Memory cards.

All memory cards used in the Solo must be micro-SD sized, so that they fit into the slot on the Raspberry Pi.

Speed ratings

Memory cards have speed ratings. It is recommended that a minimum of “Class 10”. This id denoted by a “10” encircled by a letter “C logo.

Alternatively the UHS-I rating of 1 also good enough. These ratings are indicated by the symbols shown.

Capacities

micro-SD cards come in sizes up to (currently) 256Gb. This table shows their capacity in terms of how many hours (or days) they can store, for lots of different recording parameters. (Remember the default setup is 16000 Hz sample rate, mono).

Recording Settings Memory card capacity
Sample Rate Channels 8GB 16GB 32GB 64GB 128GB 256GB
8kHz mono 112 hrs /
4.7 days		 251 hrs /
10 days		 529 hrs /
22 days		 1085 hrs /
45 days		 2196 hrs /
91 days		 4418 hrs /
184 days
stereo 56 hrs /
2.3 days		 112 hrs /
4.7 days		 264 hrs /
11 days		 542 hrs /
22 days		 1098 hrs /
45 days		 2209 hrs /
92 days
16kHz mono 56 hrs /
2.3 days		 125 hrs/
5.2 days		 263 hrs /
11 days		 524 hrs /
22 days		 1098 hrs /
45 days		 2209 hrs/
92 days
stereo 28 hrs /
1.1 days		 62 hrs /
2.6 days		 132 hrs /
5.5 days		 271 hrs/
11 days		 549 hrs /
22 days		 1104 hrs /
46 days
44.1kHz mono 20 hrs /
0.8 days		 45 hrs/
1.9 days		 96 hrs /
4.0 days		 196 hrs/
8.2 days		 398 hrs/
16 days		 801 hrs/
33 days
stereo 10.2 hrs /
0.4 days		 22.8 hrs /
0.95 days		 48 hrs/
2.0 days		 98 hrs /
4.1 days		 199 hrs /
8.3 days		 400 hrs/
16 days
192kHz mono 4 hrs /
0.19 days		 10 hrs /
0.43 days		 22 hrs /
0.91 days		 45 hrs /
1.8 days		 91 hrs /
3.8 days		 184 hrs/
7.6 days
stereo 2.3 hrs /
0.09 days		 5.2 hrs /
0.21 days		 11 hrs/
0.45 days		 22 hrs/
0.94 days		 45 hrs/
1.9 days		 92 hrs/
3.8 days

Arithmetic details:

The equation behind the table above is:

hours of recording = 10e9 * (CARDCAP - 1.5) / (2 * channels * sample_rate * 60 * 60)

Where:

  • CARDCAP is card capacity in GB
  • 1.5 is the space (in GB) lost to the Solo Operating System
  • The 2 is bytes per sample (a 16-bit sample is 2 bytes)
  • the sample_rate is in Hz (16000 for example)
  • the 60s are seconds per minute, and minutes per hour.
  • The result is a reading of hours on the card. Add a 24 to the bottom line of the equation to get the answer in days.

Note: that the above numbers are unaware of how long your power supply will last - make sure you choose a power supply to match the intended record duration.