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Not a problem I thought ... then problem, I thought! Then they mentioned that they are required to keep an archive of VCR tapes for three months, and would therefore require the same in a digital format. It is also a highly sensitive building and they have requested full frame digitally signed images. I did suggest digital tapes for additional storage but the customer prefers to keep all the information on the unit's hard drives in one box if possible.

To keep them in line with existing camera refresh rates, what storage capacity would I need to achieve the above?

The solution: Ah, the long term digital storage issue … Although it is possible to store video images for many months or more on hard drives, it is still a relativity costly exercise as it involves the use of additional drives, usually in the form of RAID, and as such RAID goes against the one box solution you require.

The good news is that the cost of hard drives is reducing while the storage capacity is increasing, so the days of long-term storage being too expensive and out of reach of most end users could very soon be a thing of the past. Full frame digital recorders are now available that can achieve in excess of two months' continuous recording at 6pps (24 Hour Mode) in one box.

We sometimes forget just how fast this market grows. Only a couple of years ago it was a major feat to get seven days' continuous recording in one box. Some boffin from Microsoft once said that hard drive storage would double every year. This has largely remained true, but in the early days when it was increasing from 500MB to 1GB and then to 2GB, it made little impact on our market as video images use large amounts of data. It wasn't until the capacities started reaching the dizzy heights of 10GB or more that manufacturers could really start to attack the traditional VCR market.

Now with 160GB hard drives so commonplace, we are starting to see the emergence of 200GB + drives. So, who knows where we will be in another two years? Maybe single drives of 500GB will allow for machines with a terabyte of storage or more?

Oh well, enough dreaming of what the future holds; what do we require for you to store up to three months' worth of recording?

To keep in line with your existing installation you would need to set the recorder to a rate of six pictures per second at an 18KB file size.

The actual formula for calculating disc storage capacities was described in detail in the November issue of this magazine. If you missed it try to get a back copy or visit the DM web site at www.dedicatedmicros.com where you can find all previous articles under "useful Info" in the support section. But here is a quick overview.

• 6 (pps) x 18KB (File Size) = 108KB per second
  
• 108KB x 60 (seconds) x 60 (minutes) =
  
• 388800KB or 388.8MB per hour
  
• 388.8MB x 24 (hours in day) = 9331MB or 9.33 GB
  
• 9.33 GB x 93 (days in three month) = 867.69 GB

So at 6 PPS and an 18kB file size you would need around 868GB of recordable hard drive space. Even in today's market this is a large amount of data to store, so we need to look into ways of reducing that total.

How fast the market grows ... only a couple of years ago it was a major feat to get seven days’ continuous recording in one box.

Hard Drive Management
A good way of achieving a longer record time is to manage your hard drive correctly. Managing your hard drive is a very worthwhile task, freeing up those gigabytes lets you increase the record rate and/or increase overall recording time.

It is also very good practice to take a close look at any site's particular needs. OK, yours requires three months' worth of recording, but do they require 24 hours a day, seven days a week? Can they switch to alarm or activity recording at night-time or out of hours? Can they select certain cameras to record only in alarm? Often the end user has just got into the routine of changing a tape a day and has never really thought about what he is actually recording. It might be 24-hours' recording of a closed door and six or seven hours of dark unusable recordings and so on. So when you are at system design meetings, be sure to discuss these different options. You can save valuable hard drive space and thus extend the recording time and reduce the overall cost. However, if you go with any of the above, it does throw a spanner in the works regarding total GB required.

Obviously, you don't know how many alarms there will be over a given time frame, so a little guesswork is needed in calculating the total storage. Every site has its own unique characteristics so study existing videotape to help get a feel for the amount of activity there might be. As a rule of thumb most engineers I speak to usually allow for about four hours in every 12 for recording alarms. This may sound a little over the top, but remember it's better to be safe than sorry. Using the above calculation, saving eight hours a day recording, would reduce your total GB required by about 290 leaving you 578GB ... now a more manageable, cost effective figure.

There are already reasonably priced all-in-one recorders available that can handle this amount of data.When it comes to providing the police with digital images as evidence, the main issue is not whether they'll be accepted, but proving the authenticity of the digital images – its origin, how it was produced and controlled.

According to draft guideline lines drawn up by the Digital Forum and PSDB, each image sequence directory should contain header data that includes, amongst other things, the camera and unit details, the date and time of recording (according to the date/time settings of the unit) and an incremental sequence number.