The new SD cards vs. SSD disks

If these new announcements from the SD Association are anything to go by, SD cards could become your primary storage medium. The SD Association has announced two major components of its new SD7.0 specification at Mobile World Congress in Shanghai, China.

SDUC

The first innovation is SDUC (SD Ultra Capacity) which is backward compatible with the existing SD form factor and features the double row of pins that was introduced with UHS-II in SHDC and SDXC. SDUC will continue to use the exFAT format, as will SDXC, so the change is in the maximum capacity which increases from 2TB to 128TB. . It’s fair to say that we can wait patiently for 4TB, 8TB and 16TB cards that have a higher capacity than the largest hard drives you can currently buy. After that, you have 32TB, 64TB and 128TB, which means you’ll be able to store 10 to 20 hours of uncompressed 8K video on a single SD card. By our calculations this will require one or two step changes in 3D NAND with an increase of at least 128 layers.

SD Express

The other announcement from the SD Association is about SD Express, which refers to the use of PCI Express Gen.3 and NVMe v1.3 as the data interface for SDUC. This is interesting since every SD card reader that has ever been used always seems to connect via USB. Modern CPUs and motherboards ship with a large number of PCI Express lanes, and since we typically use a single graphics card, it is common practice to use the extra lanes for SSD storage. In this context, it seems to make sense to allocate a PCI Express lane or two for your card reader.

The data rate for SDHC and SDXC cards using the UHS-III interface is 624MB/sec in full duplex, while the new SD Express interface is rated at 985MB/sec, so we are talking about a speed increase of around 50 percent over the current maximum. Having said that, you will often find SD cards and card readers that are rated considerably slower, around 100MB/sec, so we should be aware of how the hardware works in practice, rather than in theory.

The combination of SDUC and SD Express offers a combination of high capacity storage with a fast interface and small form factor. These SD cards are expected to rival SATA SSDs in terms of speed and make them a natural fit for external cameras and recorders from the likes of Atoms, Blackmagic and Lilliput.