V30, U3, A2: only one of those numbers matters for video
The card says 170 MB/s on the front. Your camera stops recording after ninety seconds. Both facts are true, because that number was about reading.
Every marking on a card measures something different. The big number on the front is best case read speed. That is the one thing video recording never needs.
What you need is a floor on sustained writing.
The marks that mean something
| Mark | Guarantees | Use |
|---|---|---|
| V30 | 30 MB/s minimum sustained write | The floor for most 4K recording |
| V60 | 60 MB/s minimum sustained write | High bitrate 4K, some 6K |
| V90 | 90 MB/s minimum sustained write | All-intra codecs, 8K, high frame rate |
| U3 | 30 MB/s minimum sustained write | Same floor as V30, older scheme |
| A1 and A2 | Random read and write operations per second | Running apps from the card. Irrelevant to video. |
A card can carry A2 and still be poor at video. The A marks were designed for phones running software off the card.
Converting your bitrate into a class
Divide megabits per second by 8. That gives megabytes per second.
A 100 Mbps codec writes 12.5 MB/s. That fits inside V30 with room to spare. A 400 Mbps all-intra codec writes 50 MB/s and needs V60.
Leave headroom. The rating is a minimum under test conditions. A card written to many times is slower than a new one.
Cameras write into a small buffer and drain it to the card. A card can meet its rating on average and still stall. The buffer fills. The camera halts rather than lose frames.
Counterfeits
Fake cards are common and convincing. They report a large capacity. They accept data. Then they lose it once you pass the real size.
- Test a new card before you trust it. Tools that write and verify the full capacity exist for every platform.
- Be suspicious of a large fast card at a small price.
- Buy from a seller the manufacturer names.
Format on the device
Format in the camera, not on a computer. The device lays out the card the way its firmware expects. That affects sustained write speed.
Do it before every important session. It costs thirty seconds. It clears the fragmentation that turns a healthy card slow.
Sources
The Rig Wire reports on streaming technology from public documentation, source repositories and vendor releases. We did not bench-test what is described here. When we have tested something, we say so.