[OZAPRS] Setting up TT4 Tx on 2m
David and Justine Olsen
davejust at bigpond.net.au
Fri Feb 19 09:58:36 EST 2010
I am afraid preemphasis and deemphasis are beyond my present
knowledge :-)
David
On 19 Feb 2010, at 08:38, Richard Hoskin wrote:
> 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
> >>
> >> _______________________________________________
> >> OZAPRS mailing list
> >> OZAPRS at aprs.net.au
> >> http://lists.aprs.net.au/mailman/listinfo/ozaprs
> >>
> >>
> >>
> ---------------------------------------------------------------------
> >> ---
> >>
> >>
> >> No virus found in this incoming message.
> >> Checked by AVG - www.avg.com
> >> Version: 9.0.733 / Virus Database: 271.1.1/2692 - Release Date:
> >> 02/17/10 06:35:00
> >>
> > _______________________________________________
> > OZAPRS mailing list
> > OZAPRS at aprs.net.au
> > http://lists.aprs.net.au/mailman/listinfo/ozaprs
>
> _______________________________________________
> OZAPRS mailing list
> OZAPRS at aprs.net.au
> http://lists.aprs.net.au/mailman/listinfo/ozaprs
>
>
> --
> Scott Evans <scott at vk7hse.hobby-site.org>
>
> -- This email and any files transmitted with it are confidential
> and intended solely for the use of the individual or entity to whom
> they are addressed. If you have received this email in error please
> notify the system manager. This message contains confidential
> information and is intended only for the individual named. If you
> are not the named addressee you should not disseminate, distribute
> or copy this e-mail. Please notify the sender immediately by e-mail
> if you have received this e-mail by mistake and delete this e-mail
> from your system. If you are not the intended recipient you are
> notified that disclosing, copying, distributing or taking any
> action in reliance on the contents of this information is strictly
> prohibited.
> _______________________________________________
> OZAPRS mailing list
> OZAPRS at aprs.net.au
> http://lists.aprs.net.au/mailman/listinfo/ozaprs
>
>
>
>
>
>
>
> _______________________________________________
> OZAPRS mailing list
> OZAPRS at aprs.net.au
> http://lists.aprs.net.au/mailman/listinfo/ozaprs
>
>
> _______________________________________________
> OZAPRS mailing list
> OZAPRS at aprs.net.au
> http://lists.aprs.net.au/mailman/listinfo/ozaprs
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.aprs.net.au/pipermail/ozaprs/attachments/20100219/ee4c2042/attachment-0001.htm
More information about the OZAPRS
mailing list