changes to using the YN622C-TX or the camera's menu. If you are only using one flash, it's easiest to use "Group A".
Beyond that, the settings for the flash need to be set to "neutral" or "default" values if you want to control a particular parameter from the YN622C-TX or your camera's menu.
- If using manual flash power, the power should initially be set on the flash's control panel to full power (1:1)
- If using TTL, flash exposure compensation should initially be set on the flash's control panel to "0" (zero)
- Flash zoom head setting should be set to "Auto"
- "First curtain" sync should be selected
If any other values are set on the flash's control panel, those values will override any settings on the transmitter or camera's menu for the parameter not set to the default value.
The entire point of using the YN622C-TX is to control everything from it, rather than from the camera's menu. I've found it much easier to use the YN622C-TX to make changes and control various groups than to use the camera's menu, especially when using more than one flash.