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[aprssig] Kenwood D700 TNC Transmit Equalization ScrewedUp! (Repost)

Robert Bruninga bruninga at usna.edu
Mon Apr 25 23:51:29 UTC 2005


Scott,
This debate is as old as packet radio.  Some insist on
discriminator connection, some insist the other way.
And it is trivial to add some emphasis back into a digi
to compormise between the two schools of thought.
Just a simple capacitor of the right value between the
receiver and the input to the TNC will do the trick.
Bob

>>> scott at opentrac.org 4/25/05 7:17:57 PM >>>
Am I the only one who sees this as a significant design flaw?  From
John
Ackermann's site:

"More important, if the low tone coming out of the speaker is stronger
than
the high tone, many TNCs based on the TAPR TNC-2 design, including the
very
popular MFJ-1270 series, will have great difficulty demodulating
packets.
The demodulator chip in these TNCs, the XR-2211, is particularly
unhappy if
it sees a low tone that is louder than the high tone. Because XR-2211
demodulators are so common, it's very important that we transmit
signals
that these units can decode, and that means signals that do not have a
a
twist favoring the low tone."
http://www.febo.com/packet/layer-one/transmit.html 

There are a LOT of digis out there that are going to be at a real
disadvantage because of this.  How many of them can realistically be
expected to adjust their deemphasis?

Has anyone brought this to Kenwood's attention?  How did these even
come
about?  I thought 6 dB/octave preemphasis had been the standard for as
long
as 1200 baud packet's been around.

On the other hand, maybe it's good for Kenwood's brand lock-in... after
all,
they'll decode each other just fine.  TNC-2 won't cut it?  Gotta buy
yourself a new D700...

Scott, N1VG
http://n1vg.net/opentracker 


----- Original Message ----- 
From: "Robert Bruninga" <bruninga at usna.edu>
To: <wa8lmf2 at aol.com>; <aprssig at lists.tapr.org>
Sent: Monday, April 25, 2005 10:54 AM
Subject: Re: [aprssig] Kenwood D700 TNC Transmit Equalization
ScrewedUp!
(Repost)


> >>> wa8lmf2 at aol.com 4/25/05 5:05:22 AM >>>
> >In the course of capturing signals..., I was shocked to
> >discover that the Kenwood D700 transmits 1200 baud "flat"
> >with no pre-emphasis...
>
> Yes, the TNC is used at both 9600 and 1200 baud and
> the 9600 baud has to be flat to work well.  Apparently
> 1200 baud is also transmitted and received flat.  This is why
> it is important to make sure in all digipeaters that there is no
> audio roll-off any worse than the usual de-emphasis. or even
> better, that the TNC is connected to the discriminator...
>
> A good compromise is to set about 3 dB of de-emphasis
> at the digis to account for both types of users.
>
> Bob


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