Swinging Needles (analog VU meters)

…finally!

It took me some time (and strong coffee), but I finally managed to have the ESP32 version of my Internet Radio drive two vintage style analog VU meters!

 

And here they are…

Web browser controlled prototype with basic display and no matrix keypad attached

Soon after I had purchased a rather obscure VU meter kit, it turned out that the driver board was completely useless because it was mono (Chinese for stereo?).

So I decided to have the ESP32 read the analog (DC-biased) line out signals from the VS1053 mp3 decoder, map the peak-to-peak readings to [0,255] scaled values and send these to the ESP’s DACs for driving the meters. However, I couldn’t figure out how to do that without having to connect the VS1053’s floating audio ground to the system ground, which, so I guessed, would either not work, cause noise or even damage something.

Then I remembered the VU meter plugin for the VS1053, enabling you to read dB levels from a memory register. But these plugin readings are leaking peak dB levels, so I started wondering if it was possible to use them for reconstructing instantaneous dB levels. Not suitable for audio purists, but I wouldn’t mind if the result was convincing. After all, these cheap Chinese meters will never behave like professional VU meters anyway.

So I wrote a simple algorithm with some tuning parameters and functions for scaling and smoothing, calculated resistor values for limiting current to my meters to 0.4 mA max, added (probably unnecessary) flyback diodes to protect the ESP32 from negative voltage spikes, and just gave it a try. Here’s the wiring that I used for the meters.

The first result looked surprisingly good: the needles eagerly followed every move and twist of the music. Since everything is controlled by an algorithm, any desired meter ballistics can be easily simulated without electronics.

For now, further fine tuning seems unnecessary. Besides, a more audiophile approach already crossed my mind: have a FreeRTOS task listen in to the mp3 stream while it’s being sent to the VS1053, and programmatically decode it. That will allow me to extract true dB levels and could even become a replacement for the VS1053 board. Like I recently read in an article about Software Defined Radio (SDR), “software is the new hardware“.

Anyway, music finally looks almost as good again as it did in my youth. Let’s just hope for an analog VU meter revival soon. And how about a global rediscovery of reel-to-reel tapedecks (with analog VU meters, of course)? That would make much more sense to me than the somewhat puzzling comeback of vinyl.