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Headphones protector, next problem: I/O levels

published Mar 06, 2019 10:40   by admin ( last modified Mar 06, 2019 10:40 )

It seems clear now that I have succeeded in building a headphones protector with a Raspberry Pi 3B+, Jackd and Rakarrack.

However some very practical problems remain, and they would actually remain for whatever solution I would have chosen, namely: How do I know I'm feeding the compressor at the right level? If the input signal is too low, the compressor/limiter may not kick in.

So I need an indicator, a VU meter that it is indeed being fed at a good level. I could go the route of having a VU meter directly monitoring the input before it reaches the Pi, but that would be a lot of tinkering.

Since Rakarrack already has built in meters for input and output, it's probably better to outfit the Pi with a small screen, and then create a viewport with xrandr that ensures that those meters of Rakarrack are displayed. And it should be a touch screen so the sliders adjacent to the VU meters can be used.

The second problem is the output level. I would really like to have an analogue potentiometer to be able to adjust the volume there, outside of the Pi. However making a voltage divider with the headset as a part in it, will introduce weird non-linear responses, since headphones vary in impedance depending on frequency. I better go and check to see how much headphone impedance does vary with frequency…