Genlock

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Contents

Overview

Genlock settings

Genlock is a multi-camera arrangement where a master camera provides timing signals to one or more slave cameras. When capturing video, the master and all the slaves are all acquiring frames using the same start-of-exposure signal. Genlock allows you to capture frames from different cameras with the shutter's opening (start of exposure) to being within +/- 1 uS of each other (depending on cabling and the number of slave cameras).

The timing signals consist of the trigger indication and the start of exposure signal. The timing signals are shared among the cameras using genlock cabling which creates a simple three wire daisy chain via the external trigger connector.

For each frame that is captured the cameras go through three phases: exposure, frame data readout and idle. As long as the slave camera has a positive idle time the slave will stay in genlock with the master. During the exposure phase and frame data readout phase a slave camera will ignore start-of-exposure signals from the master.

There are limitations:

  • All cameras have to be configured for compatible timing (see below). For one-to-one frame lock, each slave has to be set to a frame rate that is greater than or equal to the master's frame rate.
  • The user is responsible to make sure all cameras can be triggered before issuing a trigger. There is no automated check that verifies whether or not a slave camera has completed saving the previously captured video, finished the calibrate operation, has available storage, etc.

When using genlock the cameras can have different duration, iso, percentage pre-trigger, and shutter speed settings.

Initial configuration

Identify one camera to be the master; it doesn't matter which one. Browse to each camera and configure each camera with identical settings, except one is identified as the master while the rest are identified as slaves. Look at the LEDs on all cameras to verify all cameras are in the run state (solid green camera LED). A blinking red camera LED indicates that the slave isn't receiving a genlock signal.

Once the cameras are wired, configured, and triggerable, go ahead and trigger the master camera to verify the slave cameras are responding to the trigger. During capture, look at the camera LED on each camera and verify none of them are blinking white. A blinking red/white camera LED indicates that camera was not able to maintain genlock. Check each captured video to verify the results are what you expected.

Customize camera settings

Browse to each slave camera and adjust the settings. There are setting limitations:

  • For one-to-one frame lock, each slave has to be set to frame rate that is greater than or equal to the master's frame rate.

Once all the camera settings are adjusted, do another trial video capture to verify the timing is compatible. The metadata file will indicate if each slave camera was able to maintain genlock.

Metadata file

There are two genlock related settings in each captured video metadata file.

Key Value Meaning
Genlock Off
Master
Slave
External
Camera genlock setting.
Genlock locked True
False
None
Indicates if the slave camera was able to maintain genlock throughout the video capture. Set to NA if camera is not configured as a genlock slave.

There are several metadata file keys whose meaning can be effected when genlock is enabled:

Key Meaning
Frame rate The master is slowed down slightly to make sure it doesn’t over-run the slave. All that is required is the master period must be greater than or equal to the min period that the slave can run at given the slave's allowed settings. This includes a little fudge factor for clock frequency error and jitter due to cabling.
Frame count The slave may capture more frames than the master, even with identical settings, because of the frame rate adjustment that ensures both cameras stay synchronized.
Trigger delay For the slave camera, trigger delay is meaningless. Trigger delay is reported correctly for when the camera is configured for external genlock.

Camera Settings

The genlock setting is stored on the camera so the value is used the next time you power on the camera. The value is also stored in the metadata file. The possible Genlock values are described below:

Key Value Meaning
Genlock Off Camera will respond to trigger events as normal and generate its own start-of-exposure timing signal.
Master Camera will provide both the genlock trigger and genlock start-of-exposure signals on the External trigger connector.
Slave Camera will get trigger and start-of-exposure from the External trigger connector.
External An external timing source is providing the start-of-exposure signal. The external trigger signal can be used to trigger the camera when configured for external genlock.

Genlock status reporting

Only the genlock slave will report genlock status information, specifically genlock slave timing error. If the slave camera is in genlock, then normal camera status information is provided.

LEDs

When a slave camera is unable to maintain genlock the camera LED will blink red/white. Once the slave camera is maintaining genlock, the camera LED will stop blinking white after a five second timeout. Entering the calibrating or saving state will clear the blinking white camera LED.

If the slave camera doesn't detect any genlock signal, the camera LED will blink red.

CAMAPI status

If the slave camera is experiencing a genlock timing error, the CAMAPI_FLAG_SLAVE_GENLOCK_ERROR flag (0x40000) bit is set. Once the slave camera is maintaining genlock, the CAMAPI_FLAG_SLAVE_GENLOCK_ERROR flag is cleared after a five second timeout. In addition, entering the calibrating or saving state will clear the CAMAPI_FLAG_SLAVE_GENLOCK_ERROR flag.

Feature interaction

  • If camera Serial console is enabled, genlock setting is forced to Off.
  • If camera is configured a genlock slave, camera will ignore all trigger events expect genlock trigger over the External trigger connector.
  • Each camera generates its own timing for the calibration cycle.
  • When configured as a slave camera, the camera will only use the slave camera's frame rate setting if the genlock start-of-exposure signal is not detected for 100 ms.

False Triggers

  • Plugging in genlock cable may trigger both master and slave cameras.
  • Unplugging genlock cable may trigger both master and slave cameras.
  • Powering off a genlocked camera may trigger any other connected cameras.

Other notes

The metadata file on the master camera will contain an entry indicating the delay from the incoming trigger and the start of the first frame following the trigger.

Technical description

  • For genlock to function properly, the slave camera(s) must support a frame rate greater or equal to the master camera's output frame rate. The easiest way to ensure this is to use identical settings on all cameras. The master camera will decrease its maximum allowed frame rate slightly to allow for the slight clock differences that are possible between the master and slave cameras.

Signaling

The camera can be triggered from three sources:

The UI / API trigger() software API, Multi-function button, and Tip of phone jack are logically or-ed together and only the master responds to the trigger event.

2.5mm phone
jack signal
Genlock
OFF mode
Genlock
MASTER mode
Genlock
SLAVE mode
Genlock
EXTERNAL mode
tip trigger input signal
(3.3V LVCMOS, active low)
trigger input signal
(3.3V LVCMOS, active low)
unused trigger input signal
(3.3V LVCMOS, active low)
ring unused genlock output signal to slave camera(s)
(combined frame start and trigger)
frame start input signal
(3.3V LVCMOS, active low)
frame start input signal
(3.3V LVCMOS, active low)
sleeve ground

You can use the cable listed below to genlock two cameras (which does not support external trigger via wired remote).

Steren 252-612 12' 2.5mm Male to 2.5mm Male

To genlock 3 to 5 cameras, or to use an switch closure to trigger the master, you will need to make your own cable. To have an external trigger,you will need to make a custom cable. The signalling from the master genlock camera can drive up to 4 slave genlock cameras. If you need to connect more than 4 slave genlock cameras, then you will need to increase the drive by using an active cable with a 3.3V LVCMOS buffer.

Electrically, the tip and ring are identical circuits, with the exception that the tip is always an input, while the ring is an output in genlock master mode and an input in all other modes. Each signal has a 4.7K pullup to 3.3V, followed by a 165 Ohm series resistor. The other end of the 165 Ohm resistor connects to an ESD diode clamping array (GND and 3.3V), an FPGA 3.3V LVCMOS GPIO and an SOC LVCMOS 3.3V IO.

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