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.