One thing you need to be careful about when analyzing specifications of a DVR with respect to "frames", "fields" or "images" per second capabilities is what are they talking about:
- The total number of frames/images per second for the entire card to be spread across all cameras (cumulative total)
- The total number of frames for each individual channel
- The maximum frame capacity of the hardware not taking into account software switching, simultaneous functions, etc. (rated hardware capacity)
- Display speed
- Recording speed
- "I" frame "B" frame or "P" frame calculation
- A combination of all of the above
- Is it really images or are they even calculating frames which also provides misleading figures
There are an infinite number of ways of presenting these numbers, many of which are misleading. Sounds like financial reporting on Wall Street! Remember that 30 frames per second is real-time / real motion video, but that is for a single video stream. So if you want to record 4 cameras simultaneously, all in real-time/real motion video, you need 120 frames per second and its full unshared resources. To drill down even further you have to question at what resolution is the real time image being displayed. Many systems can only record real-time if the resolution is lowered.
The frame rate issue is a very tricky one. The fact is the speeds that manufacturers quote are usually the "maximum" obtainable, meaning under ideal conditions, and does not take into account anything else the PC, software, or video card might be doing. In other words it's like the MPG sticker on your new car - good luck trying to get that mileage; maybe downhill in neutral with a strong tailwind.
To add further to the confusion are some manufacturer quotes "IPS" (image per second). An "FPS" (frames per second) why do they do this-because 2IPS=1FPS. Therefore, it takes 60IPS to equal 30FPS or a single real-time image. It becomes more convoluted because in "images" per second there are "initial" frames and subsequent frames which refresh only changed portions of the image. Confused yet! Well make sure they are talking "Frames" per second and not "Fields" per second, as similar to IPS there are two fields to each frame.
We keep calling them "capture" cards because they are "capturing" and recording video, but what plays back and displays the video on the screen? The answer is the capture card. Even though it is "capturing" (encoding) the video, it also handles the video display on the card (decoding).
Some other math we need to learn. If we already know about the alleged maximum recording frame/image rate of the board, what about the video display? Sorry, but yes we have to add that into the equation.
So, now that you are displaying and capturing at the same time, the performance may proportionately diminish, as they may be sharing the same components to accomplish different tasks. One function must wait for the other or both perform at a reduced rate. To counteract this problem some companies use a separate video display card (decoder), which generates real-time video on all channels all the time. A live video display card usually has separate chips for each group of channel of video displayed. Each chip is capable of generating a true 30 frames per second image per channel across multiple channels. It is a live feed from the board directly to the monitor and each channel transmits its own video, without the need for the software to compress the video signal. This should not be confused with the VGA video card, which is entirely different. Today, there are also several newer technologies that are capable of providing real-time images and multiple channels without the need for a live display card but they come at a price.
Let's jump back now to, how the heck does this capture card work. After all, raw video uses a tremendous amount of data and we are talking about transmitting as many as 64 simultaneous camera images from a single machine. Making practical to send, receive and store huge amounts of data requires video compression before it can be transmitted.
There are 2 types of physical compression; hardware and software. It is actually a case of compression and decompression. Compress it to travel down the network and decompress to transmit when it comes out the other side, so the name "Codec." When you use hardware compression there is no loss of efficiencies, as all the work is being done on the board by the hardware components. That is of course if you have all the right components.
Software compression utilizes software to perform specific operations. When performed simultaneously in conjunction with other functions it has a taxing effect on overall system performance. The software uses the available hardware resources such as processor and memory to complete its task.
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