Brian Karas: February 2012 Archives

Recording Overview

The most common operational mode of the iCVR is to record video at a rate suitable to yield a usable image for basic viewing and verification, and then to do event-­‐based scaling to a higher resolution stream when an analytics rule is violated. We generally refer to the stream that represents the continuous recording rate as the "LowQ" stream, and the stream that represents the higher quality video as the "HiQ" stream.

The iCVR can also be configured to do continuous recording at the highest possible quality, or to initiate events, but not scale the video resolution up during these events. Although these are both perfectly valid use cases, they represent an atypical deployment, and will not be covered in detail in this document.

Because the iCVR's storage usage is dependant on the number and duration of events in a given time period, it can sometimes seem difficult to estimate the recording time you will achieve from a given camera model. In reality you can often calculate the expected storage time in a straight-­‐forward manner, this document is intended to assist in the understanding and calculation of iCVR storage times.

If you were to visualize the continuous LowQ recorded stream, it would look like the following diagram, essentially a video stream recorded at a near-­constant bitrate:


Now, if we factor in the analytics-­‐driven events, the recording timeline would be visualized like this:


The LowQ stream continues to record, and as events occur the iCVR records a secondary video stream (the HiQ stream) for the event duration. It starts to become apparent that total recording time becomes a function of the LowQ recording rate AND the number and duration of alarm events, along with the HiQ recording settings.

Understanding Recording Calculations

Because the LowQ recording is continuous, and generally the primary consumer of disk space, it's often helpful to first calculate the longest possible storage time, which would be a scenario where there were no alarm events (or where alarm events did not use the increased resolution stream).

The first rule of thumb is that every 100Kbps of recording rate uses ~1.1GB of hard drive space per day.

Note: The iCVR manages streams primarily by bitrate, not by resolution or framerate. You can of course configure different frame rates and resolution rates (depending on product model) for a target bit rate, but the bottom-­line factor is the bitrate.

It also stands to reason that a D1 5fps stream at 100Kbps would look better than a 960x540 15fps stream at 100Kbps, as the camera would be attempting to fit less information in the data "container" at the D1 (704x480px) resolution than at the 960x540px resolution. The iCVR attempts to maintain a near-­constant bitrate, however in cases of extremely static scenes the bitrate may dip BELOW the configured settings, and in extremely active scenes the bitrate may burst ABOVE the configured settings for brief durations. However, the average over time tends to be very close to the configured settings.

For any given hard drive in an iCVR camera or encoder, approximately 15GB is reserved for the video index database and other logging functions, the remainder being left for actual video recording.

Example: An iCVR SD Dome camera with a 160GB hard drive would have approximately 145GB for actual video recording. The LowQ stream on SD resolution iCVR products is 100Kbps, which equates to 1.1GB per day of recording space. Thus:

145GB / 1.1GB = 131.8 -­‐ We would get 131 Days, or a little over 4 months, of continuous recording if there were no alarm events.

The default alarm event recording rate for the SD units is 500Kbps. This is 5.5GB per day if recorded continuously, but alarm events are sporadic events throughout the day. 5.5GB/24Hours is .23GB, or 230MB per hour. Every hour of alarm events per day uses 230MB of space, in addition to the space the LowQ stream is using. One hour of alarm events per day would make the daily hard drive space consumed: 1.1GB + .23GB = 1.33GB.

145GB / 1.33GB = 109 -­‐ We would get more than 3 months of continuous recording if we averaged 1 hour of alarm events per day.

One hour of daily alarm events has reduced the total storage time by 22 days. As total alarm event time goes up, total retention time goes down proportionately.

Calculating Storage Time and Choosing Storage Settings

Various combinations of LowQ and HiQ recording settings will yield different amounts of hard drive consumption. The following charts show how much hard drive space would be consumed per day for .25,1,2, and 4 hours of alarm events per day. A site using the iCVR primarily as a video intrusion detection device will typically see very low amounts of alarm activity (even 15 minutes per day is a high average), conversely a site setup to create a higher volume of alarms may reach 1-­‐2 hours of average alarm events per day. 4 hours of alarm events per day is rarely encountered, and is illustrated here to give an idea of minimal storage times in different configurations.

To use these charts:
oFirst, reference the chart that most closely resembles your expected average daily

alarm rate

oSecond, find the column under the LowQ settings that correlates to the recording rate you will use for non-­‐alarm (LowQ) video

oThird, look across the HiQ row for the alarm recording rate you will use and find the intersection of LowQ column and HiQ row. This is the average daily hard drive consumption in gigabytes (GB) that you should anticipate.

oLast, divide the number from step 3 above by the hard drive size in your iCVR to get the anticipated days of storage.

Example: Using the 1 Hour chart with an iCVR-­‐HD camera. The LowQ recording rate is set for 750Kbps and the HiQ Rate is set for 3000Kbps, the intersection of these rates is 9.5GB. A 250GB iCVR-­‐HD would give ~22 days of storage (250GB HDD - 15GB reserved space = 235GB. 235GB/9.5GB per day = 24.7 days)

15 Minutes/Day Average Alarm Activity


1 Hour/Day Average Alarm Activity


2 Hours/Day Average Alarm Activity


4 Hours/Day Average Alarm Activity


You can also use these charts to find LowQ and HiQ recording options to ensure that you will select the appropriate hard drive size for a desired amount of storage time.

Example: You want 14 days of storage from an iCVR-­‐HD camera. You anticipate your environment will have about 1 hour/day of alarm activity. We can consider a 500GB unit has 485GB of available storage space, and a 250GB unit has 235GB of available storage space. 485GB/14Days = 34.6GB/day and 235GB/14Days = 16.8GB/day. Using this data, we would look for settings on the 1 hour chart that correlate to 34.6GB or 16.8GB (or less) per day. We can see there are several options that use 16.8GB or less, meaning we could easily expect 2 weeks of continuous recording time from a 250GB iCVR-­‐HD with most settings, even if some days the activity level is higher than anticipated.

Common Settings

The most common settings for the iCVR-­‐SD Cameras and Encoders is LowQ at 100Kbps (on the SD models, the LowQ stream is always 100Kbps) and the HiQ stream at 500Kbps (factory default). The 250, 500 and 750Kbps HiQ recording options on the SD cameras have been boxed off in the 100Kbps LowQ column for easy reference when looking up values for those models. For most applications, the difference in drive space consumption per day at 750Kbps vs. 500Kbps is negligible, so it may be worthwhile to bump up the settings to the higher rate to get more detailed video.

For the iCVR-­‐HD Cameras, the most common settings are LowQ at 750Kbps, and HiQ at 3000Kbps. Using the reference charts above, we can see that 10-­‐11GB/Day is a good estimate for iCVR-­‐HD drive space usage. 

About this Archive

This page is an archive of recent entries written by Brian Karas in February 2012.

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