[OZAPRS] Setting up TT4 Tx on 2m

Ray Wells vk2tv at exemail.com.au
Fri Feb 19 13:56:27 EST 2010


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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>>  
>>
>> -- 
>> Scott Evans <scott at vk7hse.hobby-site.org 
>> <mailto:scott at vk7hse.hobby-site.org>>
>>
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