How to choose the right TF card for home surveillance cameras

How to choose the right TF card for home surveillance cameras

Home surveillance cameras need to choose a suitable TF memory card. The performance is too powerful, and it is not worth the waste. Poor performance and poor quality will result in the inability to record video normally (the writing speed is too poor, the speed is unstable), or the video cannot be read normally during playback (the reading speed is too poor, the speed is unstable), or even plug in the camera Not recognized later (too bad quality). How to choose a suitable TF memory card for home surveillance cameras? Let's start by getting to know the TF card.

Name

TF (T-Flash, TransFlash) card is the old name. It was jointly developed by Motorola and SanDisk. For the Micro SD Card, the reason for the new name Micro SD is because it was adopted by the SD Association (SDA). Therefore, the appearance of the TF card is generally marked with the name logo of Micro SD.

storage specification

The storage specifications of TF card are divided into HC and XC. HC stands for High Capacity, and the file format adopts FAT32. XC is eXtended Capacity, and the file format adopts exFAT.

HC, the upper limit of capacity is 32G. XC, the current common maximum capacity has reached 1TB (2TB is also available, not much). If a TF card is marked as Micro SD HC, but the capacity is marked as 128GB, there must be a problem.

High-speed transmission protocol interface

I and II are UHS (Ultra High Speed) high-speed transmission protocol interface identifiers, which are divided into three levels.

oUHS-I: write up to 95MB/s, read up to 104MB/s, guaranteed minimum transfer speed of 10MB/s,

oUHS-II: write up to 280MB/s, read up to 312MB/s, guaranteed minimum transfer speed of 30MB/s,

oUHS-III: On the basis of UHS-II, the bus speed in full-duplex mode has been enhanced, and the SD card switching from the save state to the active state has been reduced.

The above identification is only the version of the transmission protocol, indicating whether high-speed transmission is supported, and does not represent the actual speed of the card.

Read and write speed standard

Under normal circumstances, the reading speed of the TF card is much higher than the writing speed, so the writing speed is generally used to divide the TF card. There are mainly these standards:

oCLASS: speed class, speed class

oUHS: Ultra High Speed ​​Class

oVSC: Video Speed ​​Class, video speed class

oA1/A2: APP Performance A1/A2, App execution speed

See the table below for details of each standard:

1. The A1/A2 logo is relatively rare. This logo means that it has excellent processing for random storage, that is, when using mobile devices such as mobile phones, it can obtain better speed. Under normal circumstances, the reading and writing of mobile phones may be random or fragmented. At this time, the test cannot be performed according to the writing speed of continuous file reading and writing, and the A1/A2 standard can ensure that the speed is still maintained in the case of random reading and writing. If frequent and trivial read and write operations are performed on the memory card (for example, using mobile phone applications to read and write data), the A1 card will be much faster than the card without the A1 logo. The minimum performance requirements for A1 level are 10MB/s Sequential (sequential read and write), 1500 Read IOPS (random read), 500 Write IOPS (random write); the minimum performance requirement for A2 level is 10MB/s Sequential (sequential read and write) ), 4000 Read IOPS (random read), 2000 Write IOPS (random write). Surveillance cameras generally write continuously, so don't worry too much about the presence of A1 and A2 logos. Of course, if the set alarm recording storage (such as motion detection or humanoid detection alarm), the TF card will not be continuously written, then the stability and reliability of using the memory card with A1 and A2 logos will be better. 2. Logos with different speeds appear simultaneously. For example: U3 logo and Class10 logo appear at the same time. The TF card has achieved the U3-level writing speed due to the improvement of the writing speed, but the reading speed is still 90MB/s of the Class10 level, so when the double sign of mutual shield appears, you need to pay attention to the specific speed of the card (generally will be written on the card's box).

Selection suggestion

With the above knowledge reserves, we return to the original question, how to choose a suitable TF memory card for home surveillance cameras.

speed

The read and write speed of the TF card needs to meet the needs of camera video recording and playback. Video recording corresponds to the writing speed of the TF card, and video playback corresponds to the reading speed of the TF card. As mentioned earlier, the normal TF card reading speed must be greater than the writing speed, so here we also only consider the video writing speed. How much writing speed is required to record and store the camera video locally through the TF card depends on the size of the camera's code stream (ie bit rate, bitrate, which is not expanded here, for details, please refer to: Security video surveillance video file size calculation). At present, the resolutions of common home surveillance cameras are mostly 2 million, 3 million, 5 million, and 8 million, and are generally H.265 encoded. The stream sizes of cameras with these resolutions are as follows: It can be seen from this that even with H.264 encoding, a full HD (1080P) surveillance camera stream is lower than 4MB/s, which means that a C4-level TF card is It can meet the speed requirements of a 1080P surveillance camera video writing. SD Association recommends that V6, V10 and V30 cards are generally used to record full HD video (full HD, 1080P); 4K video (8 million) is recorded using V30 and V60; V90 is designed for recording 8K video.

capacity

How much capacity TF card to choose depends on the resolution (pixels) of the camera, the recording time and the recording method. According to the code stream of the camera, it is very convenient to calculate the storage space required by a camera for recording a day. The formula is: unit time (s) file size (Byte) = code stream (bps) ÷ 8. (1Byte=8bit). 24 hours a day video file size = (stream ÷ 8) × 60 × 60 × 24. Taking the 1080P, H.265-encoded camera mentioned above as an example, its average bit rate is 2000kbps, and the storage space required for 24 hours a day is 2000 ÷ 8×60×60×24=21.6G. (Of course, the actual storage space also involves the camera code stream control method, frame rate, monitoring scene and other factors, which will not be expanded here.) Through the above simple calculations, we can save according to the resolution/code stream of the camera. Time, select the TF card of the corresponding capacity. A very realistic situation is that the capacity of the TF card is relatively limited, and the video recording may not be stored for a few days. In order to prolong the number of days of video storage, we can choose dynamic alarm video, that is, the video will be recorded only when there is alarm information in the monitoring screen. The common ones are motion detection alarm video and humanoid detection alarm video. The video will be recorded and saved only when there is movement in the monitoring screen, such as wind and grass, vehicle body movement, etc. The humanoid detection alarm recording filters out the interference factors, and only the movement of the human body will trigger the recording. This recording method can prolong the storage time of the recording when the capacity of the TF card is limited. At the same time, because the recorded videos are all images of human movement, it is more valuable than the 24-hour uninterrupted recording, and it is more convenient to playback and search.

other

It is not necessary to use the so-called special TF card for video surveillance. In essence, this kind of card is not much different from the ordinary TF card. It is just a method promoted by the manufacturer. More attention should be paid to the points mentioned above, the logo and performance of the TF card itself. The picture below is a special TF card for video surveillance launched by a manufacturer. The core of our concern is actually the logo and performance of the TF card itself. The maximum capacity that the camera can support is limited by the hardware performance (mainly the encoding chip). The manufacturer will generally indicate the maximum capacity of the TF card that the camera can support. It is best not to use a TF card that exceeds the recommended maximum capacity. Stability and other unexpected situations arise. Using genuine TF cards is actually the key. If you use TF cards from unknown sources and are not produced by regular manufacturers, then everything mentioned above is useless. If you are worried about the insecurity of TF card storage in cloud storage, such as camera damage, TF card damage, theft, etc. The cloud storage service can also be opened, and the video recordings are saved in the cloud. Even if the camera, TF card is damaged, or stolen, the previous recorded videos can still be played back from the cloud.