Hello again. I realize that I am a couple of weeks behind on our continuing quest to go digital, but I have a very good excuse worked up and it's ready to fertilize the nearest garden.
The fact is that I have taken on a new position with a formidable company. This all started some five months ago and I needed to get caught up on the basic fundamentals of working for a corporation again. It has, after all, been over 25 years since I received a check with a signature other than my own in the authorization slot.
For those of you that have been emailing me with comments, questions and the such, I apologize for not responding. I am not trying to be rude; I have just been inundated by everything as of late, so please accept this as my blanket apology to all -- especially to Jocelyn Getson and the crew of Vision Technology International, who wrote and asked specifically if I would be continuing with this series on going digital.
Please note that I have a new email address for you to write to (see the About the Author footer at the end of this column). Please keep the comments and such coming and I will respond; I promise.
Does this job situation mean that I am no longer printing/distributing my books and training programs? Absolutely not. LeapFrog Training & Consulting is still alive and well and our technical library and various products can be found at www.LTCTrainingCntr.com. LeapFrog is slowly narrowing down its offerings, but this training center is alive and well just the same. I too am still very much alive, well and available for any/all that are having questions and problems, or for those who just want to comment.
OK, this is a good place for a review of our current standing on the process of going digital. The opening article, the one that actually started the series, concentrated on the whole definition of what going digital means and covered the general principals of designing CCTV systems with purpose. Article II ran through the process of choosing the proper camera for your application. Article III discussed lighting situations. Article IV covered analog and digital resolution ... the key differences and problems with both. Article V discussed choosing lenses and some of the intricacies of math and vision. Article VI, the last posted message before I had to take a break, ran through cabling and transmissions.
This week it seems quite natural that we start to discuss storage of the video image. Please understand that this is a very involved conversation and will end up taking two or even three separate articles. But no long road is ever walked without the first step.
A little history to digest
History ... again with the history. I remember when video was recorded on reel to reel units (Yes, I really am that old). This was before the advent of the cassette players of the mid-1970s. Everything that we hold as fact for recording video images today dates back to these original systems and opens our general perception of video recording. The major topic of any storage system today, be it VHS, DVR or hard drive, rotates around our understanding of the beginning. The first subject is called "real" time.
With the introduction of digital recording units, the first mad-capped statement coming out of the manufacturer's lips was that the security industry did not need to record in "real time", but that two to five images per second was plenty. I was one of the first to stand up and mock this statement, and I did so adamantly! I wasn't upset by the fact that the new, modern digital format recording systems could only record two or three images per second. I was upset that anyone would use such a blatant, blanket statement about visual security.
I was also upset that the world was moving on. Over the next couple of years, I heard more and more manufacturers claiming that their digital unit could record in "real time". Finally, it hit me. I wasn't upset about recording fewer images per second, I was upset by the misuse of the terminology "real time".
If you break down what is meant by real time according to common use of the terminology, then the first problem you run into is that real time is a matter of interpretation. If you are in the U.S., Canada, Japan, and parts of South America, your understanding of real time is different from the rest of the world. This is because you all work on NTSC standards, while the rest of the world works with PAL standards. This means that your interpretation of real time is 30 images per second, while the rest of the world perceives it as 25 images per second.
What real time really means
The fact is that both sides of the world are wrong. Here's some more history for you. Real time is a slang expression that came from the amount of time it took to use a full reel of tape on a reel-to-reel recorder. It was adapted to the cassette player and somewhere along the way, people slowly forgot the lessons of their grandparents and misspelled the word "reel" as "real". With that new spelling came a new misconception of reel/real time.
Lesson one in digital image storage: What is real time? Real time is relative to the application and should never be confused as a standard. Be it one image per second or 25 images or even 30 images per second, no set number of images per second constitutes real time.
So the next question that should come to your mind is: "How many images per second do I need to store?" And the answer is that I don't know. What is your application? If you are mounting a camera in a bank lobby and the purpose of the camera is to produce an identifiable image of all persons that pass into and out of the lobby, then your math works out as follows:
- The average time for a person to walk across the lobby is three seconds while the fastest time that a person can run across the lobby is one second.
- The application requires that one image of all persons entering or exiting the lobby is recorded.
- There is a slight blind spot directly under the camera that will account for a half second of lost imaging.
All right, based upon the information provided, what is your opinion on how many images, per second you should record of this bank camera Is it just one? Do you need two per second?
Personally, I would recommend between three and five images per second, three images being the minimum and five images being slightly overkill. Three images per second guarantees that I will have at least one image of a person running. Since I always plan for worst case, I am covering my bases. In the event of an alarm of some sort, I could have this image rate increase exponentially. If the lobby is a high-risk area, I could start recording 30 images per second. If the lobby is a "so-so" risk, I could leave it at three or five images per second. Again, the application will drive the design and requirements. This is the process that is used to determine the minimum required images that are stored for each camera in a system.
This is also the first point of consideration when recording with digital systems. If your system design requires flexibility because of multiple, intertwined applications, you will look for features in your storage system that allow for independent programming of images per second, per camera. Additionally, if you are automating your system through integration with alarms and/or other such triggers, you may also want or require that the storage rates of images per second is able to be increased or decreased on a per camera basis during specific events. This whole concept is the strongest advantage of digital storage systems as opposed to VHS.
What part of the video to store
The next logical question that comes to most minds is: "Why can't I just record 30 or more images per second from all my cameras?" This answer also dates back all the way to the reel-to-reel recorder. First, the timing sequence of images that are or were produced by cameras was and still is based primarily upon the frequency of the input power. Those areas that work on the NTSC standards are areas where the locally provided power is 60 hertz. In PAL portions of the world, the power is 50 hertz. Divide either by two and you have your standard images per second as created by the camera for each standard. Do we have cameras that produce more than the standard 25 or 30 images per second? Yes, we have units that can produce 10,000 or more images per second. However, these are not the typical cameras that are found in most security situations.
Let's get back to the relationship between reel-to-reel and modern digital recording. The first step to recording a video image is to understand that each image takes up space. Think about one image as one gallon of water. How big of a container do you need to hold it? One quart? One gallon? Ten gallons? For a half-inch video tape on a reel, let's consider it to be a three quart jar. Unfortunately, we have to fit the gallon image onto the tape. So, we do a couple of things. First, we dehydrate the gallon a bit (we lose some resolution). Second, we put the recorded image on a slant across the tape so that instead of it being a half-inch wide, it's more like 2/3-inch wide. Next, we restrict the number of images per second (the final portion of our water volume) to 25 or 30. Bring this concept forward to digital storage and the same concepts hold true. We only have so much room to store our water. So the first point of dehydration is to restrict the number of images that we store per camera, per second. This "dehydration" will remain so as long as the cost of a 10-gallon jug far exceeds the cost of the 1-gallon jug.
I know some of this may seem pretty basic (and pretty historic), but it's a good overall start for our trek of going digital with video storage. I will see you in about three weeks with a very detailed column on digital storage systems. Who knows, there might even be room enough for a frank discussion on compression factors. Until then, I remain faithfully in your service.
About the Author: Richard R. "Charlie" Pierce has been an active member of the security industry since 1974. He is the founder and past president of LRC Electronics Company, a full service warranty/non-warranty repair center for CCTV equipment. In 1985, Charlie founded LeapFrog Training & Consulting (Formally LTC Training Center), a full service training center specializing in live seminars, video-format certification training programs, plain language technical manuals and educational support on CCTV. He has also recently become the director of integrated security technologies for IPC International, Corp. He is an active member of ASIS, ALAS, CANASA, NBFAA, NAAA and SIA. He is the recipient of numerous security industry awards, and is a regular contributor to Security Technology & Design magazine. Look for his columns to also appear regularly via SecurityInfoWatch.com and this website's e-newsletters. He can be contacted via email at [email protected].
