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TSI BBQ 07-09-10 OK

Simon Lambert, immortalized by Rick Coleman

 The Security Institute (TSI) had its annual conference earlier this week at the very agreeable Oakley Court hotel in Windsor. Very nice. The Queen has a house nearby, you know. I was fortunate to be invited to join the great and the good for their barbeque on Tuesday evening. 

The day’s earlier torrential downpours held off during the balmy evening on the terrace and lawn with the Thames flowing by. Amongst the chums with whom I ate, drank and waxed lyrical, putting the security world to rights, were Gordon Tyerman of CCTV Training Ltd, Brian Sims the editor of Info4Security, James Walker of Dallmeier, an old colleague from Reliance Electronics in the early 1990s, and Mike Bluestone of TSI, formerly a fellow council member at the ASC, the Association of Security Consultants

Thanks to Carly Huckle of TSI for being so kind as to invited me to an enjoyable gathering. I must get around to actually joining TSI one day soon! 

One highlight of the evening was receiving a fine caricature by artist Rick Coleman. He did the assembled crowd proud with some wonderful drawings and a nice chap to boot.

Annoying Aspects of CCTV (Part 3)

One of the most interesting jobs we independent CCTV consultants are hired to undertake is the comparison of digital video recorders (usually abbreviated to DVR) especially on behalf of our customers who might be non-technical people. We enjoy getting into the nitty-gritty of testing the performance and foibles of various manufacturers’ DVR offerings, while the aforementioned customers usually avoid it like the plague! It’s an ‘Anoraks Only’ zone.

Squashed by the GUI. Ouch.

This work has highlighted the third of the annoying aspects of CCTV; particularly regarding DVRs and their software-driven interfaces hosted on a pc screen. The problem is that digitized CCTV images (both live and recorded) are displayed to the CCTV operator by the machine’s GUI (graphical user interface)(pronounced “goo-ee”, if you were wondering) on a computer screen. That display is made up of an array of pixels, each capable of showing almost any colour you care to name. Wouldn’t it make sense to achieve best picture clarity for the viewer by arranging for each pixel in the footage to correspond with a single pixel on the display? Surely, yes, because the alternatives reduce picture quality. Here’s how they do it:

If the image is displayed using fewer pixels than the footage actually contains (we technical people call that ‘smaller’) then many of the picture details simply get ‘ignored’ by the display because the software driving the display has to recreate the image (‘downsample’) so that each of the display’s pixels knows what colour to show. The downsampling software can sully the CCTV image too.

By the same argument, if the image is displayed using more pixels than the footage actually contains (we technical people call that ‘bigger’) then many of the picture details simply get created by the display equipment. Significantly, the software driving the display has to recreate the image (‘upsample’) so that each of the display’s pixels knows what colour to show. The upsampling software can sully the CCTV image too. It is simply fabricating its detailed information.

Widened CCTV image

Waaaayyyy too wide. Dragged within the GUI. Looking ridiculous.

In order to arrange the CCTV images on the GUI, be it a single image or maybe 25 images in a ‘mosaic’ of ‘cameos’, the software almost invariably shrinks or expands the images as described above, thereby reducing clarity. Sometimes the software will ‘auto-fit’ the array of images to fill the screen and change their aspect ratios in order to fill it, giving rather unhelpful distortion of the footage. Sometimes it will allow the operator to arrange and squash the images that they wish to watch into the available screen space by dragging and sizing floating ‘windows’ around the screen, leading to the same unhelpful distortion.

When testing DVRs for my customer I asked each product’s attendant salesman if their GUI included a button that simply set footage to the ideal 1:1 pixel assignment or, at least, something to force the correct aspect ratio to be restored. Their machines didn’t. In fact, only when I spoke to the actual designer of one well-established DVR did I get the response, “What a good idea! We’d not thought of that.” In the words of the average American teenager… “duh?!”

Ok, it’s easy for me to play the smart-arsed armchair expert here, but surelythe people who design these things should be thinking about these things. It strikes me that far too little thought often goes into these products from the point of view of the people who actually use them to do their job. So, the lesson here? Ask this question of your prospective DVR supplier. Heck, if you’ve already got one, ask them too and see the look on their face 😉 and suggest that they get one up on their competitors by putting this into their next software update.

Until next time; stay focused.

Annoying Aspects of CCTV (Part 2)

Following on the heels of my previous post I can’t ignore some other shortcomings in ways that CCTV footage is displayed by the machines sold in our market.

My point this time is that pictures are often displayed as too narrow. Ok, let’s not lose too much sleep over this one as the effect is not dramatic, but my beef is that it shows a disregard or ignorance of correct technique by those who design the CCTV equipment. This is usually when showing digital video on a computer display. Here’s the explanation:

Woody from Toy Story

Narrow headed Woody. © Disney

The normal 4:3 picture when digitized (D1 sampling) is 576 pixels high and 720 pixels wide. Yes, for those good at quick mental arithmetic, 4:3 would strictly mean that the 576 picture lines (from a PAL camera) would dictate that there be 768 pixels horizontally. For reasons of how best to digitally sample TV signals the world some time ago agreed that the sampling process will be designed to actually give 720 pixels. So, to avoid distortion when the picture is displayed they agreed that each pixel will be rectangular. If each pixel is about 7% wider than it is tall then we re-establish the correct 4:3 picture on the screen. Voila!

However, many bits of CCTV software simply display these rectangular pixels as square ones on a computer screen. So, the result is a display where images that are squashed sideways by 7%!

Back to the police radio: “the suspect is an IC1 male wearing yellow and with a really narrow head….Yeah, looks like Woody off that Toy Story movie”?!

The solution to this wrong practice? Be aware of the issue and ask your supplier to show you that their machine does not have this shortcoming. It’s quite likely that they will have this problem, if what I’ve seen around the market is anything to go by! However, the more we as independent CCTV consultants pressure them to get it right the likelier it is that they’ll put some effort into fixing their products when they realise that thier customers not as dumb as they assume.

Until next time; stay focused!

Annoying Aspects of CCTV (Part 1)

A photograph in a recent issue of that fine CCTV Image magazine, the organ of the CCTV User Group, showed a spanking new CCTV control room. The proud owners had invested in a bank of monitors often known as a video wall. Nothing unusual there, you might say to yourself. Indeed. (My thoughts on how to save money, power consumption and cooling costs in your control room by avoiding the need for a video wall will wait for another day.)
CCTV image with 4:3 aspect ratio

CCTV image with 4:3 aspect ratio

In CCTV where, disappointingly, end-users’ technical advice can come from an ex-policemen who doesn’t really know a pixel from a pixie or from a salesman who thinks “codecs” is spelled “codex” (because he once read The Da Vinci Code), we often see regular CCTV images being inappropriately stretched across widescreen monitors. Please, no.

Look at it, then think about it. Take the normal 4:3 CCTV picture and stretch it over a typical 16:9 LCD and you end up with a sideways distortion of 33%. That’s a whole 1/3!

CCTV image with 16:9 aspect ratio

CCTV image with 16:9 aspect ratio

Why should this annoy us? Because it’s surely ridiculous to take a picture from a normal CCTV camera with its 4:3 aspect ratio and dumbly stretch it over a big LCD or plasma screen designed for 16:9 widescreen pictures. Stewie Griffin from Family GuyThere’s no advantageous level of detail being created; just a profound sideways distortion that indicates a disregard for proper system design. So, is any benefit whatsoever? Nope. Can you imagine the CCTV operator calling the police on the radio and telling them, “the suspect is a IC1 male wearing yellow and with a curiously wide head….Yeah, looks like Stewie Griffin off that Family Guy cartoon”?! But more seriously, why tell responders to look for a “fat man” when it’s the video display that has falsely added the weight to his appearance?

This common arrangement, to me, shows ignorance. But that doesn’t stop it being a very common sight in CCTV control rooms around the UK and probably the rest of the world too. Ok, if they’ve bought widescreen LCD monitors in anticipation of displaying HD CCTV images in the future (which are 16:9) then that’s acceptable forward planning. But, please, in the meantime show 4:3 pictures properly. At worst, doing it will give you a black margin down each side of the screen. It’ll be correct for the humans to watch and it will look fine. Honestly.

It flabbergasts me that our industry so rarely bothers to get this right. Of course, some diligent folks do exist, but they are rare, and the aforementioned so-called ‘consultant’ numpties can fool an unwary client just enough to get themselves hired. Please, for your own benefit, make sure you hire CCTV consultants who know what they’re doing. One advantage is that your pictures will be shown to your CCTV operators accurately. The pictures are the reason you bought CCTV in the first place.

Keeping come back, folks. Who knows, one day I might post something that not purely a dig at the other practitioners in the field! 😉  Still, I think these points need to be made by someone. 

Until next time; stay focused.

Designing CCTV using 3D

 

Security Institute logoI must start with an apology for the long gap between blog posts. June simply flew by and much of it was spent writing, just not here! I was writing for a presentation I was invited to give at a seminar on 17th June sponsored by The Security Institute as part of their CPD programme (for those sporting a quizzical expression – Continuing Professional Development). It’s important for me to thank Nicky Stokes, MD of ISD Tech Ltd., who invited me to participate and his team who put a lot of work into hosting the event at Dunsfold Aerodrome which is where they film the astoundingly popular Top Gear television programme.

The ISD Tech team at Dunsfold (Photograph © Simon Lambert. All rights reserved)

The subject that I was asked to speak on was my work as an independent CCTV consultant using 3D graphics technology to design CCTV and security systems. This is something that I’ve actively pioneering since 2000. At that time the software was expensive and had a long learning curve, even for a techie type like me. My message to the audience in 2010 is that there is a great array of free software that slowly, slowly others are beginning to use in the security and CCTV industries.

To summarize what I told them: freely available 3D modelling applications include:
Google Sketchup
trueSpace 7.6
Second Life (virtual world)
Blender
Unity (video gaming engine)

A package that I intend to trial one day is CCTV Cad which is specifically designed for use in the CCTV industry. However, it comes at a significant price so warrants closer evaluation.

So, why would anyone bother using 3D CGI (Computer Generated Images) in CCTV?
My experience has shown:
– 2D is often vitally misleading. 3D shows the reality.
– A picture replaces 1000 words, and everyone understands.
– CCTV designer, installer and their customer’s confidence is high.
– The costs of fixing failed expectations are kept to a minimum.
– 3D software tools are getting cheaper and more amazing every year.
– Even in 2010 the “wow” factor with 3D is still undeniable.

If you’d like to read more here is a copy of an article that I wrote for ‘CCTV Image’ and ‘CCTV Focus’ magazines explaining the real benefits of using 3D in CCTV design.

Until next time; stay focused.

“On CCTV 300 times a day.” Rubbish. And here’s why.

I was  prompted to write this today owing to a TV news report that stated each of us is likely “caught” on camera “330 times a day”. What?!  

First, a subsidiary point in this outrage is that when we’re viewed by a CCTV camera we shouldn’t feel “caught”. Why the negative wording, eh, Mr. TV Reporter? Are you “caught” when a bobby-on-the-beat sees you walking along in accordance with the law of the land? No. So why say that about CCTV? Sensationalist agenda from the mass media? Hmm. Anyway, on to the main point of this blog post.  

300 cctv cameras in one day

Thomas thought it would save time to go and get his "300 a day" over in one go. Image © Simon Lambert

I couldn’t help but notice the ubiquitous urban myth that each of us is view by 300 CCTV cameras per day in the UK has been inflated in this tv report to 330. Where did the additional 30 times a day spring from? I imagine someone decided that the figure must have gone up since it was originally ‘calculated’ and that 10% seems like a reasonable wild stab in the dark to a journalist looking to advance their career with breaking news, eh what? Tosh.  

So where did the 300 figure come from in the first place? Can it be relied upon as the truth of the matter? The media certainly trot it out as ‘fact’ at every opportunity, and we know how trustworthy they are 😉  This was addressed recently in respected magazine Wired in ‘A Sharp Focus on CCTV’ by Heather Brooke. She wrote that Simon Davies of Privacy International walked London from Blackfriars to Bond Street in the earlier 1990’s and counted cameras before multiplying this up. How on earth can that represent the whole of the UK? It’s laughable, so let’s dismiss that.  

Heather mentions a much more illuminating article by David Aaronovitch in The Times in 2007, so I went hunting for it. It’s quite a long article, so here are the salient points in short form with due credit to Mr. Aaronovitch for his diligence.  

  • The Information Commissioner’s 2006 “Report of the Surveillance Society” states “a person can be captured on over 300 cameras each day”.
  • The media reword this as “the average person is”, as distinct from the original, “a person can be”, which foments far less unrest.
  • The ICO quotes 1999 book “The Maximum Surveillance Society” as the source of this number 300.
  • This book describes a fictional journey around London where the hero visits in one day:
    – his home estate that has a drugs problem
    – two schools
    – a hospital maternity wing
    – his work place
    – shops
    – a railway crossing
    – several car parks
    – public transport
    – Heathrow airport
    – a football stadium
    – a red-light district
    – a speed camera in his car.
  • It seems you have to work very, very hard to be viewed by 300 cameras, even in this fictional construction.

So, the figure really is rubbish. I hope you’re gobsmacked by the apparent truth behind the hackneyed headlines. Being viewed by 300 cameras a day is demonstrably an urban myth with practically no foundation in fact. It’s the product of story about a fictional fellow making a wholly non-average journey in London. The actual number has not been determined and let’s not forget, London isn’t Swansea, which isn’t Inverness; so don’t generalize for everwhere in the UK. It has been ‘laundered’ from one academic tome to another until it’s origins in a fictional piece are deeply buried. Then it has been misquoted by the media so many times that it has become an almost irresistable virus. Rubbish though it be, I’m astounded to see that this now ‘de facto’ statement has just been arbitrarily given a 10% boost with no discernible justification. Don’t stand for it. Say something to the people who need to know. 

Until next time; stay focused.

The CSI Effect. Not a good thing.

Forensic investigators all over the world know about a modern phenomenon called ‘The CSI Effect‘. The popular television shows such as ‘CSI’ which hit US screens in late 2000 have given people an unrealistic impression of what forensic investigators can achieve. Generalizing here, it seems that the realities of forensic investigation are far less impressive and certain that the television dramas would have us believe. Want some real-world examples from a seasoned video forensics investigator? Read this. It’s a hoot.

x4 enlargement shows that the detail simply isn't there But on CSI? Of course, it would.

In my role as an independent CCTV consultant I’ve never ceased to be appalled by the number of clients who are willing to accept low quality or poorly designed CCTV systems because, they tell me, “If anything happens the police can enhance the pictures.”

That’s where I see the CSI effect day to day. The first time was on one of my earliest major assignments when the security manager of a world famous site said it. He was getting tired of poring over large numbers of layout drawings with me. We were creating an ‘Operational Requirement’ according to the then British Standard BS EN 50132-7 with regard to the Rotakin target criteria. We were trying to design CCTV to protect the sort of premises that is regularly attended by VIPs at the highest level. Skimping on the design and accepting poor image detail did not strike me as a good idea and I had to insist that we complete the process properly.

It seems that there is a huge body of evidence, unfortunately much of it only anecdotal, supporting the realization that CCTV owners, installers, police and members of the jury really don’t understand the technologies limitations and make poor decisions as a result of their crazy expectations. I feel strongly that these expectations need to be put straight. I guess that’s one reason why I do what I do. If you agree, do let me know.

Until next time; stay focused.

Why hire CCTV Consultants?

If you can afford for your CCTV to let you down just when you need it to deliver, fine, don’t waste more money hiring a consultant who’s capable of making sure it’s working properly. However, should that thought cause you sleepless nights the next five minutes reading are definitely right up your street. 

So how will hiring a consultant change things for the better? Well, let’s remind ourselves of some very common issues.

How often are CCTV pictures found to be poor or useless? Often a CCTV specialist’s customers have a naïve expectation borne of watching tv drama like ‘Spooks’ and ‘CSI’ that poor pictures can be enhanced enough to be useful. The vast majority can’t, so ‘those responsible’ may become the focus of their disappointment. Sub-standard pictures result from badly adjusted lenses, poor lighting, inadequate recording equipment, etc. Unrealistic design briefs, inexpertly compiled technical specs or shoddy installation and maintenance are often to blame.

Simon Lambert trapped inside a monitor

To avoid these pitfalls you need a competent expert on your side. You can still play the Lone Ranger and claim the glory if you’re brave enough, but you’ll be glad that a Technical Tonto is also there to quietly save you from hidden quicksands.

But, I hear you end-users cry, our CCTV company provide that expertise as part of their service. Think. If your installer/maintainer is so reliable then how come they let you suffer the aforementioned embarrassments? After years conducting training courses I’d estimate that 95% of engineers coming into the classroom simply don’t understand how to set up your CCTV properly. This would be less of a worry regarding your paper towel dispensers, for instance. But this is security, so we should all be very concerned at this sorry state of affairs.

People are often oblivious to their problems. An independent expert can unearth all sorts of important issues that have gone unnoticed and threaten to deliver you a nasty surprise one day. CCTV is undeniably technical involving electronics, physics, operational and legal issues. Systems are expensive to establish and run so the cost of this ‘insurance policy’ will be small beer in comparison, and a very wise investment. Indeed, it can more than pay for itself by saving the costs of unnecessary replacements and upgrades touted by partisan salesmen. Does their product do what the glossy leaflet promises? Very often not, but only impartial CCTV consultants skilled and properly equipped to test it will uncover this and present you the real figures before you decide to part with real money. These are very rare creatures, so look carefully.

Beyond taking all these tasks off your plate, and carrying the can for them, there are other advantages. It’s strange but true; things you’ve told your organization for years but get consistently ignored are somehow heeded when the consultant arrives and says the same things. Albeit galling; now your message gets through. Result.

So, how do you choose the right consultant? Type “CCTV consultant” into Google and all sorts will come up. Those listed range from science graduates to ex-police/military. Clearly their individual experiences may suit different assignments. Unashamed ‘techies’ good at hunting salesmen’s snake-oil and the engineers’ shortcomings have trained differently from ‘non-technical’ people who are more used to writing Codes of Practice or team management. Be aware that when they are vying to win business even consultants can be prone to claiming glory for projects to which they made a limited contribution at best. Caveat emptor.

So, can having the guts to hire a good consultant fix your CCTV’s shortcomings? Highly likely, if you heed their good advice. If ignored and trouble ensues please don’t make them scapegoats. Worryingly, some bodies shun “consultants” on the basis of one unsatisfactory case. Isn’t that simply cutting off their nose to spite their face? Now you know how to sleep soundly.

Until next time; stay focused.

No! Image degradation is not ok.

Thanks go to George Reis of Imaging Forensics for highlighting this matter in his blog. I was prompted to put finger to keyboard upon reading his post because just last week I was banging on about this very subject in my presentation to the Association of Security Consultants in London at their Business Club meeting. If you’ll permit me to blow my own trumpet for a moment, I was genuinely, pleasantly, astounded at how many of the audience came up to me afterwards to say how much they were glad to hear someone speak up with the truth of the situation. They tell me it is very rare.

I was calling for users (and suppliers) of digital video recording systems to deliberately set up their machines for ‘Forensic Fidelity’ if there is any chance that their footage will be used in a forensic investigation. Might their recordings be used to identify an assailant or a murderer? Or might it even exonerate an innocent person? Put yourself in the shoes of the latter. Would you want to be that person relying for your liberty on a disgracefully unclear video that comes from your CCTV system or, if you’re a supplier, the CCTV of one of your customers?

Ask a forensic investigator like George how often his efforts to glean vital details from security video are thwarted by a lack of clarity brought on by excessive image degradation owing to digital compression. More and more often, I’ll wager. Manufacturers, installers and end-users are, in the vast majority of cases, far too ready to cut corners for economic reasons. They compress data far too heavily in order to save on storage capacity which costs money. Probably no independent CCTV consultant worth his salt has been asked to check that all is up to scratch for the task in hand. Big mistake. Again, the salesman’s smoke, mirrors, and head-in-the-sand view of his product’s inadequacies will rule the day and the reputation of CCTV advances one more inch around the U-bend.

The magazine article that George refers to is touting the apparent virtues of H.264 video compression and says “In surveillance, you can compress it without [worrying about] distortion of the image. Say you store the images of a parking garage overnight. If there’s a bit of distortion in the image, it’s not a problem.”

What? What?! Look at the opening paragraphs of this post and you’ll understand my exclamation and shock. It is a problem. Often a very serious one. Don’t shrug it off so glibly.

Later in the article: “They might say, ‘I want to keep very high quality 2 megapixels per second.’ That’s low for broadcast, but good for surveillance. You don’t see the degradation.”

At risk of seeming to repeat myself… What? What?!
No, sir, it’s not good for surveillance where you will see the degradation in a great many situations. You, sir, and the rest of the CCTV industry, should stop telling people such things. It’s tosh that sets the bar so low that when I ask suppliers to give us a reasonable ‘forensic fidelity’ at 4 or 6 or even 8Mbps they look at me like I’m barmy. No, penny pinching  and a lack of appreciation of the vital realities are the only barmy elements in this common situation.

This industry needs to wake up and realise what a huge disservice it is doing the owners of CCTV, the police who hope to use it as evidence, the forensic investigators who struggle to glean the information they want, and the people in the CCTV pictures who either deserve to be locked up or set free so that justice is properly served. Image distortion is a problem.

More on H.264 and on snake oil salesmen another day.

Until next time; stay focused.

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.