What's the GENLOCK for?

[Steve House]

There's sync and then there's sync. Think of the frame as a printed page in a book. Sending timecode from one camera to another and/or to an audio recorder records the precise time to the frame from a common clock when each device recorded its data so they can be aligned again later. But remember that the video frames are rasters made up by scanning from one side to the other along successive lines down the page, just like we read text. Sending Genlock insures that each camera is looking at precisely the same word in the same line on each page at precisely the same time. When the master camera or the house clock says "Read line 1!" every camera in the group reads it exactly in unison. That way cuts, dissolves, etc between devices can happen seamlessly without jumps in the scan. If camera 1 is looking at line 5 at the same time camera 2 is looking at line 200 even if it's within the same frame, that frame would be jumbled when one mixes the scan from one camera with the other. Genlock makes sure when the master is scanning along line 5, ALL of the devices are scanning line 5 exactly in lockstep.

[A. J. deLange]

Steven,

If you genlock only then the time codes will be in sync in the sense that they run at the same rate i.e. each time one camera's TC increments by a frame the other ones' do as well. The time codes won't read the same however unless they are "jammed" to a common source. With the XL2 it was possible to use the remote control to bring two cameras to the same time code (but not possible to keep them locked). If the XL H1 has the same capability then two cameras can be brought to the same time code and kept together without any equipment other than a cable and the remote control. If, OTOH, it is desired to set a particular time code then an external device must be obtained to supply it.

I don't consider myself qualified to recommend any particular pieces of equipment as I'm not in the business and there are so many options available about which I know nothing other than what is on their web sites. I do use MOTU equipment to obtain word clock from an XL2 and that seems to work out for me but may be entirely unsuitable for your purposes. An XL H1 should plug nicely into this system but then again it should be possible to genlock an XL-H1 to an XL2 with just a video cable (no time code alignment without the other stuff)

[Steven Blye]

Thanks A.J. So, just the video out (composite?) from the XLH1 to the Genlock in on the 2nd XLH1would be enough to genlock?

However, the length of cable would affect a lag in synchronization? Therefore an outside source with cables of equal lengths would assure one of a solid sync?

Am I correct in my understanding?

[A. J. deLange]

Yes, that's correct on both counts.

WRT cabling - the sync will be solid in either case but with, for example, 100 feet of cable there will be a delay of about 200 nS which is about .004 line. Some equipment has adjustable delays of up to a line or 2 to compensate for cabling. Whether the XL H1 does or not I don't know. It's pretty clear that for short distances this isn't a problem.

[Steven Blye]

Thanks for the help A.J. and all.

[A. J. deLange]

I just looked at the video demo from the Resfest and one of the first things I spotted was a genlock adjustment menu item showing a negative number. This implies that the camera can be set to trigger either before sync or after thus allowing lots of flexibility with respect to sync cabling.

[Steven Blye]

That's very good news.