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Poor Audio Quality, FlightCom 403 Stereo Intercom



 
 
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  #1  
Old April 13th 06, 12:31 AM posted to rec.aviation.owning
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Default Poor Audio Quality, FlightCom 403 Stereo Intercom

FYI: I am helping a 210 owner install a FlightCom 403 stereo
panel-mounted intercom. After straightening out some
shielding/grounding/wiring/input-level issues, and while testing the
unit, I noticed that the music quality through his headphones sounded
very "tinny", was obviously lacking in bass response, level was too low
to be useable when the engine was running, and it has quite a lot of
distortion. I'm used to good sounding stereo in my own a/c, so I have a
basis for comparison. btw, I homebrewed my own stereo music switcher
from scratch..

I went and got my stereo Lightspeed headsets, no improvement. I went
and got my Sony CD DiscMan CD player to use as a music source; no
improvement. Next we called customer service at FlightCom. They were
very helpful, but I had already tried everything the technician
suggested, with no improvement.

During the call, the technician mentioned that they had recently done a
design change because customers were complaining that the entertainment
level was too low relative to the intercom and/or aircraft radios,
especially when using MP3 and IPOD players. Without really saying so,
he hinted that I might want to undo the ECO change, and offered to
email me a schematic/parts-placement diagram.

I used the supplied data to "reverse-engineer" their intercom. It
became obvious to me that their design change was in-fact responsible
for the distortion. They added two resistors to increase the gain of
the "input" amplifiers, thinking that would make MP3/IPOD players
louder, but what they actually did was to drive the input ampifiers
into saturation at normal listening levels. Undoing the change
(removing two resistors) reduces the gain back to where it used to be,
eliminating the saturation (clipping) and the distortion is gone.

However, that left the entertainment level at the headphone outputs too
low. I added two resistors (parallel with existing resistors) in the
summing network at the main Left & Right output amplifiers, thereby
increasing the level of the entertainment without changing the relative
loudness of either the intercom or aircraft radios.

That still leaves the missing bass response. I analyzed their circuit,
and sure enough, each entertainment channel has three coupling
capacitors in the path that are too small by an order of magnitude
(0.1uF should be 1uF or more, in six places).

Sweeping the audio response from the entertainment input to the stereo
headphone output in an unmodifed unit shows that the low frequency
response is down -3db at 300Hz, and -55db at 20Hz. That is what makes
music sound like it is being pushed through a taught string! I sent an
email to FlightCom to suggest that they need to hire me as a consultant
to fix their design...

In summary, the unit as it comes from FlightCom is unacceptable for
music quality. Even though you could fix it, how many of you are
willing to go through the hassle. In its present state, I can't
recommend it to anybody...

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  #2  
Old April 13th 06, 10:00 AM posted to rec.aviation.owning
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Default Poor Audio Quality, FlightCom 403 Stereo Intercom

Mikem,

In summary, the unit as it comes from FlightCom is unacceptable for
music quality.


One name: PS Engineering.

--
Thomas Borchert (EDDH)

  #3  
Old April 16th 06, 04:52 PM posted to rec.aviation.owning
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Default Poor Audio Quality, FlightCom 403 Stereo Intercom

mikem wrote:
FYI: I am helping a 210 owner install a FlightCom 403 stereo

[snip]
Sweeping the audio response from the entertainment input to the stereo
headphone output in an unmodifed unit shows that the low frequency
response is down -3db at 300Hz, and -55db at 20Hz. That is what makes
music sound like it is being pushed through a taught string! I sent an
email to FlightCom to suggest that they need to hire me as a consultant
to fix their design...

In summary, the unit as it comes from FlightCom is unacceptable for
music quality. Even though you could fix it, how many of you are
willing to go through the hassle. In its present state, I can't
recommend it to anybody...


Thanks for that writeup Mike, I am shopping for a new intercom (for my
210 no less) and will avoid that one!

--
Cheers,
John Clonts
Temple, Texas
N7NZ
  #4  
Old April 16th 06, 06:36 PM posted to rec.aviation.owning
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Default Poor Audio Quality, FlightCom 403 Stereo Intercom

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.

Given a 3 dB corner (I would have preferred a 9 dB corner for reasons that
will become obvious) and a 20 dB/decade drop per pole, it would appear that
there are three poles between input and output that are band limiting. I
subjectively agree that two of these are unnecessary, but I'd just as soon
start rolling everything off below 300 Hz., albeit not quite this fast.

Given your measurements below and postulating three poles lets me guess that
the 9 dB (3 dB per pole) is going to be somewhere around 150 Hz. and then
the 60 dB point 15 Hz. That is my best guess at what the designer was
trying to achieve.

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 {;-). BTW, if what you said was in fact the case,
going from a 0.1 uF capacitor (mylar) to a 1 uF capacitor (electrolytic)
saves me 3 dB in board real estate and 6 dB in cost. Hell of a tradeoff for
better performance -- smaller and cheaper (if the topology allows capacitor
polarization).

Jim

mikem wrote:
FYI: I am helping a 210 owner install a FlightCom 403 stereo

[snip]
Sweeping the audio response from the entertainment input to the stereo
headphone output in an unmodifed unit shows that the low frequency
response is down -3db at 300Hz, and -55db at 20Hz. That is what makes
music sound like it is being pushed through a taught string! I sent an
email to FlightCom to suggest that they need to hire me as a consultant
to fix their design...



  #5  
Old April 17th 06, 07:30 AM posted to rec.aviation.owning
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Default 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!

-----------------------------------------------------------------------

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...

  #6  
Old April 17th 06, 04:40 PM posted to rec.aviation.owning
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Default Poor Audio Quality, FlightCom 403 Stereo Intercom

Mikem,

The 403 does NOT acheive this requirement!


Again, IMHO you got what you paid for. If you want an "entertainment
channel" in your aircraft, not an intercom, then you'll have to shell
out the dollars for it.

--
Thomas Borchert (EDDH)

 




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