The advantage is a device that is specifically manufactured to record audio and video from multiple inputs is probably going to be set up ready to use and running on a box that probably has a proprietary version of Unix or Linux in it. It will run and run and run. It will probably cost more than a PC based system, but will be more dependable. One box, one function.
A PC based system will require you open a box, install one or more video cards and then install and set up the software. It will probably be windows based and will probably require maintenance tasks. It will probably not be as reliable as a dedicated device. The advantage is, if you already own a PC, you just need to purchase the video capture cards that usually come with their own software. It is much less expensive than a dedicated device, but usually can't compare to the video quality, frame rate and resolution of a dedicated box.
Both types of devices should have built in motion sensing, which will detect any changes in the video screen and save the recording. this eliminates the need for hardware motion detctors. Good devices will always record, but just keep the stuff that is triggered by motion. This iliminates any lag time that would occur if it didn't start recording until it detected motion. Also, some systems let you mask out areas of a picture so that, for example, you can have a camera system in a pet shop, but mask out the hamster cage so Tiny doesn't trigger motion detection when he runs in his wheel but the counter area will trigger by motion. Some systems will also let you set the sensitivity so that small changes, such as dappled sunlight coming through a window on partly cloudy days won't trigger the motion all the time.
At work we use a dedicated device from
www.dedicatedmicros.com called the Digital Sprite 2. You access it through a web browser on any PC in the network. It is pricey but works flawlessly.
For PCs acting as a DVR I have used a program called Active Web Cam. It works well for a few cameras, but the more you have, the slower it gets if they all are recording. A neat feature of it is it will record at a slow frame rate until motion is detected, then increase the frame rate until the motion ceases.
I have a funny story to share about video surveillance at my home....
I set up a camera to watch the back door for deliveries, etc... I installed hardware motion sensors in the area(I didn't have the video motion detector software at the time). It was set to detect motion and record for 5 minutes. When I got home from work, a four hour tape in the VCR was full! After review, I found that a Robbin would fly up and land on the gate, poop on it and fly away. Then 4:55 of video of nothing. Then the Robbin would come back from the other direction, land on the gate, poop and fly away and another 4:55 of nothing. This went on all day, and to top it off it was mulberry season. You know what a mess that can make! /forums/images/graemlins/tongue.gif