[OZAPRS] Setting up TT4 Tx on 2m

David and Justine Olsen davejust at bigpond.net.au
Fri Feb 19 19:11:05 EST 2010 

Thanks Ray

It seems the MT8000 has just PWR and DEV adjustments. So it was DEV I  
was adjusting.

After adjusting deviation by listening to change in audio, I've now  
managed 1 kilometer range out of the 8 watt tracker with rubber  
antenna, sitting on the dashboard of my car. I am not sure how much  
better I could expect with this set up, but I was hoping for quite a  
bit better. My digi is picking up mobiles from 20k away.

David

On 19 Feb 2010, at 17:32, Ray Wells wrote:

> David,
>
> I don't know the rig or the tx audio circuit so my only thought is  
> that
> the audio limiter/compressor is being badly overdriven and its
> compression characteristics are frequency sensitive.
>
> I'd attack it from a different angle and start with as little audio as
> possible (certainly less than 50mV into the microphone input) and then
> raise the level, all the time watching the CRO. Then the
> limiting/compression point will become obvious on the CRO as that  
> point
> where received high tone no longer increases whilst the low tone still
> increases.
>
> It's also worth checking the audio level into the tx occasionally to
> make sure something stupid isn't happening there. Murphy is your
> constant companion when testing something.
>
> Ray vk2tv
>
> David and Justine Olsen wrote:
>> Ray
>>
>>
>> Thanks. I got that info from the previous link I posted.
>>
>> Now with the Microtrak8000, I am looking at the traces on a  
>> software CRO
>>
>> I get the 8000 to transmit, and I have the audio from a receiver
>> going into my PC and viewing the trace of a software CRO
>>
>> Here is the interesting thing. As I decrease drive of the 8000, the
>> amplitude of high frequency tone becomes much smaller than that of
>> the low frequency tone.
>>
>>
>> David
>>
>>
>> On 19 Feb 2010, at 12:56, Ray Wells wrote:
>>
>>
>>> David,
>>>
>>> Put simply, the high frequencies (high tone for packet) are  
>>> boosted in
>>> the tx at the (usual) rate of 6dB per octave, meaning that each
>>> time the
>>> audio frequency is doubled , the tx deviation is also doubled. If a
>>> 1khz
>>> tone produced, say, 2.5khz deviation, a 2khz tone would produce  
>>> 5.0khz
>>> deviation.
>>>
>>> In the rx the reverse process takes place to restore the audio to  
>>> what
>>> it was prior to tx preemphasis. The effect of pre/deemphasis is to
>>> improve the signal to noise ratio of the recovered audio.
>>>
>>> Television audio and FM radio both use pre/deemphasis for the same
>>> reason.
>>>
>>> Ray vk2tv
>>>
>>>
>>> David and Justine Olsen wrote:
>>>
>>>> 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
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>>>>>>> http://lists.aprs.net.au/mailman/listinfo/ozaprs
>>>>>>>
>>>>>>>
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>>>>>>>
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>>>>>
>>>>>
>>>>> -- 
>>>>> Scott Evans <scott at vk7hse.hobby-site.org
>>>>> <mailto:scott at vk7hse.hobby-site.org>>
>>>>>
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