[OZAPRS] Setting up TT4 Tx on 2m

Fri Feb 19 14:27:28 EST 2010

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
>>>>> 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 <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
>>
>
> _______________________________________________
> OZAPRS mailing list
> OZAPRS at aprs.net.au
> http://lists.aprs.net.au/mailman/listinfo/ozaprs