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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...