Today I had a customer ask why digital backs used firewire and dSLRs use USB. It’s an explanation I give frequently but I thought I’d take this chance to blog about it to all you loyal readers. The below represents my personal unabashed view of why USB sucks and FireWire rocks (in high-end applications such as tethering high-res cameras).
USB and FW are both great specs on paper, but there are some very technical differences between them. Without getting too deep in technical jargon here are some differences:
- FireWire can provide more electrical power than USB over-the-cable
- Firewire 400 connectors are make a very tight physical connection* mini-USB as used on most dSLRs fall out very easily on their own and are therefore often accompanied by a screw-in holder of some kind, making it much harder to “unplug and go”
- Even in ideal situations the maximum sped of FW400 is higher than USB, the speed of FW800 is far higher (ignore the specs which say USB is slightly faster – this is the theoretical max speed and no USB device, not even high performance ones, come anywhere close to this speed).
- FireWire supports Daisy Chaining which is a fancy way of saying you can connect a FireWire device to a computer by connecting it to another FireWire device already connected to that computer. This has a lot of cool applications that most people don’t need or use.
- Finally the greatest difference of all may be the way the connection is managed. With USB the computer is asked to manage the transfers of data which means the CPU has to be monitoring/managing the connection anytime data will be transferred. With FireWire there is a chip on board the device which can communicate directly with the FW board so that no system resources are needed. As long as the computer has available system resources (meaning it’s not working very hard) the difference between these methods is minor. However when the computer is taxed (e.g. by heavy continuous shooting of a high-resolution camera and the processing/rendering required to show previews of those images) the difference can be huge because USB requires the CPU to “pay attention” to it to transfer new data, and if the CPU is swamped it cannot pay full attention to the USB. Whereas with Firewire the device and Firewire hub can continue to manage the transfer of new data by themselves. The practical result? Plug in a digital back like a P40+ and hold down the trigger and it will continue shooting the same speed for many dozens, or even hundreds of images in a row without changing speeds. Plug in a dSLR and hold down the trigger – you’ll have to wait until the camera has hit its buffer limit so that you’re watching the speed of transfer from the camera to computer rather than the speed of the camera to the camera-buffer. If your computer is fast enough to manage the connection to the camera and do the calculations of proxies (and anything else which is open and running) and still have headroom to manage the USB connection than the rate will not change. If however you have a slower computer or you start pushing the computer hard (checking focus at 100%, processing images, making adjustments to a batch of images, opening files in the background in photoshop etc) then the downloading of new images will crawl to a stand still making the camera feel lethargic and non-responsive.
So all of the above points make it sound like FireWire should be everywhere and USB nowhere. What’s up?
There is price to the last point. Since FireWire devices must manage themselves they must include the size/weight and most importantly cost of an onboard chip. These chips aren’t very expensive, but for many consumer electronics there are only cents worth of profit per item and the money spent on the FW chip just doesn’t make sense. Likewise adding FW ports to a computer is more expensive than USB ports and since most of the PC world is extremely price conscious many PCs did not include a FW port.
When Canon changed from the firewire port on the 1Ds II to the USB port on the 1Ds III it was a sad day for me. I understand why; Canon makes general purpose cameras used by a huge variety of users and many 1Ds III will never take a single tethered shot, and in the general market place many computers do not have a FW port. Cheaper and more readily available connectors won out over the better (but ever so slightly more expensive) technical solution.
*The older SCSI and Parallel/Printer port connectors were even better but were designed to be plugged in once rather than plugged and unplugged and were designed before hardly any “mobile” devices were available.
See more: http://www.usb-ware.com/firewire-vs-usb.htm