About Digital Video
From IPTrains
First off, let's define our terms. In an IPTrains context, digital video is streaming video from a web cam. Equally important, transmission of our digital video is over TCP/IP, the core protocol of the Internet. Why is this important? You may have seen some of the analog video available from other vendors. Analog video is fun, that's how we started, but it has several important problems that become apparent when you start to try to use it seriously.
A brief discussion of analog video over ISM band radio
- Analog video is, well, analog. It has no noise immunity in transmission. This becomes very obvious with a weak signal or in the presence of interference. Think over the air analog TV and you're pretty close... Like this ->
- Analog video from a moving G scale railroad engine needs to be sent over AM or FM radio, without protocol support or any sort of error correction. This shortcoming manifests itself as static, noise and loss of picture.
- There are four analog video slots (channels) in the 2.4Ghz ISM band. These are not the same thing as 802.11b/g (WiFi) channels and in fact compete with WiFi and cordless phones, microwave ovens, etc. for crowded unlicensed spectrum. Analog video loses every time because of the afore mention lack of protocol support and error correction.
- All four of the analog video ISM channels can not be used at the same time because of guard band issues, only 1 and 3 or 2 and 4 can be active w/o interfering with each other. Each analog video channel in use requires a separate receiver ($$$).
- One analog camera to one ISM video channel, period. That's two train cams, max. With digital video we often run more cameras than that on one train!
- Once it come out of the receiver analog video is still analog, it must be viewed on a TV or analog video monitor. You can not look at it on your computer, save pictures or movies or send it over the Internet w/o additional digitization equipment, for each camera. If you want to bring the signal inside, you'll need expensive and fussy plumbing (low loss coax, video amps, etc).
Digital Video over WiFi
- Digital video over WiFi takes advantage of the power of TCP/IP, the same magic that makes the Internet work reliably around the world. You will never see static or frame tears on digital video, the worst that can happen is a momentary 'frame freeze' if signal is totally lost (in a tunnel or other radio hostile location). This is all possible because of the error detection and correction made possible by protocol support of every frame sent over TCP/IP.
- Digital video is, well, digital. Various resolutions are possible at various frame rates, up to 1280x1024. That last resolution can approach a good digital camera for still pictures from the cab. Look at the thumbnail picture on this page, that was taken by Trainvision with no additional equipment required, just 'frame grabbed' directly to the computer.
- Multiple digital video streams from multiple trains, can be sent over the same WiFi channel, simultaneously. All to the same access point (AP). Unless you are a club operation or have a huge layout you will never need more than one AP. If you do have a very large layout (longer than 500 linear feet [not trackage, LOS distance] end to end) then an AP at either end will cover the entire space and allow the locomotives to 'roam' and always be in coverage without loss of quality
- The signal is digital (IP) from the time it leaves the engine and can go to your computer screen and/or on to the Internet for viewing by others. You can literally show off you pike to people on the far side of the world, in real time. Most model railroaders love to show off their layouts, I know I do, now your friends don't have to be there or look at static pictures on a web site, they can see the real thing, live, from anywhere on the Internet, as viewed from the cab of the locomotive.
- Some basic math about video streams. Our cameras run from a low of about 780Kbps (320x240) to a high of about 3Mbps (1280x1024). An 802.11g channel is rated at 54Mbps. Divide by 2 as 802.11 is a half duplex protocol and round down, call it 25Mbps of available bandwidth per AP. Round 780Kbps up to 1Mbps. That's 25 simultaneous video streams on one WiFi channel, on one AP. In reality that's a conservative calculation as more than 25Mbps of over the air bandwidth is normally available 'upstream' as there is not a lot of downstream traffic in the system. Bottom line? You will not outgrow an IPTrains digital video system.
