microSD Cards for Audio

The WAV Trigger uses microSD cards very differently than does a digital camera, computer or smart phone. In order to play and mix low-latency, polyphonic audio, the WAV Trigger must perform "just-in-time" reads all over the card. If any one read takes too long, then there will be glitches and artifacts in the audio. With a camera, if one read or write out of hundreds of thousands takes a long time, who would notice?

The WAV Trigger expects multi-sector reads to take something around 1 ms, give or take. Anything over 20 ms will cause a glitch while playing a single 44.1kH stereo wav file. Many multiple reads over 5 ms may start to affect polyphony.

To determine which cards are suitable for polyphonic audio, I've written alternate firmware for the WAV Trigger than will measure and report the read times of any formatted microSD card. The card need not have any files on it - it just needs to be formatted as FAT16 or FAT32. If you'd like to get the firmware and test your own cards, scroll down to the bottom of this page for details.

Here's a list of cards that have been tested so far and the corresponding results. If the model number is not indicated, it's because I just have the card itself without any packaging or purchase information, or because the results were reported to me without this information. A yes indicates excellent WAV Trigger performance. A probably indicates that despite some higher than normal numbers, in all likelihood you'll never hear a problem. A no means that you will definitely notice occasional artifacts in the audio and to not use these cards in the WAV Trigger.

Interestingly, the lesson at the moment appears to be to stay away from the newer UHS speed class cards (a "U" with a 1 or 3 inside of it.)  While these cards have apparently been optimized for use in cameras, doing so has a detrimental affect on their random access times and makes them unsuitable for the WAV Trigger.