[OZAPRS] Setting up TT4 Tx on 2m

Fri Feb 19 23:20:58 EST 2010

DEV = Audio

VK3HAU


----- Original Message ----- 
From: "David and Justine Olsen" <davejust at bigpond.net.au>
To: "Australian APRS Users" <ozaprs at aprs.net.au>
Sent: Friday, February 19, 2010 8:00 PM
Subject: Re: [OZAPRS] Setting up TT4 Tx on 2m


> Ray
>
> Thanks for that info. I can do all of that except the last step (7).
> The MT8000 has a TT3 chip and Tx on the same board and there is no
> adjustment of audio to the Tx from the TT3 chip.
> The only adjustments available are PWR and DEV.
>
> David
>
> On 19 Feb 2010, at 18:51, Ray Wells wrote:
>
>> David,
>>
>> This is what I would do with just a CRO as available test equipment.
>>
>> 1. Monitor your rx with the CRO
>> 2. If you have a handheld or other 2m rig and, with it on the correct
>> frequency, produce some loud Shhshhing noise into the microphone.
>> Generally higher frequency audio produces the maximum deviation, hence
>> the shhishing. Most commercial amateur rigs will produce close
>> enough to
>> 4.5/5khz deviation, the standard level in the amateur service in Oz.
>> 3. Establish a reference point on the CRO from the shhishing.
>> 4. Connect your TNC and if you can switch between tones, feed the
>> 2200Hz
>> tone into the tx at a reasonably high level, and adjust the deviation
>> control for a tad under the reference level you established on the
>> CRO.
>> 5. Don't touch the deviation control after this is set.
>> 6. Reduce the output of the TNC to its minimum value and enable
>> both TNC
>> tones
>> 7. Increase the audio input to the tx until you again see the
>> reference
>> level on the CRO and then back it off just a tad.
>>
>> Provided the tx has "standard" preemphasis characteristics you
>> should be
>> pretty close to ball park settings and, the decoded tones as seen
>> on the
>> CRO should appear at approximately the same level, if you're
>> looking at
>> deemphasised audio (say the top of the volume control) If you're
>> looking
>> at pre-deemphasis audio the high tome should be approximately twice
>> the
>> level of the low tone but this is not important here. What's important
>> is that you set the maximum level to just a tad below your CRO
>> reference
>> level.
>>
>> I usually set packet rigs for 4.0khz deviation on the high tone. Since
>> you have no way of measuring deviation with just a CRO, the reference
>> point minus a tad method should be reasonably satisfactory.
>>
>> Don't confuse deviation with tx power, there is no relationship. A
>> given
>> tx will produce the same deviation regardless of whether it's power
>> output is 1mW or 1KW. Only the range of that tx will change, not its
>> "loudness".
>>
>> Ray vk2tv
>>
>> David and Justine Olsen wrote:
>>> 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
>>>>>>>>>>
>>>>>>>>>> _______________________________________________
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>>>>>>>>>>
>>>>>>>>>>
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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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>>>>>>>>
>>>>>>>>
>>>>>>>>
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