Digital without the jargon

To

When a video image is captured it is electronically scanned into a digital format of about 250 KBytes of data. Most recorders then compress it into a smaller file size.

Why compress?
40,80 Gig hard disks are now commonplace and are ideal for your average PC package. However when you are storing digital video data this space can be easily swallowed up. So it is of the utmost importance to keep the amount of data stored to a minimum. Let us say for instance you compress at a ratio of 10 to one, your drive will hold nine times more data, or in video playback terms, 10 hours instead of one.

   The way in which digital recorders compress can differ. Compression is generally done in two ways, Full Frame and Conditional Refresh. Full Frame grabs a complete picture every time so its file sizes are mainly static, making it easy to work out storage etc. Conditional Refresh stores the moving part of the picture up to a certain percentage then stores a full image. This leads to sporadic file sizes because every image stored can be different. Using this system it is only possible to average out the storage time.

Both systems have their pluses and minuses. I'll just stick to Full Frame for now and perhaps cover them in detail at a later date.

This compressed data is referred to as your image file size, these are usually programmable functions within the recorder. This image is then sent to the hard drive to be stored. The speed of these images being sent to the drive is your pictures per second (PPS). This again is usually a programmable function within the recorder.

Time-lapse mode v PPS
For years people have been used to 12hr, 24hr mode etc. These times arrived from stepping down a three-hour VCR tape and reducing the number of images recorded. One tape can hold about 270,000 images. These images have to be then divided up to cover your time-lapse period. This is known as your update time or PPS. So PPS is not new, it has been the standard for years.

The thing that has changed is the storage device; no longer are we ruled by a three-hour tape. We now have the capacity to use hard disks, which can be singular or stacked allowing us to determine our own storage size. It is for these reasons that digital recorders use PPS.

For a comparison with time-lapse recording, in 24hr mode on average most video recorders will put down to tape about six pictures per second, so setting your digital recorder to the same will give you the equivalent PPS as 24hr mode. This is a useful reference for comparing analogue against digital recording update times.

One of the main areas people get confused about is how long they can record on the hard drive without over-writing. This is done by calculating together your PPS, image file and hard disk sizes. Most manufacturers use part of the hard disk for their operating system, logs etc. An 80-Gigabyte recorder may only have 79-Gigabyte of recording space, so check with your supplier.

Let's say we have 40GB of hard disk space for recording, a picture file size of 20KB and a recording rate (PPS) of six. We first have to calculate how many KB there are in 40GB.

This is achieved by doing the following calculation.

These calculations may seem daunting but most manufacturers will have data sheets with the above calculations already worked out for you.

I hope this has cleared up some of your doubts about digital recorders and has given you the confidence to approach your first installation. There are many benefits these machines can offer and my advice is to contact the manufacturers and get a demonstration.