[OZAPRS] Setting up TT4 Tx on 2m

Fri Feb 19 09:38:05 EST 2010

Ray / All,

Several years I did some extensive testing on the effects of deemphasis on
received AX25 signals at APRS digipeaters.

I found that the digipeaters reception and decoding of data worked best (1
to 2 db better) when the deemphasis was bypassed. This was the case for both
preephasised and non-preephasised transmitted signals. This is partly
explained by the fact that most TNC (especially the MFJs) require the high
tone of the received audio to be louder than the low tone for correct
decoding. Especially for weak signals.

I also looked at the output of the Kenwood D7 & D700 and found that they
only used a 3db per octave preephasised / deemphasis on the data TX / RX.

All VK3 digipeaters have been modified to bypass the preephasised /
deemphasis to obtain the better receive performance. The other advantage of
this is that we bypassed the audio volume control and squelch circuitry
which makes the digi on the hill less prone to accidentally adjustments.

Cheers

Richard

VK3JFK

  _____  

From: ozaprs-bounces at aprs.net.au [mailto:ozaprs-bounces at aprs.net.au] On
Behalf Of Ray Wells
Sent: Thursday, 18 February 2010 1:15 PM
To: Australian APRS Users
Subject: Re: [OZAPRS] Setting up TT4 Tx on 2m

David/All,

I'm not suggesting your problem is tone level related but failure to grasp
the following is arguably responsible for more TNC decode issues than any
other cause.

The need for "twist" adjustment depends on the radio hardware involved so a
brief discussion is probably in order.

The "norm" for an FM/PM radio is to apply 6dB (x2) per octave of preemphasis
in the transmitter. For packet, this results in the high tone (2200Hz)
having twice (near enough) the deviation as the low tone (1200). In the
receiver deemphasis is applied which restores the audio to a "flat"
condition, i.e. both tones going to the TNC have the same level.

If TNC audio is fed into the microphone socket you can depend on 6dB per
octave preemphasis. However, if the TNC audio is fed in further down the tx
audio chain, such as the audio injection point for tone signalling,
preemphasis will not be applied in the tx and should be applied externally -
with twist, in the case of the TT4.

Going to the receiver again, if it receives a signal without preemphasis, it
will still apply deemphasis (assuming audio is taken after deemphasis),
resulting in the high tone being just half the level of the low tone when
fed to the TNC. FSK demodulators based on the the  exar pair 2206/2211 will
not decode if the high tone is lower in amplitude than the low tone, by even
a very small amount.

If rx audio is taken from the demodulator in the radio, i.e. before
deemphasis, the preceding is not a problem because audio that is transmitted
"flat" will be decoded "flat".

The exar chip set (found in genuine TNC2 TNC's)  is not disturbed by high
tone being considerably higher in amplitude than the low tone.

In summary, every tx should employ preemphasis, regardless of it being
generated internally, or externally. That will eliminate any decode problems
in TNC's with finicky chip sets because high tone will always be the same
level as, or higher than, the low tone coming out of the rx.

David, is TXD 60 60mS or 600mS? Although TXD provides time for your tx to
stabilise before valid data is sent, it can also have an impact on the
remote rx in cases where muted audio is being used for the TNC. There are
some very sluggish receivers kicking about and TXD needs to be long enough
for that rx to unmute. My Icon handheld scanner is pathetic in this regard,
with the mute being so slow that I can miss at least half of the digi
beacon, even when I'm at the digi site.

Get that CRO working and save your hair, lest you should end up like me :-) 

Ray vk2tv

David and Justine Olsen wrote: 

I am presently running TxD of 60 and TXTWIST of 50. I've tried a range of
TXTWIST but none sound too different 

David

On 18 Feb 2010, at 00:36, Scott Evans wrote:

ABAUD is just the speed set for serial port 1 ... what TXDELAY are you
running? because depending on how quick you radio is you may need to
lengthen the delay a little, also check the TXTWIST setting too, this
controls the level between the 1200Hz & 2200Hz tones.

On Wed, 2010-02-17 at 21:51 +1000, David and Justine Olsen wrote: 

ABAUD is set to 19200

On 17 Feb 2010, at 21:37, Norm, VK3XCI wrote:

> Sort of like 9600 baud?
> 
> 
> 
> 
> 
> 
> 
> 73 de Norm, VK3XCI
> Mildura, Australia
> The Wintersun City
> QF15bt.
> 
> David and Justine Olsen wrote:
>> I've gone through all of the TT4 audio adjustments as explained in
>> the help docs by listening on another set, but still no success being
>> received by any digis.
>> 
>> One thing that has me curious, is the the squak from my Tx sounds
>> nothing like those from others. The others are a deehdaaah kind of
>> sound with two distinctly different tones ate the beginning and end
>> tone, whereas mine is a sqaaaark :-) of mixed tones at the same time.
>> 
>> David
>> 
>> VK4MDX
>> 
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