This is the third and final installment of a complete illustration on how to calculate the hard drive capacity of a Digital VCR.
by Germán Alexis Cortés*
As stated in the previous installments, many times in the electronic security industry, it happens that we sell CCTV systems without doing the proper analysis. In fact, only a few people know exactly what calculations should be made and I believe that very few of them actually do. The rest are content to choose a brand of equipment and check which model offers a lower price.
So it is very useful that we analyze in detail the other aspects that influence the calculation, in this third installment we will include five other points that must be taken into account:
9. Activity in each scene
This is a fundamental variable that is often ignored and if we calculate it, we make the hard drive smaller than it apparently should be, saving a lot of money. The percentage of activity tells me how static or changing the scene is.
To give an example, imagine a camera that observes a hallway in a university, while classes are taking place (15% Activity). The same corridor, but at the exit of each class (100% activity). The same corridor at dawn (3% of Activity). Therefore we must segment the day into several types of scenes and calculate the percentage of activity of each one.
Again the visit to the site (Site Survey) is very important. Just by asking the end user which are the peak hours and which are the ones with the lowest traffic, we can segment the day. Then in the visit in the peak and valley hours, we are located in the place where we would place the camera and we can take an average of activity, with a stopwatch that we let run while our eyes see movement and we stop it as soon as it ceases, we repeat the process as many times as necessary during a certain time, for example 5 minutes.
At the end we will see that on our clock there are marked, for example 3 minutes 12 seconds. The percentage of activity is calculated with a simple rule of three, as it is a totally linear and proportional relationship. So when unifying units (seconds) we see that the percentage ratio is 192/300, which indicates that the activity was 64%.
The same exercise, takes time and calculation we must do in the moments of valley (little traffic). And what is all this for? To decrease the capacity of the hard drive and not have to buy it so large. Mathematically multiply by a percentage less than 100%, will always give less Hard Drive.
Some Hard Drive Capacity calculators do not take into account the percentage of activity and others do not allow you to segment the day. Be careful, because not having these factors, will result in a Hard Drive larger than necessary. And let's understand that manufacturers are not interested in being bought less. Sometimes, just to give an example, the necessary hard drive is 10 TB, assuming constant activity of 100%, but in real life, doing the activity calculations and segmenting the day into different ways, we get that the disk needed for the same number of days is only 4TB is just an example.
Would you risk not doing the math and allowing your competition to do it, winning the project?
10. Recording Method
Current technologies allow countless ways to record images, among which we can highlight:
a. Continuous Recording. Widely used in high-risk sites, such as casinos, government areas, prisons, among others. It is about recording at the maximum speed of the standard (NTSC: 30 FPS).
b. Time-Lapse Recording. That is, record at fewer frames per second than the commercial standard. The speed depends on the risk and importance of the scene being stored, but above all on what you really want to get out of the image. In this way, they record the vast majority of DVRs and NVRs today.
c. By events. In this case, an event can be caused internally in the DVR (video loss, sudden image change, camera lock, image change detection, among others) or it can be generated externally, through the alarm input ports (traditionally one for each video channel). The concept of intensive recording is then handled when there is an alarm situation caused by an event. That is, it goes from low speed and resolution to high resolution and speed for a certain time. It is really the best way to program your DVR.
For example at night, record at very low resolution (QCIF) and speed (0.5 FPS), to create a good sense of security of the owner and the Head of Security, but schedule to record at resolution (D1) and speed (30 FPS), when the image change detector detects a strange condition.
This only lasts a few seconds, and ensures that in case of high-risk situations, the DVR is attentive to recording the best possible information. The same can happen, with other less demanding scenes, during the day. In this way the way of recording will depend on the conditions of the scene and not on a cold static parameter that is often not enough or that sometimes devours huge amounts of Hard Drive. This factor alters the formula in the number of Frames that are recorded per second.
11. Amount of Information You Wish to Store
In this case, we are referring to the pre-alarm, alarm and post-alarm. Many DVRs/NVRs can work only for events. That is, they do not record until there is a change of image, however in real life, they are recording continuously. An internal algorithm of the software, monitors if there is an internal or external event.
If it does not occur, it overwrites in memory the last seconds of video, but if an event happens, these seconds are adhered to the alarm video and something very valuable is achieved for the security area, and that is the possibility of seeing the alarm clip including the seconds before the event. Sometimes these seconds are enough to identify the face of the offender or to know the real origin of the incident. In DVRs/NVRs this possibility is called pre-alarm and the amount of time prior to the event itself can be configured.
The same happens after the event (external or internal) has passed, where the DVR is told to record a few extra seconds, after everything returns to normal. It is obvious that the more seconds you have to burn, the more hard disk we will need. However, there is nothing better than having the complete information of an abnormal situation. When calculating the Hard Drive, these extra seconds must be taken into account, which directly affect the actual percentage of activity.
12. Importance you want to give to each scene
It is all that many times the security expert can determine. However, this is not a factor that applies in the formula, but a parameter that determines the other variables. Each scene may be important at a certain time of day or week, and yet not be important at other times.
Being fully aware of this way of segmenting the day and configuring the programming of the video recorder, is the golden formula, to save hard drive, without sacrificing quality in the information. Not all DVRs allow you to do it, but in many that you can, I have seen that it is not done.
The scene showing the entry of employees into a factory is not as important as the scene showing emergency doors; however what happens if you generalize and then in an evacuation situation do not record what corresponds? Or worse, always record the door motionless for fear of losing valuable information? This requires that there is a mechanism that changes the hierarchy of the cameras, according to a schedule or the presence of an event (internal or external).
If your DVR doesn't have these capabilities, you're likely to incur cost overruns for a very large hard drive or feelings of frustration for a very small hard drive, which won't allow you to record what's required.
With these thirteen (13) factors analyzed, then we return the formula again, to calculate a second of video. Notice that everything analyzed, directly or indirectly affects any of these three factors. By segmenting the day, the number of hours for each scene is analyzed and we can then add how much is spent daily and per month.
For example, we are told that a DVR on average occupies 8 KBytes per frame, in D1 resolution. We want to record at 20 FPS in a scene where we have measured 45% activity, during the day (7:00 a.m. At 6:00 p.m.), but at night the activity drops to 15%. If we want to record for a full week, what hard drive do I need?
We must apply the formula twice, when it is day and when it is night:
For the day case (11 hours), the formula is: 8KBx20x0,45x3600x11x7=19 GB
For the night case (13 hours), the formula is: 8KBx20x0.15x3600x13x7=7.5 GB
To the total we must add 15% for the Operating System, virtual memory and others. Therefore the Disk we need for that camera is 30 GB, if you assume that there are 16 similar cameras that we must record a month, the necessary disc will be 2TB approx.
Final Notes
Be wary of rudimentary and easy calculators. Only use those that are complete, according to the segmentation of the day. Prefer to do the calculations in a simple spreadsheet and present them to the end user, showing the different alternatives studied and along with him, change some parameters. I assure you that if you show knowledge and experience, you will gain the trust of the end user.
In case the end user does not have the entire budget for the Hard Drive calculated, then decrease the number of days to record. In no way agree to decrease quality, resolution, speed or other alternatives, which only impoverish and deteriorate the stored images.
In the next opportunities, we will see how to back up a large hard drive and how are the options of external hard drives type RAID, in NAS or SAN configuration.
* Germán Alexis Cortés H. is a Colombian Electronic Engineer, with a postgraduate degree in Engineering Management Systems. Consultant of the National Institute of Standards and Technology – NIST, in Electronic Security and Building Automation. CCP of ASIS, and CISSP of ISC2. He has over 20 years of experience in the electronic security industry. Renowned lecturer and university professor on high-tech issues at the Latino level. He has been a director of several companies in the sector and is currently a partner and directs Insetrón Ltda, an engineering and technical consulting company in electronic network projects for security, communications and automation. He has successfully participated in more than 150 projects at the Latino level.
