Poor Audio Quality, FlightCom 403 Stereo Intercom

RST Engineering wrote:
Mike, with all due respects, the designer can only do what the requirements
group tell him or her to do. The design doesn't need "fixing", the
requirements need to change.

For voice quality intercoms, a bandwidth of 300-3000 Hz is considered
entirely adequate. No, it won't reproduce the cannon in the 1812 Overture
worth a damn, but it will give entertainment quality sound. It is NOT tin
can quality, it is a bit better than telephone quality which is what an
intercom was intended to do in the first place.

It is YOUR subjective opinion that a low frequency corner of 300 Hz. is
inadequate. That may have been the design goal -- intercom quality with
lowered rumble and hiss to the music. Purely subjective.

Maybe subjective, but here are my requirements for the entertainment
channel
in my airplanes:

I use Lightspeed ANR headsets. They do a great job of reducing the
ambient
cockpit noise in the range from 20 to 250Hz; above that, the noise
reduction is equivelent to good passive headsets. Since I am of
advanced
age and am loosing my high frequency hearing, music sounds best when I
can
hear the bass, and the bass sounds particularly good when the ANR gets
rid
of anything that would compete with it.

I have taken the Lightspeeds home and compared them to my old expensive

Koss (dating myself) stereo headphones that I use for classical music
listening after the wife goes to bed. The Lightspeeds are almost as
good as the Koss. btw-David Clark and other brands suck in this
comparison.

If I connect my Sony Walkman CD player to the Lightspeeds, they sound
better (richer, fuller bass) than the little headphones that came with
the
Sony. The level is a little low, but I'll talk about that later. The
quality is as good as listening through my home CD deck and Stereo amp.


When I use the same Lightspeeds and CD player in the aircraft, I dont
want the sound to be made worse by having an intercom in between the CD

and the Lightspeeds. The 403 does NOT acheive this requirement!

I agree that the frequency response of the aircraft-to-headphones and
intercom-to-headphones channels of an aircraft intercom should be
constrained to 300-3000Hz, the music channel should not.

...
The wonderful thing about being the requirements department in the morning
and the design department in the afternoon lets me take the heat for the
whole damned product {;-).

I dont care if it was the Sales or the Engineering dept that screwed up

the 403; it is definitely screwed up!

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More on the Sony output being too low to drive the Lightspeeds: The max

loudness is just short of what you would like it to be in the aircraft.
It
is fine in a quiet living room. The Sony player runs on just two AAs,
and
can produce a maximum of 3Vp-p (1Vrms) into the LightSpeeds. The
Lightspeeds require about 5Vp-p to drive them to an approprate loudness

for a noisy cockpit, even with the ANR enabled. The Sony headphones
have
a lower impedance than the Lightspeeds, so they sound louder...

I learned this when I tried to use the Sony CD player with the
Sigtronics
SPA400 with add-on RES400 stereo music switcher in my Pacer. The RES400

uses a DPDT relay to switch either the aircraft and intercom audio, or
the
music to the headphones; no amplification, no frequency shaping.

It sounds good on the ground, but at takeoff/criuse rpms, the level is
just a little low. I ended up building a stereo amp with a gain of
about 3 and putting it between the CD output and the input to the
RES400.
It works so well that I built a similar stereo amp/relay switcher for
the 182. When I plugged the same CD player and Lightspeeds as I use in
my
aircraft into the friends 210, the missing frequency response is
glaring...