fons@xxxxxxxxxxxxxxx wrote: > On Sat, Oct 24, 2009 at 12:16:58PM +0100, Folderol wrote: > >> Very interesting information. It's fascinating how complicated something >> like a 'simple' metering system is! > > Well, it's not really complicated... > > 'Traditional' VU meters had no active electronics > at all, just a bridge rectifier and a resistor. The > dynamic response was determined only by the mechanics > of the meter. This means that a 'real' mechanical VU > was not just a V or A meter with a different scale, > it had to be engineered to have the right response. > > Now such a moving coil meter is just a classic mass + > spring + damping system, which corresponds to a second > order low pass filter. So to get the correct response > from a software VU you take the absolute value of the > signal and apply the low pass filter. For a VU the > filter was defined in the time domain: with a step > input It should reach 99% of the final value in 300ms, > overshoot to between 101% and 101.5%, and finally > come to rest at 100%. The overshoot means sub-critical > damping, so the filter is 'ringing' (but not much). > The -3dB frequency for a VU filter is around 2Hz. > > The filter used in jkmeter is similar, but has exactly > critical damping, it does not overshoot, and the -3dB > frequency is around 1Hz. The other difference is that > the input to the filter is not the absolute value of > the signal but the square of it. This means you can't > direclty compare the 'speed'. For short signals (less > than 125 ms) the K meter is faster than a VU would be, > for longer ones it is a bit slower, and it also falls > back slower than a VU. If anyone is interested in the > exact time and frequency domain curves just ask off- > list. I started off my working life with 17 years in radio broadcasting, so the "traditional" vu meter and it's rigidly defined characteristics were very much a staple of life. I think that experience still colours my thinking. Just to drift back to the zyn/yoshi metering for a moment - with 0.031 I've gone back to the standard zyn characteristics, and just eliminated the need to grab the master lock. I think it's better now. Thank you Fons, your generosity in sharing your knowledge is very much appreciated. cheers, Cal _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/mailman/listinfo/linux-audio-user
