[OZAPRS] Setting up TT4 Tx on 2m
Ray Wells
vk2tv at exemail.com.au
Fri Feb 19 19:51:25 EST 2010
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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>>>>>>
>>>>>>
>>>>>>
>>>>>> --
>>>>>> Scott Evans <scott at vk7hse.hobby-site.org
>>>>>> <mailto:scott at vk7hse.hobby-site.org>>
>>>>>>
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>>>>>>
>>>>>>
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>>>>>>
>>>>>>
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