envelope follower

[Martin Czech]

Design idea for narrow bandwith envelope follower:

Usually, envelope followers detect the amplitude of a signal
via rectifying and lowpass filtering.

Problem: 

To convert the rectified output to "dc" some lowpass filter is
required. For proper filtering at low input frequencys high steepness/
high order and low cutoff frequency are necessary.

On the other hand, for good transient tracking the cutoff should be as
high as possible, with no overshoot. This means also low steepness.

It is not possible to get both with a simple recitifier/lp
arrangement.

I think a better way to handle this is to seperate the audio signal
into severall bands and to process each band seperately like it is done
in compander systems.

Any experience about that out there?  Do you know any follower that
utilises this kind of idea ?

For very narrow bandwith systems, like vocoder channels, the signal can
be assumed as sinusoid, fixed frequency. I remember the usuall ac/dc
converter scheme for 3-phase-power-distribution with 6 diodes giving
rather good dc with little ripple. Now, this can be used for the above
situation:  I used three precision rectifiers, the first "normal", the
second with simple highpass filter before the input, the third with
simple lowpass filter before it's input.  The filters are set up in
such a way, that +-60 deg phase shiftet signals get into the
rectifiers. The rectifiers outputs are summed such that the amplitude
loss for the filtered branches is compensated.  

Result : 
The circuit output gives pretty good "dc", the peak to peak ac part is
only 13% (100% with standard rectifier).  Furthermore the main ac
component has a 6 times higher frequency than the audio input (only 2
times higher in standard case). This makes it much easier to find a
proper lowpass filter suitable for all applications.


Example : 

Lowpass is 4-th order Bessel (best choice for no overshoot) with fc such
that it needs one audio signal cycle to settle.

"3-phase" system yields -70dB ac suppresion

"standard" rectifier -31dB ac suppresion

In case of 10% frequency or phase shifter mismatch

"3-phase" system yields -60dB ac suppresion

"standard" rectifier -34dB ac suppresion

This means 30dB improvement on the expense of tripple rectifiers
and two resistors and two low tollerance caps. The filter fc
could have been a bit lower in the example, so that -80dB are possible
for a reasonable follower.

m.c.