Automating recording

I work for a small school. We would like to record all lectures given in our main lecture room. The presenter always uses a microphone and I have a way to tap the audio out of our loudspeaker system at line level. I would like to make the recording process as automatic and simple as possible since we have many technically challenged people in the school.

My thinking is to simply record the audio out of our speaker system every day from 8am til 11pm (we have weekend and evening lectures) with a headless Mac mini and then extract audio from these recordings as desired. I understand Audacity would allow me to strip out the silence fairly easily. Is this a reasonable approach? Is there a good way to do this automatically with Audacity?

Thanks :slight_smile:

I suspect it would be easier and more reliable to use a stand alone digital recorder. Typically they record onto a memory card, which can then be copied onto a computer for editing.

The problem is there’s not necessarily anyone on-site for each event to activate the recording. When a guest lecturer comes they simply switch on the microphone and give their presentation. I can’t rely on that person to manage starting and stopping the recording. If the switching on of the mic would start the recording, this would work. Can stand-alone digital recorders do this?

With something like a Zoom H2n (other makes and models are available), the device has it’s own built in microphones, which are reasonably good quality. You could probably replace your current mic with such a device, and connect it’s headphone (monitor) output to the speaker system. Permanently attach the instructions for how to switch on the “mic” to the lectern.

If I were approaching the task this way, I’d probably not use Audacity for the recording part, but rather use a simpler app that can write audio files (WAV for best quality. MP3 or OGG if I need to conserve disk space) directly to disk.

I’m more familiar with Linux than macOS, but I’d expect a similar approach would be available on macOS. On Linux, I’d set up Chron jobs (scheduled tasks) to run a Python script multiple times per day (coinciding with period breaks). The Python script would start a command line recorder (such as “arecord”) to record a WAV file direct to disk, and ensure that each file has a unique name (probably using date and time in the file name).

Can stand-alone digital recorders do this?

That’s Sound Activated Recording. That can sound like a surveillance recording, not a pleasant lecture or theatrical presentation. It’s generally choppy with smaller than normal gaps between phrases or at worst, ideas running into each other.

“Thank you for your attentionI’ve enjoyed being hereToday I’d like to discuss the problems of interval scheduling.”

So the problem then is not to capture the valuable work, but turning it into something useful later—which could be a career move or a foreshortened pathway to the loonie bin.

We had a similar problem recording the sound from videoconferenced meetings. In order to get all sides of the conference, we had a separate “mute” performer with no microphone and who didn’t speak, but recorded everything. Routinely, nobody would stop the recording at the end of the meeting.

You have one of those impossible jobs. It’s 93% unattended and never gets to 100%.

You could throw Engineering® at it. Make the act of turning on the microphone happen through a control panel rather than just a switch on the microphone cable. Obviously, the control panel then also starts the recorder.


No lectern, unfortunately. They have a head-mounted mic and (sometimes) walk around with it during the lecture.

This is a good idea. I thought I might be able to call Audacity to make the recording using such a method. But I could live with a Python script. I guess I need to see what’s available for recording from the command line on a Mac.

You understand exactly my situation. Notice I said the presenter turns on the mic, but not that they turn it back off. Our solution for the presenter leaving it on and the batteries running out overnight is to have rechargeable batteries ready in a charger. Now if I could just get people to put the dead batteries into the charger properly (ie. not backwards) I would be all set.

I would love a solution where the switching on of the mic also triggered the recording to start but I can’t come up with a bulletproof way to do this. Hence just recording everything.

The disadvantage of using Audacity in this way, is that Audacity’s “native format” is the “Audacity Project” (not an “audio file”).

Audacity projects are typically much bigger than an ordinary audio file due to the very high precision 32-bit float format, plus all the extra stuff that projects support that ordinary audio files don’t.

Audacity projects are more difficult to manage, because they have multiple parts (the “.aup” file and the associated “_data” folder that contains the data).

Until an Audacity Project is “Saved”, the audio data only exists in a temp directory. If the computer shuts down (for any reason) before the project is saved, the entire recording session is likely to be lost (destroyed).

To create a normal audio file from Audacity, you have to “Export” the audio. This adds complexity to the task which is unnecessary if you use a recording app that can write an audio file direct to disk.

Perhaps SoX

You may be able to use Applescript / Automator rather than Chron and Python (I don’t know much about Applescript, and have only rarely used Automator).

I’m going to go out on a limb here and recommend a commercial solution.

Audio Hijack from Rogue Amoeba can be programmed to start and stop recordings at specific times and name the recording with the date and time. If you know the schedule for the lectures, you could program the Mac Mini to record during those times. Audio Hijack also has silence monitoring, so you can program it to start a new recording after it detects silence for a certain duration.

You don’t want to record one long file from 8 am to 11 pm. That’ll be way to big to bring into Audacity.

I did this years ago when I worked at a community radio station. We were required by law to record everything that was broadcast and retain those recordings for 30 days. I used a headless Mac Mini to make recordings one hour long.

– Bill

Audio Hijack looks like a perfect fit- thank you billw58!