The silent death of the SD memory card in photography

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At the same time as the boom in digital cameras in the early 2000s, SD cards became established in photography. Less than 25 years later, the responsible SD Card Association seems to have finally reached a dead end and the format has lost any prospect of a rosy future. Meanwhile, the competing CFexpress standard is doing much better.

Anyone interested in technology may have come across terms such as UHS-III (not to be confused with the video speed class U3) or SD Express, but more in the news than in the shops. For seven years, no new SD Card Association standard has been able to establish itself on the market.

UHS-I and UHS-II were still real successes

In the world of technology, it usually takes several years for new standards to establish themselves on the market. Example: USB 4.0 can be found on many new desktop PCs and notebooks five years after its introduction, while external SSDs and other end devices are still rather exotic.

In contrast, the UHS-I interface for SD cards took off almost like a rocket, more than quadrupling the maximum possible transfer speed from 25 to 104 MByte/s: Version 3.01 of the SD specifications was released in May 2010, Toshiba presented its first UHS-I memory card at IFA in September of the same year, Nikon followed suit a month later with the D7000 DSLR camera and by the turn of 2011, the entire product chain of cards, card readers and cameras was on sale.

Even if the comparatively simple implementation certainly played its part - the clock rate of the interface increased from 50 MHz (SDR25) to 208 MHz (SDR104) and UHS-I SD cards were now allowed to consume four times as much electrical power at 2.88 W - a mere six months until market availability testified to good cooperation between all those involved.

One year later, in 2011, the interface was completely redesigned: with UHS-II, SD cards were given a second row of contacts, LVDS-based transmission (Low Voltage Differential Signaling), a tripling of the transmission speed and the same power consumption. Nevertheless, it "only" took around 2.5 years for Fujifilm to introduce the first UHS-II camera with the X-T1 and for suitable SD cards to reach the market at the same time.

In 2024, UHS-I is de facto the minimum standard for cameras, card readers and SD cards. UHS-II has also made it into around 100 different system cameras - only the smaller microSD cards never caught on with the faster transfer rate. Neither smartphones, regardless of price range, nor mobile games consoles such as Steam Deck or Nintendo Switch allow more than 104 MByte/s (UHS-I).

UHS-III, SD Express 7.0 & 8.0 exist on paper

UHS-II was followed in 2017 by UHS-III with a further doubling of speed to 624 MByte/s. At least in theory, because not even a mock-up from a manufacturer made it to one of the numerous technology trade fairs.

The situation is not quite as dire for SD Express: when the interface was completely redesigned again in 2018 – just seven years after UHS-II, mind you – the responsible SD Card Association also opted for PCI Express (3.1) including the NVMe protocol and raised the limit for SD cards to an impressive 985 MB/s - even tiny microSD cards could outperform SATA SSDs. This was followed in 2020 by the quasi-mandatory update to PCI Express 4.0 and up to 3,938 MB/s (PCIe 4.0 x2 with third contact row), but only for standard-sized SD cards.

Although there are a handful of SD Express memory cards and card readers, they tend to be from second-tier manufacturers (Adata, DeLock, Ritz Gear, Sabrent). Industry giants such as Sandisk and Lexar have so far confined themselves to demos and announcements, while Samsung's microSD Express card is due to be released later this year.

If you ask around at photography trade fairs such as ^CP+ in Japan, you will mainly hear resignation: A camera with SD Express is not to be expected, according to circles of memory card manufacturers, who usually work closely with camera manufacturers to have suitable and compatible cards on the market at the camera's market launch. There is also a consensus as to why.

The bumpy upgrade path

PCI Express and UHS-II/III use incompatible signal transmission methods in the physical layer: High-Speed Current Steering Logic (HCSL) and Low Voltage Differential Signaling (LVDS) respectively. The SD Express standard does not provide a mechanism for either end devices or SD cards to switch between the two transmission methods on the controller side.

Both UHS-II/III and PCI Express occupy the bottom row of contacts on SD cards. This results in a compatibility problem: companies have to decide whether to offer a camera, a memory card or a card reader with UHS-II or PCI Express as an interface, never both (in one card slot). Only the top row of contacts is always available for UHS-I with 104 MByte/s.

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Memory cards with UHS-II and SD Express 7.0 compared (2 Bilder)

There is therefore generally no sensible upgrade path for customers. Anyone who buys a future-proof SD Express card today will have to make do with UHS-I speed until a suitable camera is available. Anyone with UHS-II cards would not only drop from 985 MB/s to 312 MB/s (UHS-II) in a hypothetical SD Express camera, but to 104 MB/s (UHS-I).

CFexpress is ready to replace it

While SD cards carry the first contact series as "legacy", which can only serve as the lowest common denominator for downward compatibility in the long term, the much more recent CFexpress standard (2017) offers PCI Express 3.0 and high transfer rates from the outset.

CFexpress is the (successor) to CompactFlash and later XQD from the professional segment, but with version 2.0, further form factors were added in 2019, including type A, which is only 4 mm shorter and narrower than SD cards. Sony uses the similar shapes creatively and offers combined memory card slots for CFexpress Type A and SD (UHS-II) in newer Alpha cameras. Canon, Fujifilm, Nikon & Co. usually rely on a mix of one compartment each for CFexpress Type B and SD (UHS-II) in the professional segment.

This gives the customer both the choice and the option of continuing to use SD cards that have already been purchased. In addition, CFexpress memory cards are now often significantly cheaper, at least for capacities from around 256 GB.

The upgrade to PCIe 4.0 presented at the end of 2023 will double the possible speed to 2 (Type A) to 4 GB/s (Type B). Corresponding memory cards and card readers are already available on the market. Even if corresponding cameras are not expected before 2025, more likely 2026, end customers will not suffer any disadvantages when purchasing such cards today. Existing CFexpress cards with PCIe 3.0 should work in future cameras at their native speed.

The less than rosy future of SD cards (in photography)

SD memory cards are likely to remain with us for a long time to come, with microSD cards in particular still being the medium of choice for many mobile devices. As UHS-II has never established itself in the compact design, the lack of backwards compatibility with UHS-II is not critical and microSD Express could have a chance on the market.

In photography, on the other hand, the future looks gloomier. Here, too, the SD card will continue to exist for many years, at least in the entry-level segment without special speed requirements. However, the trend is towards higher-quality cameras, in which CFexpress is playing an increasingly important role. In 2023, Germans will have spent an average of 1676 euros on mirrorless system cameras.

Meanwhile, there is no foreseeable way out of the development trap for SD cards. There are no known efforts to make SD Express and UHS-II/III upward and downward compatible after all, nor is a radical redesign such as the switch from CompactFlash to XQD/CFexpress to be expected.

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(mma)