Can AIS catcher support writing IQ audio files

[iceofwater]

Hi jvde-github！
I use Airspy R2 to receive AIS signals, but I have a requirement to write the IQ signal of AIS as a WAV audio file. However, my verification is to play it again through the playback file of AIS catcher, but I don't know how to implement this. Do you have any development plans for this? Or can you give some suggestions on how to develop this area?

[jvde-github]

Hello!

At the moment, not yet. It is in the works though as it has been requested a few times. Also have thought about how to implement it, just need to do it.

What you can currently do is record with SDR# at 162 MHz, write as a WAV file and then read it in AIS-catcher with -w.

Can you say a bit more about your use case? Are you ok with a downsampled signal (to 192K) or looking at the raw signal coming from the AirSpy (say at 6MHz)?

Thanks! Jasper

[jvde-github]

Added something experimental. It can be activated via -m 9 -go file output.wav.

It is essentially a decoding model that only writes the IQ stream to file. So, to enable this decoding model and the regular one and take input from a file (posterholt.raw for testing):

$ ./AIS-catcher -r cu8 posterholt.raw -m 9 -go file output.wav -m 2 -q -v
AIS-catcher (build Jan 12 2025) v0.61-211-g85b86377
(C) Copyright 2021-2025 jvde-github and other contributors
This is free software; see the source for copying conditions.There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Device    : File (RAW)
Settings  : rate 1536K format CU8 file posterholt.raw loop OFF
Model #0 -> (Src: 1, Grp: 1): [Export output]
Model #1 -> (Src: 2, Grp: 2): [AIS engine v0.61] ps_ema ON afc_wide ON droop ON fp_ds OFF dsk OFF

----------------------
[Export output #0-0]                    total: 0 msgs
[AIS engine v0.61 #0-1]                 total: 38 msgs

Now I can decode this wav file as follows:

$ ./AIS-catcher -w output.wav -v -q
AIS-catcher (build Jan 12 2025) v0.61-211-g85b86377
(C) Copyright 2021-2025 jvde-github and other contributors
This is free software; see the source for copying conditions.There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Device    : File (WAV)
Settings  : rate 1536K format CU8 file output.wav
Model #0 -> (Src: 1, Grp: 1): [AIS engine v0.61] ps_ema ON afc_wide ON droop ON fp_ds OFF dsk OFF

----------------------
[AIS engine v0.61 #0-0]                 total: 38 msgs

This has not been thoroughly tested.

[iceofwater]

Hi，thank you for your response，please tell me how many the commit number, and yours code can translate the airspy callback content for float data to a wav file success?

[jvde-github]

install the latest and add these to your command line for AIS-catcher at the end: -m 9 -go file output.wav

[iceofwater]

When i used the latest codes, it shows me an error then end the running:
input 9 out of range [0,5]
So i ask you how many the commit number, i think the latest isn't i need?

[jvde-github]

It sounds like you are not on the latest edge version indeed. Just re-install with the script (see docs) or download the Edge version for windows.

It is in the edge version indeed.

[iceofwater]

OK, i was tried to run precompiled AIS-catcher, it can write wav file, but the wav file's samplerate is 2.5M, it is too big, can let the samplerate reduce, for example the AIS-catcher can support the smallest samplerate is 96k? Because the file use 2.5M samplerate will occupy the disk space very fast.
When store the wav file, then need to palyback use -w to restore the signal data.

[iceofwater]

I want to directly obtain float type data from the callback function of Airspy on AIS catcher, convert this data into a 96k file (preferably the smaller the file, the better, but the minimum supported in AIS catcher code is 96k), and then this file can still be replayed through the - w parameter of AIS catcher.
Then I also discovered an issue, which is that AIS catchers have high requirements for the playback of WAV files, as shown below:
if (header.wFormatTag == 3 && header.wBitsPerSample == 32)
Device::setFormat(Format::CF32);
else if (header.wFormatTag == 1 && header.wBitsPerSample == 8)
Device::setFormat(Format::CU8);
else if (header.wFormatTag == 1 && header.wBitsPerSample == 16)
Device::setFormat(Format::CS16);
else
throw std::runtime_error("format not supported");
However, when creating WAV files, I found it difficult to achieve Format 3 and Bitesimple 32. Such files may have additional PEAK and FACT fields in their content, but AIS catchers cannot handle them. Therefore, I added the process of skipping PEAK and FACT to correctly replay WAV files.