More space for less money: SSD instead of CFast with Blackmagic

Archive notice: This article was originally published on May 10, 2018. Links and embedded videos are preserved as part of the historical record.

Let’s go into more detail: 12bit RAW with 4608 x 2592 pixels (4.6K for short) requires around 427 Mbytes per second at 25 fps, and at the sensor’s highest frame rate of 60 fps it is around 1025 Mbytes/s. The interface for CFast 2.0 can transfer a maximum of 600 Mbyte/s, so the 60 images cannot even be recorded uncompressed with just one card. This is why the camera has 2 slots, which can be used alternately if required. In fact, the Ursa always uses lossless compression, similar to .zip or .rar, the data is simply better sorted and optimised. But in the worst case – as with pure noise – this would be the full data rate. However, a compression with slight losses is also offered for RAW: Then, at 3:1 for 60 fps, only just under 342 Mbyte/s have to be stored away; at 4:1, 256 Mbyte/s still remain (the data rates of ProRes 444 XQ are even higher). The RAW compression is at best visible through a slight deterioration in the fine structure of the noise, especially in dark areas of the image.

Unfortunately, a CFast card suitable for the Blackmagic Ursa series cameras already costs more than a Tbyte of storage space in the form of an SSD at 256 Gbyte. In addition, a maximum of 512 Gbyte is currently available at horrendous prices, while 2 Tbyte would be easy to find as an SSD if you need to make long, uninterrupted recordings. In principle, SSDs should be suitable, as the SATA 3 standard has a theoretical limit of 6 Gbit/second, which corresponds to around 768 Mbyte/s. If the 8b/10b coding is taken into account, the maximum uncoded transfer rate should be 4.8 Gbit/s or 600 Mbyte/s. In practice, good SSDs seem to reach their limit at 540 Mbyte/s, but you must be aware that manufacturers like to specify data rates with the small addition “up to”. Some SSDs use internal compression that does not apply to video, or their speed drops between individual write operations, while we cannot do without a single frame when recording video.

SSD recorder from Blackmagic

The manufacturer (BMD for short) has also recognised the need and offers an SSD recorder that is placed between the camera (one of the two Ursa Mini) and the holder for the battery. Although it is supplied with power in this way, the connection for the image is made via the rear BNC connections. Nevertheless, recording in RAW is possible, as in this case internal data is sent via the interface and not a video signal in the SDI standard.

The SSD is inserted into the slot from above and can therefore be quickly removed and taken to the copy station. The entire control is transparent from the camera. The rear card slot is considered occupied and no CFast card can be used there. As the data rate for a single SSD cannot be increased any further, uncompressed RAW and ProRes 444 XQ can only be used up to 30 fps at full image size. Higher frame rates can still be recorded in split mode with an additional card in the front slot. Parallel recording as a backup is not possible. In addition, the rear SDI connections are no longer available for external monitors or an ATEM switcher. Apart from these minor disadvantages, for around 400 euros you get a solution that perfectly matches the camera for longer recordings on more cost-effective media.

CFast to SSD

Even before the SSD recorder from BMD was available, a number of people were already thinking about how to get an SSD on the camera. These include Tom Antos and João Felipe Khury, whose suggestions we draw on here in the test. There was also a device from Atoch called C2S CFast to SSD, which, despite its high price of 600 US dollars, looks more like a hobbyist solution. At the moment, several Chinese suppliers have very similar versions starting at around 275 euros on the internet (plus duty and tax). They even claim to be compatible with ARRI Amira, ARRI Alexa Mini, Canon XC10, Canon C300 Mark II and Canon 1DX Mark II in addition to Ursa – with the nice addition: “in theory, as long as the device supports CFast support”. We have not tested these very similar-looking devices.

The blue adapter cables from Micro SATA Cables recommended by João Khury are only available from the USA. Instead, we used a Russian offer from Videotech on Ebay, which can even be made in the desired cable length for 45 euros. It arrived within a fortnight and makes a neat impression. Our cable connects the CFast slot to an eSATA connector, which in turn fits small hard drive housings with a corresponding connection. This is where the first technical pitfall becomes apparent: an enclosure that only supports 3 Gbit/s via eSATA is not sufficient. A test with an inexpensive Icy Box (type IB-231STU3S-G) did not lead to success; only the S251SMU33EP from Startech, which is also used by João Khury, is suitable thanks to its SATA III interface; it is also available in this country for just under 40 euros.

Unlike USB, no power is supplied via SATA. However, such enclosures usually come with a cable for separate power supply from a USB port. As many suppliers now equip their V-mount batteries with USB as well as a D-tap, you don’t necessarily need an additional source. With the Patona battery used in the test, we first had to briefly press the charging indicator with the device connected in order to activate the power via USB.
Not bad in itself, because no power is consumed beforehand, but at first glance it appears to be a defect. Now the LED on the housing also lit up and the inserted SSD was recognised by the camera as a CFast. Since a D-Tap is always present on the camera’s battery plate, a voltage converter from D-Tap to USB can be used if there is no USB, which can be found cheaply as a plug unit with a looped-through D-Tap or as a cable.

In practice

The initial euphoria at the almost 3 hours of recording capacity displayed for a 1 Tbyte SSD in UHD in ProRes 422 HQ was followed by disillusionment: the recording stopped after a few seconds, even with less demanding formats. A search on the Internet revealed that some other users had also had similar experiences, but that the adapter worked perfectly for them in the other slot. In fact, it also worked for us when we inserted the adapter into the rear slot of the camera. Now we were finally able to write a Samsung SSD 850 Pro in full 4.6K with uncompressed RAW at 25 fps up to the limit without a single dropout.
To rule out a problem with the housing used, we also tried direct cabling from eSATA to the drive plus power supply via Molex connector.

The result was the same: Nothing works in the front slot, in the rear slot it works as expected. A defect in the slot can be ruled out, as one of the CFast cards recommended by BMD (the Transcend CFX650) works perfectly in both slots. We do not have a reasonable explanation for the phenomenon, but it can be assumed that the slots in the camera react differently to the additional cable route – after all, we are dealing with very high frequencies at 6 Gbit/s. Anyone considering such a solution should therefore order cables that are as short as possible and expect that only one of the two slots will work.
Compared to the solution from BMD, this has the advantage of significantly lower costs and free SDI connections. We were also able to make recordings in uncompressed RAW at full image size up to 60 fps in parallel operation without any problems. This provides a very universal and yet inexpensive solution, as a smaller CFast is certainly sufficient for the relatively rare cases where this ultimate image quality is really required.

We would be sceptical about the variant currently offered by Innotech from London for 75 pounds with two SSDs directly connected, not only because of the observed problem with the second slot, but also because of the power consumption. As a rule, USB adapters are only designed for a good 2 amps, which is likely to be critical with two SSDs in split mode – our 1 Tbyte is specified by Samsung at 1.4 A.
Another advantage is the low weight of the overall solution. We shortened the top of our Startech case by approx. 1.5 cm so that the SSD can be easily inserted and removed, and attached the whole thing to the back of the battery with Velcro. Alternatively, you could also attach several enclosures in this way and use them alternately with permanently mounted SSDs, as the USB 3 interface also provided enables fast copies and the SSD would be better protected.
On the other hand, all the complete solutions mentioned at the beginning have the disadvantage that they sit between the camera and the battery and therefore shift the centre of gravity considerably to the rear, which can be a problem with lightweight lenses. A final objection to our tinkering would be that the internal monitor cannot be fully utilised due to the cable and therefore continues to consume power.

We have also tested this: With just the internal monitor at normal brightness and continuous recording to the SSD, our Ursa Mini Pro ran for two hours and ten minutes on a 95 Wh battery. With an additional viewfinder monitor from Alphatron, it was still one hour and 50 minutes when we set the brightness of the internal monitor to minimum. This is not a significant handicap when you consider the power consumption of some other cameras with comparable image quality.
Incidentally, the flap with the monitor on our camera can be opened just enough to switch it off – the only way to be sure is with a very inelegant solution such as a
very inelegant solution such as a rubber band. At least you are better off here than with the Canon C200, where the CFast slots are not activated at all when the flap is open.
A solution such as the very thin adapter cables from an Ebay supplier in Italy, with which the flap supposedly closes, hardly seems more sensible to us. The cables would obviously be clamped after all, as there is very little space at the edge, no matter how delicate they are. Both are likely to be detrimental to reliability.

Comment

Admittedly, this is tinkering and you should expect that it will not work with every component and perhaps not even every camera. But the advantages in terms of cost, weight and flexibility are attractive, and under the conditions we have specified, the whole thing also works in continuous operation without image loss. For over 300 euros saved in this way, you can easily find a good 1 Tbyte SSD.