Running on RPI Zero W and other devices with performance limitations

AIS-catcher implements a trick to speed up downsampling for RTLSDR input at 1536K samples/second by using fixed point calculations (-F). In essence, the downsampling is done in 16-bit integers performed in parallel for the I and Q channels using only 32-bit integers.

This feature can activated with the -F switch and is only available for RTL-SDR running at a rate 1536K per second (the default). To give an idea of the performance improvement on a Raspberry Pi Model B Rev 2 (700 MHz), I used the following command to decode from a file on the aforementioned Raspberry Pi:

AIS-catcher -r posterholt.raw -s 1536K -b -q -v

Resulting in 38 messages and the -b switch prints the timing used for decoding:

[AIS engine v0.31]  : 17312.1 ms

Adding the -F switch yielded the same number of messages but the timing is now:

[AIS engine (speed optimized) v0.31]    : 7722.32 ms

On an RPI Zero W this will bring down CPU load to ~40% and avoid buffer overruns.