# How does an RMS detector work?

Discussion in 'Electronic Basics' started by MRW, May 30, 2007.

1. ### MRWGuest

I'm currently delving into audio stuff. I was wondering how exactly
does an RMS detector work. I can picture how a peak detector works,
but if you send a speech signal to an RMS detector, how does it
compute the RMS value?

Also, does attack time dictate the time period for RMS calculations?

2. ### John PopelishGuest

It squares the signal (converting it to a unidirectional
signal by the transformation), averages that with a low pass
filter, and then takes the square root of the low passed
signal. RMS means square Root of the Mean of the Square of
a signal. Mean is a way of saying average over some time.

3. ### BobGGuest

The RMS volts is the same as the DC volts that gives the same watts.
In fact, the teletronix RMS limiter used an incandescent light bulb as
the rms calculator. There is certainly a 'time constant' associated
with it... get an old speaker voice coil, hold it between your fingers
and run music thru it and turn the volume up and down and it gets
hotter and cooler. The 'crest factor' is the peak to RMS ratio. I was
surprised to see that the RMS seems to track the avg but 3dB higher
for most of the music I've run thru my rms and avg calculator program,
so either you can use an RC which gives a good avg and just add 3dB to
get the rms, or my program is messed up. Anyone else want to run a
wave file thru an rmsser and an averager and see what comes out?

4. ### CharlesGuest

One way is to dump it into a resistive load and measure the temperature rise
of the load. By the way, equal dc and rms voltages cause the same
temperature rise in a given load.

Also, check this out: http://www.national.com/ms/LB/LB-25.pdf

5. ### Phil AllisonGuest

"MRW"

** Where did you come across an "rms detector" in audio gear ??

If so, their use of the term is 100% bollocks.

** A " true rms to DC converter " has a slow response time when used for
audio band signals, ie 100mS at least.

Not must use in audio, unless maybe you are trying to compute the heat
dissipated in a speaker's voice coil.

........ Phil

6. ### ChrisGuest

A really great and thorough response to your question is in Analog
Devices' "RMS-to-DC Application Guide", Second Edition. It's
http://www.analog.com/en/content/0,2886,773%5F866%5F15010,00.html

The whole thing is over 10MB in .zip file format. Most of your answer
will be in secion 1: theory:

Take some time, and read it. You'll definitely learn the answer to
your question, as well as a lot of other stuff. It's presented in the
old Analogue Dialogue method, so even students and newbies can get a
lot out of it.

Good luck
Chris

7. ### MRWGuest

Wow! Thanks, Chris! I've been looking for something like that. Time to
hit the books.

8. ### Phil AllisonGuest

"MRW"

How about you tell us what the heck you are on about.

Is it a " dbx " comp / limiter - or not ?

........ Phil

9. ### JasenGuest

one way is to digitise and then arithmetically compute the RMS

another is to use a logarythmic amplifier

another is the feed it to a resistor and see how hot it gets.

Bye.
Jasen

10. ### Rudolf DrabekGuest

meter.
Take care for the crest factor!

11. ### Phil AllisonGuest

** LOL !!

Love to feed * Jasen the Fuckwit Jerkoff * a 120 watt soldering iron
where the sun don't shine.

....... Phil

12. ### BobGGuest

So I'm starting to get the picture... the Aussies dont like the Kiwis,
the Norskis dont like the Finns, the French dont like the Bosch, the
Yanks dont like the Wetbacks. Thus is the wonderful peacful
civilization of humans.

13. ### MRWGuest

Can't we all just get along? I thought the Internet was a place of
homogeneity, but I guess our IP addresses deceive us.

14. ### meGuest

Not only off topic, but glaringly obvious throughout human history....

16. ### MarraGuest

0.707 of the peak value ?

17. ### redbellyGuest

Except during "National Brotherhood Week" . . .

18. ### redbellyGuest

It seems that standard practice is to filter out or somehow exclude
the DC value when doing rms measurements. I.E., a DVM or
oscilloscope set to "AC" will read Zero when measuring a constant DC
signal.

Is there anything like a standard, agreed-upon cutoff frequency used
by DVM's and 'scopes for AC/RMS measurements? I'd guess mainly you'd
want it well below the lowest audio frequencies of ~20 Hz, so that
audio and higher frequencies are included in the measurement.

Mark

19. ### Phil AllisonGuest

** ROTFL !!

Love to feed * Jasen the Autistic Fuckwit Jerkoff * a 120 watt soldering
iron
where the sun don't shine.

....... Phil

20. ### JasenGuest

yeah, it makes sense to filter out the DC when doing an AC measurement,
and it hard not to filter out the AC when making a DC
measurement.
certainly rolloff would want to be below mains frequency.

probably anything that's too high in frequency to read on the DC scale should
appear as AC.

ideally the sum of the AC and DC readings should be the RMS of the
whole signal (assuming that the DC is advertised as mean and not RMS
reading) , I guess that puts the cutoff frequency down below 3Hz somewhere

Bye.
Jasen