Low Latency Syncronization over the internet

Previously, ActiveDeck was able to stay in sync within about 4 seconds between the PowerPoint computer and it’s neighboring iPads. However, this wasn’t good enough for us- our background is in show control systems and frame accurate video playback systems. We have a curse of over-analyzing every video playback system we see for raster tear, frame skips and sync problems. Given that ActiveDeck solely relies on the internet, we thought 4 seconds was pretty good, considering. But we wanted to make it much, much better.

We thought about the best way to improve the latency, and our initial thinking leaned towards a local network broadcast originating from the computer running PowerPoint, but that introduces issues on WiFi networks, especially those you would find in hotel ballrooms. VPN to the cloud service would be another option, but adds lots of complexity.

We ended up using pure HTTPS communications (no sockets, no VPN, no broadcasts) to and from the cloud servers with the use of some clever coding. If the iPad has internet connectivity, it will be in sync.

Check the video out, this is over a cable modem internet connection and a plan Linksys WRT54G access point. Our Windows Azure servers are at least 13 router hops from our office. The beautiful part is that the sync messages are tiny and this will scale to hundreds of iPads.

100 iPads

Have you ever wondered what 106 iPads look like when packed as densely as possible? Here is a picture:

For a recent project, we developed a synchronized iPad display app. The project was to support a presentation with some new method of interacting with the participants. The designers liked the idea of handing out ipads to which they could “push” content they wanted, when they wanted.

So, we fired up xCode and built the iPad app. The application is made up of several modes of operation while the main mode is to display content driven by the presenter, so that on cue all of the iPads display new screens without any interaction by the person holding the iPad. This looks pretty awesome when it gets triggered and you can see all of the iPads change their screens at once.

Other modes are sort of like tests or drills, where the users complete a quiz and then submit that data to the presenter. We have another application there that creates graphs based on the statistics from all of the iPad users to which the presenter can speak to when projected on a large screen in the center of the room.

When we set out to design the system, we had to think through the potential bottlenecks. Our main concern was network latency, so after some research we specified the best wireless access points we could find- Ruckus Networks. See these links:



We ended up with 5 access points and a network controller on a gigabit network. Worked great (a little bumpy the first day of the presentation to due to a faulty access point).

Next, we created a back end system where the content would be stored locally, yet able to be updated during the presentation. Using IIS, we posted the images and XML files on a local (to the event network) webservice. We then wrote a multi-threaded socket server on another computer that was dedicated to triggering page turns, mode changes, and initiating fresh content downloads to the iPad.

Here is a video during some initial sync testing, this is all running from one access point, and triggered by Chris and his PC.