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Size Matters


A file is a collection of data. File size refers to how much data is contained in this single unit of measure. The larger the file size, the more bandwidth (the amount of data that can be transmitted in a fixed amount of time) and computer resources are required to display, record and transfer a file.

Conversely, a smaller file, of the same amount of data, will use fewer computing resources, display and record more effectively and will transfer across networks more efficiently.

The size of a file of video data can be affected by image size (resolution), bitrate, motion, compression and various other factors, as previously discussed. Ultimately this can affect the amount of video you will be able to store on the DVR’s hard drive and overall performance when viewing and recording multiple cameras simultaneously. As cautioned previously, beware of performance claims; they are usually referring to results achieved under the most optimal conditions.

As a rule, the higher the compression ratio, the smaller the file size. The smaller the file, the less sharp the image, the quicker the transmission speed, the less storage space and processing power required. Conversely, a lower compression ratio results in a larger file size, sharper image with a slower transmission speed and more storage space and processing power required.

A good way to understand compression is to think of the printer attached to your computer. If you print with a higher resolution the quality is better but it takes longer to print because:

• there is more processing power required
• more memory to buffer
• more dots per inch to process
• more ink (data) to lay down

Using a lower resolution which generates lower quality is faster because:

• there are fewer dots to process
• less memory to buffer
• less ink to lay down

This reverts back to a previous issue of speed claims. When they say your printer prints 20 pages per minute, it’s usually not based on the highest resolution or with full pages of ink coverage, but the maximum possible on low resolution with the manufacturer’s definition of “normal” print coverage. Again, like the MPG ratings!

The fact is, older compression technologies produce larger file sizes, since they are not as efficient as the newer technologies. Accordingly, they require more processing power, which begs the question; can it generate enough power to provide a working solution with such large file sizes? The answer in most cases is a resounding, no. This is why many manufacturers will use multiple pieces of hardware with fewer channels of video to try to compensate for the shortfalls.

Newer compression technologies can compress video more efficiently and therefore result in less power required across the board. This is why in many cases older technologies are more expensive than the latest; because the storage requirements can be as much as 3x greater, as well as the amount of equipment.

Large file sizes bring several interesting challenges for your DVR solution. One primary concern is the majority of DVR users want to be able to watch video remotely. So the video must be transmitted across a network. As such, bandwidth becomes a major concern and in some cases a mathematical improbability for older technologies.

We return to the garden hose analogy. If the garden hose can only accommodate one gallon of water per minute (e.g. your network connection) and you are trying to pump water from a source (e.g. the video data stream) which requires 3 gallons per minute we have ourselves a physics problem.

The water (video data) backs up (latency). Now let’s add to the conundrum, what if you wanted to receive water from another source at the same time (video data from multiple DVR’s simultaneously) – the result is the throughput has been further cut. In terms of video what you end up with is chopped up and sometimes frozen images.

The bit rates below are for uncompressed color frames at 30 frames per second NTSC:

CIF 36.5 Mbps
2CIF 73.0Mbps
4CIF 146.0Mbps

What do these numbers mean? A single 4CIF uncompressed image is 146Mbps. Typical network cards can only accommodate a maximum of 100Mbps. Therefore, a single camera would ostensibly surpass the capacity of the network. So what happens when you have to transmit multiple cameras? The answer is, it cannot handle it. While there have been improvements in network cards which now approach Gigabit speeds (1000Mbps), there is still the issue of the capacity of the encoding and decoding cards. They simply cannot handle the task. The processing power is too great. Therefore, in order to transmit camera images compression technology is utilized.