DVRs are inherently different from their analog ancestors by nature of how they record and their ability to manipulate video images. Compression is a significant factor in deciding the resolution and size of an image. It is also a factor when determining the rate of recording – or ips. Take a look at a good comparison of DVR features and specs and you’ll see a variety of terms listed for compression modes – extended, normal, fine, super fine – high quality, medium, standard – basic, normal, enhanced. Anyway you interpret them, it amounts to good, better and best. In plain English: the lower the compression, the better the image. The higher the compression, the less detail and quality.
This rule of thumb applies when evaluating motion video as well, except now quality, size and rate (still measured in ips) are issues to contend with. As a result, you must determine and/or define acceptable image detail and quality when deciding whether or not you wish to record images in real-time – or 30 ips.
Basically, there are two types of compression employed in all DVRs: interfield and intrafield compression. Interfield compression compresses full images so that if you were to record at 30 ips, you would actually record 30 pictures per second. Intrafield compression only records the portion of an image that changes from one frame to the next. For example, if you were to open a door in a scene, only the portion of the image showing the door would be recorded. The concept behind intrafield recording is to minimize data storage requirements, which in turn extends recording capacity. Both forms of compression offer benefits for specific applications. Interfield recording provides higher quality which is required in more advanced systems – especially those employing pan/tilt/zoom systems. Intrafield compression is ideal for systems with fixed cameras since movement is limited.
Therefore, DVRs with interfield compression offer better image detail and quality for real-time recording applications – but you will sacrifice storage capacity. The reverse holds true with intrafield recording, you’ll sacrifice image quality and detail, but increase recording capacity. It’s a trade off either way. New DVRs offer both interfield and intrafield compression so you can decide what works best for you based on your specific application.
The positive side to this is that DVRs are very smart machines that can be programmed to change compression rates under certain conditions – such as alarm activation or motion detection. This allows you to decide what cameras get recorded in real-time and when.
Of course, there is another consideration: storage capacity is an issue. With analog real-time recorders, the storage parameters are very simple – 24 hours on a standard T-120 tape and 40 hours on a T-160. After 24/40 hours, you simply replace the tape with a new one. As you would expect, it’s a bit more complex with DVRs.
Internal storage limitations will dictate how often you need to download your information to some other location for later review and archiving. Once again, the combination of low compression with a fast transmission rate can quickly consume storage space. Therefore, it is important that you consider your storage capacities – internal and external – when determining compression resolution, size and rate. In addition, the proliferation of networking DVRs also offers the option to distribute storage requirements across a network as needed. But this too is a recent development.
The bottom line is that there are numerous recording devices available today that can provide real time recording. Once your system parameters are clearly defined, you should explore the options available to you by speaking with various manufacturers or consulting with your systems integrator.