In the old day’s video was recorded on VCR and require rotation between video cassettes as they have limited recording time and poor quality after a while. Today the DVR is recording on a hard drive and the size of the hard drive determines the recording time. Video quality is much higher using hard drive and does not loose from its quality over time.
The DVR is recording frames (images) and does not records continues video stream. We can control the frequency of capturing the images. High frequency (more frames per second) gives better continuous images and it looks like video stream but it lowers the recording time because it records more images on the hard drive. In low frequency mode (less frames per second) the video is not continues as with high frames per second but you get more recording time.
Video compression has a big effect on recording time and quality. In order to save space on the hard drive the captured frames needs to be compressed when recorded and decompressed when viewed. The DVR does it automatically for you. Good compression algorithms shrinks the image file and allows you to save a better quality image for less space on the hard drive. This gives you more images to store before the hard drive gets full. Video compression technology available today is Mpeg-3 and Mpeg-4 when Mpeg-4 is better. When the hard drive gets full the DVR gives you the option to overwrite the old images with new ones.
There are 3 ways to record images and in some DVR’s you can also determine for each camera individually video quality, and the frequency of the images on each one of the following option:
1) Schedule - you can determine for each day the starting and end point.
2) Continues Recording - records 24 hours.
3) Motion detection - you can control each camera sensitivity to motion and you can also determine which part of the image will trigger and which is not. For example you can set low sensitivity when recording outside and don't want to record when the wind blows on the trees or if your video is also capturing a part of a sidewalk and don't want that every person on the sidewalk will trigger the recording so you can exclude the grids that points on the side walk.
Few of the DVR’s allow combining the options I mentioned above. Example if you have a store and choose to record from 9am to 7pm in 5 frames per second all the time and during the night to record in 1 frames per second to save space on the hard drive but on the same time you would like to set the frames per second for 7 if the motion detection is Triggered so you get full coverage all the time. This feature of combinations does not available in all type of DVR’s.
Archive - when you want to save part of a video you need to copy it from the hard drive before it gets overwritten when it becomes too old. There are few ways to record: CD or DVD recorder, either external or built in, flash memory card, USB memory, and swappable hard drive.
Network connection - most of the DVR’s have network connection. This way it can be configured to view remotely from any computer over the internet or from inside your local network. You can also download part of the video and save it locally on your pc or on a CD.
Recording and display rate - the DVR records images and not continues video stream. Each DVR has a limit of how many images per second it can records and how many it can display per second. The higher the rate per second is, the better the DVR. The display or recording rate per second is for the entire cameras. Example a DVR with 120 frames per second display rate and it displays 16 cameras so each camera gets 7.5 frames per second max for display regardless of the rate it was recorded.