Recording the decoded audio

[pjsg]

Would anybody be interested in being able to record the decoded audio to (say) a .wav file. I'm interested in using the web interface to tune to an interesting frequency, record the audio, and then feed the audio into other tools for further decoding.

Anybody else want this as well?

[fventuri]

@pjsg - I think it is a very good idea and it can be useful to others too.

I don't know if you are already familiar with the program 'pcmrecord' that comes with ka9q-radio (https://github.com/ka9q/ka9q-radio/blob/main/pcmrecord.c).
Since ka9q-web requires an install of ka9q-radio, I wonder if perhaps what you are suggesting could be achieved with a new button that triggers a 'fork+exec' of 'pcmrecord' with the suitable command line arguments.

Franco

[pjsg]

I was thinking about using pcmrecord, but that would have to be run on the server, and then the output transferred to the browser. The browser already has the audio in a suitable format, so it shouldn't be difficult to save it to disk as well.

[fventuri]

Good point about the web server.

Since WAV is just a very thing wrapper around the PCM stream, I agree with you that it should be easy to write to file.
If I remember correctly one of the fields in the WAV header is the length of the data stream, so you would have to know how long the recording is going to be before the file can be written to disk (or perhaps you would be able to write that field at the end using 'seek()', but I am not sure if browsers support that).

Franco

[ka9q]

I was thinking about using pcmrecord, but that would have to be run on the server, and then the output transferred to the browser. The browser already has the audio in a suitable format, so it shouldn't be difficult to save it to disk as well.

What do you mean by "suitable format"? I don't know how audio streaming works over the web, but I suspect it might be compressed (I think Opus is common). So the browser may not already have the audio in PCM, if that's what you want.

[pjsg]

I think that, in this case, we actually have the WAV file -- well we have 16bit 12000Hz mono data stream that can easily be written into a file. This appears to be sent over the websocket connection to the web server program.

There seem to be two options for recording the audio from a user perspective:

1: (Conventional - single button): Click a button to start recording, get a dialog to pick a file to save to. When recording is done, click the same button again (which now says 'stop recording').

2: Alternative - two buttons: Click 'Start' button to start recording (then the button changes to 'restart'). Other button is 'save' which opens the file save dialog and saves the recorded audio.

In both cases, cancelling out of the 'File save' dialog would discard the data / not start the recording.

These buttons would probably only work if the audio was currently playing.