SD Card For Surveillance TutorialBy Brian Rhodes, Published on Mar 21, 2012
Axis's VMS elimination solution uses SD cards to store video on the camera. This makes SD card selection and specification a key part of a surveillance design. In this note, we examine SD cards, key criteria and descibe which types to best apply.
Here are the key elements reviewed:
- Speed Class
- Backward Compatibility
- IP Camera Application
SD card memory is an 8 or 9 pin solid-state memory format, using flash memory chips packaged in narrow form factor. This memory format has gained wide acceptance since its introduction in the early 2000s for a variety of electronic media storage, including music, pictures, and video clips. It is commonly available, inexpensive, and widely distributed. The low cost of the memory format has made it an ubiquitious storage medium for many data recording devices.
A number of SD card formats exist, and they are not entirely interoperable. Care should be taken to ensure the proper media is used according to the type storage device supports.
- SD cards - too small for widespread use in IP cameras.
- SDHC cards - the most common format accepted by IP cameras
- SDXC cards - not yet commonly accepted by IP cameras
The storage capacity of these card formats vary:
- With the older 'SD' format supporting a maximum size of 2 GB. This small capacity is not practical for use as onboard storage.
- The 'SDHC' format is widely accepted by many IP camera manufacturers and supports a range of size denominations from 8 - 32 GBs. This format is limited to a design maximum of 32 GB and is cheap enough at 32 GB to be a good fit for edge storage use.
- The newest 'SDXC' format is designed to support capacities from 64 GB to a theoretical maximum of 2 TB per card. This format is not yet widely adopted by IP camera manufacturers for onboard storage, but due to the larger capacities supported, we expect to see edge storage moving to this format.
An interesting theory surrounding the slow release of SDXC memory has been that "there is no real need for larger card". We consider the development of edge storage using SD format storage to increase demand for larger denominations and their eventual introduction into the market.
'MicroSD' cards are a smaller form factor of memory using the same storage technology. The full scope of micro SD, microSDHC, and microSDXC cards exist, and the same storage limits apply to this form factor. MicroSD card are available in the same denominations and with similar prices to SD formats. The microSDHC format is popular in smaller dome IP cameras like the Axis M50 series, Vivotek FD series, and Panasonic's WV series.
Speed class is a critical consideration when recording video to SD cards. The 'speed class' notes the data transfer rate supported by the card. Since SD card formats can be built using different types of memory, the speed class designates how fast the included memory chips will transfer data for storage. Common SD Speed Class marks are "2", "4", "6", and "10" and they refer to the speed in Mb/s (e.g., class 2 = 2MB/s, Class 4 = MB/s).
While Class 2 might be enough for many IP cameras, it is safer to use Class 4 or higher. For instance, Class 2 max speed is 2MB/s, equivalent to 16Mb/s. This should be enough for most HD cameras, however, VBR streams might spike and approach the max of this class. By contrast, Class 4 provides double the speed, up to 32 Mb/s, which should be safe for almost any H.264 camera stream/configuration.
An SDXC card can be used in an SD or SDHC reader, but it first must be formated for use in the lower format. This results in losing the benefits of the higher format. An expensive SDXC card can be purcahsed and used in an SDHC device, but at the expense of losing the additional SDXC capacity.
IP Camera Application and Storage Capacity
A number of manufacturers are adding SD storage onboard cameras, including Mobotix MX Control Center, Axis Camera Companion, Genetec Trickle Storage, and Milestone Edge Recording [link no longer available]. Sizing the storage properly based on both capacity aand speed class is a critical consideration.
Once a card is installed, it should be fortmatted by the device before use. Despite the transportability of the memory card, it should not be removed from the device to recover video, but video should be downloaded from the camera via the web interface.
The storage capacity of the card, in terms of days of storage, may be insufficient. Many manufacturers claim multi-week storage capabilities with larger capacity SD cards (e.g., Axis Edge Storage Whitepaper). However, these settings may be too low to reflect practical usage. For example, Axis claims 19 days of 720p video on a 32 GB card. However this presumes a 10 FPS framerate and only 20% motion detection. Also, if using Axis in its default VBR mode at night, bandwidth could spike far higher. Real life results will vary wildly depending upon how these variables impact recordings.
Recording each camera to its own SD card eliminates storage advantages of aggregating cameras on a single recorder. With an NVR or DVR, if some cameras required a lot of storage and others required little, the storage averages out between cameras (e.g., one camera required 50GB and a second only required 10GB, only 60GB would be needed for both). However, with SD recording, the capacity is limited to the capacity of the card. The end result is that some camera's SD card storage might be insufficient for its particular needs, resulting in fewer days of storage.
Warning on Max Storage
At the current time, most IP camera manufacturers support SDHC but not the SXDC format. SDHC limits card size to 32 GB per camera. The greatly impacts how useful SD storage is as a primary method of storage. This capacity may not be sufficient enough to record video for many applications - continuous recording or high motion scenes or high resolution, etc. This hard capacity limit will be expanded to 2 TB once SDXC formats are supported, which is an 'upcoming adoption' or roadmap release from many camera manufacturers. However, it is not possible to field upgrade cameras with SDHC to SDXC. As such, any camera that only supports SDHC is stuck with that 'forever'.
Due to the capacities required to be useful for recording onboard video, an SD denomination of less than 16 GB will be marginally useful. While smaller denominations of storage can be purchased, the pricing is not significantly less to warrant purchasing less storage.
- Standard SD format cards (limited to 2 GB sizes) can be purchased for around $2 per card.
- SDHC format cards with a 4 Speed Class can be purchased for about $35.00 per 32 GB card.
- SDXC format cards with a 10 Speed Class can be purchased for about $80.00 per 64 GB card.
SD card pricing is very volatile, and the commodity nature of this memory results in steep discount specials and 'loss leader' pricing from many online vendors.
We expect to see this trend continue and eventually be a common feature. Understanding the camera and software's ability to use this storage is only half the equation. The other half of understanding the accompanying media will ensure the right product is specified.