On 30 Aug 2006 18:10:23 -0700, "nrp" wrote:

On inquiry, I was told by technical people (I suspect it is a
near-garage shop operation) at Headsets Inc that their rejection system
is all analog. Being somewhat familiar with analog and digital noise
cancelling systems, I can verify that from the HInc system that I have.
Oddly enough the final performance seems somewhat similar.

It is easy to talk about a 180 degree cancelling pressure waveform
generator (per Roger above), but that is an over-simplification of the
problem that isn't acheiveable from a real control loop stability
standpoint.

True it is a bit of an over simplification and it doesn't give the
ultimate noise cancellation of digital overall or predictive over
specific ranges.

OTOH it does work very well as any user who has turned off their
analog ANR while in flight and then turned them back on. The
difference is astounding and it is substantial so the analog phase
canceling works. Also this is not an area where you are looking for
control loop stability, but rater effectiveness and they are not the
same.


Digital systems get around this by a slow optimization in the frequency

Remember the add hype and the claims that the new Telex has this
exclusively. We have to remember too that when you look at the
difference in db it doesn't appear much different. OTOH db is a ratio
and every 3 db is a doubling, or cutting in half depending on the
direction you are headed.

domain over a wide (i. e. hi) frequency range at the expense of being
able to cancel random noise, whereas analog does it in the time at the
expense of bandwidth, limiting it to a few hundred Hz, but making

My day-to-day experience says the bandwidth limitation is not nearly
that drastic, but is on the order of one full magnitude wider at 3,000
Hz where the fall off becomes noticeable.

analog better at random uncorrelated noise rejection. The Headsets inc
website posted the rejection capability of their system At low
frequencies it is about as good as the best digital system.

At high frequencies (above say 300 Hz), the passive rejection
capability of a good headset (mine is a David Clark H10-40) is more
than adequate to the task for my ears.

I've been trying to get HI interested in random motorcycle helmet noise
cancellation (generally under 100 Hz) but they don't seem interested -
or they may know why the analog control system can't be made stable.

A system that works on random noise by nature isn't stable, or we are
using the word differently. However a low frequency repetition rate
does not necessarily mean a low frequency sound. The bark of an
exhaust, supersonic prop tips, or a gun shot are all sounds with very
short rise times which makes them a high frequency sound, but with a
low repetition.

From my engineering experience, a 300 Hz analog response loop is an
impressive acheivement.

They sure do a good job on that big Continental exhaust bark at low
RPM. All you hear are the accessories running.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com