How to check for reliable USB Direct Recording

Compared to a usual computer, the front USB port of the UFX has a limited buffer to compensate for interruptions to the write process. The effect of the buffer depends on the amount of data to be transmitted, which is the combination of sample rate and channel count. The higher the data rate, the shorter the available buffer time. When a medium causes a pause that is longer than the buffer time, data is lost.

To check the performance of your medium the UFX shows the USB load and the Max Read/Write Time, and in case the USB device is too slow it will also show errors. Checking is easy:

- At 96 kHz sample rate enable 34 channels for recording. This is the worst case scenario. It reaches the highest data throughput (9.79 MB/s) with the smallest internal buffer size (around 306 ms). Most external hard drives will have no problem to continuously record at this rate until the drive is full, with zero errors. It is nearly impossible to find a USB stick that can do the same, but some come close. If your medium shows only a few errors after some time, simply reduce the number of recorded channels to 32, then 30, and at some point it will work. Many USB sticks, even the latest super-fast USB 3 ones, suffer from repeated write interruptions of more than 500 ms, therefore will fail. You can see that directly when looking at the Max R/W Time value.

Reducing the number of channels decreases the data throughput and increases the buffer size. With 30 channels you reach 8.64 MB/s, 347 ms buffer, which is exactly the same as 60 channels in 48 kHz.

- Many sticks on the market have a ridiculous 2 to 5 MB/s write rate, which then limits the number of channels that you can record. If the USB load goes up to 90%, errors are near or already shown, meaning the stick is too slow and you have to reduce the number of recorded channels. Better sticks with more than 10 MB/s will allow for the recording of nearly all 60 available channels at 48 kHz! Try it. You will never need that, but if your stick passes this test you can be sure you will never get problems with less channels.

A more realistic example might be 30 channels input plus 2 stereo mixdowns = 34 channels = 4.9 MB/s, which gives an internal buffer size of 612 ms. Sounds like nothing special, but you will be amazed how many of the USB sticks you have will fail even here.

It is impossible to give recommendations as the manufacturers change the internal hardware of their sticks all the time. But I still want to give some personal experiences to show what the above means in real world:

The old Corsair Voyager GT 4 GB can record 56 channels at 48 kHz, write time around 120 ms (bravo!). The latest Sharkoon Flexidrive Extreme Duo USB 3.0 (32 GB) fails as it stops every half a second for half a second (I immediately sent it back). The Mach Xtreme FX USB 3.0 (16 GB) would record even 16 channels at 192 kHz (write time 80 ms!), but at specific points it suddenly stops for about 400 ms (what the....). Well, still usable with a few less channels. Despite an average write rate of 11 MB/s the Sony MicroVault (should have been Fault) 8 GB regularly interrupts the recording stream by more than 600 ms. So it is limited to record no more than 10 channels at 48 kHz. Uh Oh, but at least they don't have my personal data.

All USB 2 and 3 disks that we tested, hard drives as well as SSDs, from Seagate, WD, Hitachi, Samsung, Corsair to name a few, were able to record 192 kHz 16 channels (= 96 kHz 32 channels) error-free. Error-free long term recordings at the highest channel count require hard drives labelled AV, which are specially designed to maintain an un-interrupted data flow. These days such disks are not much more expensive than standard ones, so really worth it if you take it to the limit.