[OZAPRS] Setting up TT4 Tx on 2m

Fri Feb 19 13:42:59 EST 2010

Richard,

I recall we discussed this some years back. I understand the advantage 
of avoiding deemphasis and given the 2206/2211 tone level issue it 
certainly makes sense. For digi's it's mostly feasible to hack into the 
radio to achieve this, if it's not already provided, but for home 
operators this may not be possible/desireable, making, in my opinion, tx 
preemphasis somewhat essential to cater for such users. Too much high 
tone can be handled but too little is a problem.

My earliest packet rx was a FDK Multi-750 (anyone old enough to remember 
them?) and for convenience I picked speaker audio. I found it almost 
impossible to decode packets with that rx and investigated. It turned 
out that to make the radio sound better on rx in SSB mode, there was 
about 10dB/octave effective deemphasis (read severe treble cut) built 
into the audio rear end. Like all of my radios, that rig gained an 
emitter follower that picked pre-volume control audio and, problem 
solved. I use that rig on the satgate today. Radios ain't radios, to 
corrupt an old add for oil.

Ray vk2tv

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 <mailto: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 <http://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 <mailto:OZAPRS at aprs.net.au>
> > http://lists.aprs.net.au/mailman/listinfo/ozaprs
>  
> _______________________________________________
> OZAPRS mailing list
> OZAPRS at aprs.net.au <mailto:OZAPRS at aprs.net.au>
> http://lists.aprs.net.au/mailman/listinfo/ozaprs
>       
>
>  
>
> -- 
> Scott Evans <scott at vk7hse.hobby-site.org 
> <mailto: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 <mailto:OZAPRS at aprs.net.au>
>
> http://lists.aprs.net.au/mailman/listinfo/ozaprs
>
>  
>
>  
>
>
> ------------------------------------------------------------------------
>
>
>  
> _______________________________________________
> OZAPRS mailing list
> OZAPRS at aprs.net.au <mailto: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
>   