[OZAPRS] Setting up TT4 Tx on 2m
Ray Wells
vk2tv at exemail.com.au
Fri Feb 19 18:32:15 EST 2010
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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