Video signal via the slip-ring
Analogue video signal from a co-rotating camera, mounted on an arm affixed to the turntable, passes through the slip-ring and out an RCA socket on the apparatus base.
The signal transmitted is video of standard definition VGA 640×480 with minimal lag, and crucially displays motion-in-view at a consistent, smooth frame-rate. Note that the signal from the slip-ring is at a lower resolution than many high definition cameras on the market. However the analog signal is transmitted steadily in real-time, and so can readily be used with, e.g., a particle tracker. Moreover, the slip-ring is a very cost-effective solution and computer equipment is not essential since the signal can be sent directly through to a TV monitor a DVD recorder or the particle tracker.
Here we describe how one can use a DVD or a computer to capture video. This video may then be viewed at one’s leisure or, with suitable formatting, played in to the particle tracker.
Recording to a DVD / obtaining a video file
Routing the slip-ring signal first through a DVD recorder, one can connect a TV/monitor to the recorder’s Video Out for a live view of the rotating frame. A blank DVD±R is recordable for a single experiment, or DVD±RW is rewriteable for recording multiple runs or sessions.
To convert these recordings to portable file, first finalize* the disc if it is rewriteable, and load it into a computer with a DVD drive for reading. Software such as Handbrake (multi-platform) can convert video on DVD to a compact format such as an .MP4 file. To play these files into the provided particle tracker, see Video for Particle Tracking.
*If you’d like to particle track from a DVD±RW, one need not finalize the disc—simply replay the recorded chapter into a laptop with a RCA-to-USB video converter as described below.
Recording on to a laptop via USB
A live view, or playback from DVD, may be passed through the Auxillary Out to a frame-grabbing device connected to a laptop’s USB port. This input can be displayed as a webcam feed.
USB-in video may be viewed and captured by stream-handling software Video LAN Client (VLC) as a DirectShow input; in VLC, a recording button is displayable through advanced menu options. Saved without compression, this format results in large files by default, so ensure sufficient hard disk space, or record salient segments of duration ≤ 2.5 minutes for manageable file sizes, easing analysis downstream.
For live analysis with the provided particle tracker, click “Connect to Camera” in the software and select the frame-grabber’s video as the source; for details, see the guide to particle tracking.
Wireless video options (with or without slip-ring)
To circumvent the slip-ring entirely, higher budget options exist to obtain time-continuous HD video out over wireless, using low latency IP cameras.
Some high definition camcorders can be controlled & monitored wirelessly by an app on computer or smartphone, with the caveat that these live previews are subject to network delay and often transmitted at a lower resolution and frame-rate than that which is available if recording occurs on-device.
A wireless controllable camera such as a GoPro may be used to record in high-definition while the system rotates. The digital preview broadcast from an action camera is often a reduced quality and frame-rate, subject to variable lags from transmission over wi-fi. Thus action camera recording / attachment serves best in parallel to the analog camera if one wishes to view an experiment’s dynamics as they progress. Whichever route, experimenters may prefer to focus on executing the experiment well and to observe the live fluid, while obtaining a quality video for acquiring data afterwards.
HD recording: the Hero & similar action cameras
A GoPro device or similar action camera may be attached to the suspended arm using a roll-bar mount in conjunction with “the Frame,” an accessory that holds the camera and provides a connecting point to the mount. Recorded video will be saved to an on-board micro SD card, accessible directly by demounting the camera and removing “the Frame” after spin-down, or read from in-camera via USB (standard to mini-USB for models up to Hero 4†, to USB-C for Hero 5) without removing “the Frame.” Connect the device to the computer and power it on (USB logo should appear) to access it as a drive and transfer recordings to disk (videos are stored in the DCIM folder.)
Leaving the action camera mounted to the turntable arm reduces the need to recalibrate its view. A longer equivalent USB cable can enable one to connect directly to the device without demounting it, to access files and recharge the device’s onboard battery. Alternatively, recordings may be transmitted wirelessly to a smartphone or computer by connecting to the camera’s built-in wi-fi network. Adding a portable power-bank (widely available) is recommended if you intend to record many experiments between recharges of the camera, or downloading large videos wirelessly.
† Methods here described have been tested with GoPro Hero 3+ SE. Guides for specifics on this and other models can be found with GoPro’s help.
Any smartphone or computer can serve as a “smart remote” for a GoPro action camera, to preview capture, calibrate settings, initiate recording, or download resulting media (some resolutions are down-converted for phones.)
To prepare a viewer’s system for the first time, ensure connection out to the internet (and not the camera’s wi-fi network.) On a handheld device, download the GoPro application available through a phone or tablet’s application store. For laptop/desktop, official and 3rd party softwares are available cross-platform to access the equivalent view – control functionalities wirelessly.
With the app installed, you can connect to action camera. In the viewing device’s settings, log on to the camera’s wi-fi network (name and password assigned in camera set-up) then load the GoPro application, selecting “Connect your Camera.” Through the remote application, one may adjust the field of view, capture type (video or time-lapse,) resolution and frame-rate of the video, before beginning the recording and initiating tracers (dots, dyes) to visualize the experiment in view. Recording can be confirmed on-screen, and on the camera itself by a red blinking LED.
Obviously, direct contact with the camera while rotating would introduce vibrations through the arm, oscillating the fluid. So, before spin-up:
- Enable the wi-fi by holding its button on the camera’s side, confirmed ON by a rapidly blinking blue LED. This allows other devices to connect to its wi-fi network, to power-on and control the camera remotely.
- Power on the camera to preview its capture frame and field of view, in order to perform manual centering and ensure the contained fluid volume’s edges are fully in view. This is especially important if the camera has been remounted between experiments or laboratory sessions.
Resources / provided software + scripts:
Connect to a video encoder’s broadcast stream
Example scripts assisting wireless playback:
Open real-time streaming protocol (RTSP) with video LAN client (VLC)
Requires hardware akin to wireless encoder.
Requires software VLC.