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How much video storage do I need?

Many years ago when the time-lapse VCR was used for bulk CCTV storage, each system’s manager or designer considered the CCTV’s operational requirements to calculate how many videocassettes would be kept in their secure cabinet.

Let’s look at their really simple maths. It’s largely the same starting point for digital video which is why I’m posting this as it seems to cause confusion with many people.

They considered the basic factors:
1) storage duration, e.g. 31 days
2) image refresh rate, e.g. 2 ips (images per second)
3) number of cameras to store, e.g. 16 cameras.

Hopefully, it’s fairly easy to see that the total number of images you need to store is:

        Number of images per day from a camera  x
        Number of cameras  x  Number of days.

I hope that it’s just as easy to see what your number of images per day from a camera is:

        No. images per second  x  60 = No. images per minute
        No. images per minute  x  60 = No. images per hour
        No. images per hour     x  24 = No. images per day.

Just multiply the three figures together for the total!

Let’s run our figures:

2 ips  x  60 mins  x  60 mins  x  24 hours  x  31 days  x  16 cameras  =  85, 708,800

Now, how many images can a standard 3-hour videocassette contain? Running full-tilt it stores 50 ‘fields’ of video per second over, (suprise, surprise) three hours. Being quick at maths you know that this is:

        50 ips  x  60 seconds  x  60 minutes  x  3 hours  =  540,000 images on one cassette.

this leaves our earlier example needing 159 videocassettes to fit in their lockable cabinet.

Ta-dah! There’s your answer*.

Ok, so let’s follow exactly the same method for our hard-disks in our digital recording system. You really don’t need to rely on that silly little bit of software that some salesman gave you on a CD or that ‘wizard’ on his company’s website. How does their calculation work? Who knows? Can you trust it? You don’t know. So, better to work this out for yourself, then you know you can trust the answer. You’ve seen how easy it is above.

Let’s go digital. Let’s assume that the footage is made up of a stream of still images (jpeg, wavelet, etc.) that are each 30KB in size**.

We find: 2 ips  x  60 mins  x  60 mins  x  24 hours  x  31 days  x  16 cameras  x  30KB
=
  2,571,264,000KB.

There are 1000KB in a MB followed by 1000MB in a GB, so this huge number of KB  =  2,571GB.

“But!”, I hear some of you shouting, “What about systems using more efficient mpeg compression?” Yes, mpeg and other temporal compression techniques do permit smaller file sizes by only carrying the changes between consecutive images (roughly speaking). Fine. I say that this efficiency gives you more of a safety margin with your disk sizes. The calculation above assumes a worst-case scenario when you’ve got rain, snow, traffic, electronic camera noise in low light, etc. all constantly creating changes from one image to the next that erodes the mpeg advantage and practically turns it back into plain jpeg, for instance, mpeg I-frame only encoding. Whether you follow this philosophy or not depends on how averse you are to the risk that your system actually runs out disk space in such circumstances and is forced to either stop recording or begin overwriting the oldest footage that is only, say, 24 days old instead of the 31 days that the system should achieve. Could you live with that? Could you explain to the Information Commissioner (in the UK) that you’ve not complied with your obligations under the Data Protection Act (1998) by ‘accidentally’ deleting data you are obliged to keep safe? My suggestion is that you rely on your own calculations and take with a pinch of salt the assurances of the salesman who shows you his own mysterious ‘calculation’ about how much storage you need. His might be smoke & mirrors to win a purchase order from you. You need to only trust your own logic. I hope I’ve explained how.

Until next time; stay focused.

* It’s not always quite that simple, as a time-lapse recorder running in the commonly used 12-hour mode actually makes a 3-hour tape last for a 15 hour duration***. However, the CCTV owner will change the tape after 12 hours for shift-based operational reasons, so a fifth of the tapes capacity is not used. More simply, we can see that they use two tapes a day in each VCR during each of 31 days! But how many VCRs will they need? That depends on how many images per second the system’s multiplexers can put onto each tape which, assuming they are capable of handling a heap of probably unsynchronized camera signals, could be as high as 50 put may be as low as 25. Aren’t you glad we don’t have to worry about this now VCRs are nearly obsolete? 😉

** I very much prefer clear pictures that haven’t had all the useful detail compressed out of them by overzealous equipment peddled by installers who seem to think that you can get a quart into a pint pot because it’s cheaper than the way their competitors do it. A CIF image (352 pixels by 288 pixels) can be approximately 30kB as a jpeg and retain come clarity. Of course, it varies depending on the scene. However a 4CIF image (704 pixels by 576 pixels) will be four time bigger. Don’t let salesmen tell you different.

*** Similarly, a 24-hour mode makes a 3-hour tape last for 27 hours. Why? The spinning video head in the VCR can only write alternating odd & even video fields onto the tape, so you can’t successfully slow a 3-hour tape by a factor of 4 (to span 12 hours) as this is an even number. It has to be an odd number like 5 which means that the tape lasts for 3 x 5 = 15 hours, or like 9 which gives 27-hour storage because a factor of 8 (giving 24 hours) wouldn’t work.

———————————–
Videotapes image by NASA Goddard Photo & Video on flickr.com by creative commons attribution.
Hard-disk image by daddo83 on flickr.com by creative commons attribution.
VCR head image by Jef Harris on flickr.com by creative commons attribution.

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