Why Doppler Effect is not an issue

Messagede F6DZP » Mer 23 Oct 2013 07:29

Receiving DVB-S from HamTV is receiving a DATV signal coming from a station that is moving at a very high speed.
We know that Doppler effect will be there and will give about 60 kHz of frequency drift.
Receiving standard digital TV DVB-S/S2 is receiving signal from broadcast satellite that is geostationary and there is no Doppler frequency drift.

The question many people ask:
Could the DVB-S demodulator accept HamTV Doppler Effect without lost of data?

the answer is: Yes, Derotator will do the job.

Derotator is a software function based on some special algorithm that can find and lock on freq+ SR from IQ signal that has been extract and digitilized outside the true frequency.
This functionality is indispensable even if received frequency is well know and very stable.

In all TX to RX chain, there are always factors that are present:
- real frequency used by modulator ( 1255 MHz could be 1255008 KHz in reality)
- LNB or converter LO could be not very precise and change with outside temperature.
- Tuner oscillator cannot be set to any frequency we ask. tuning is no continuous, there is a minimum frequency step that is used.
for TT S2-3200 card that uses STB6100chip, tuner step is 52,73 kHz
when I ask freq 1255 MHz, tuner will be set to 23800 x 52,73 = 1 254 974 kHz
for TT S2-1600 card that uses STV6110 chip, minimum step is 843,75 kHz
when I ask freq 1255 MHz, tuner will be set to 1487 x 843,75 = 1 254 656 kHz

As we can see, we never get the good frequency and I and Q signals are get and digitilized with bad zero tuning.
So there is an algorithm ( Das Costas is the more often used) that will search in digital IQ data where is the "zero" tuning and compute the offset
we can see all that using Tutioune in Expert mode.

My DATV TX is set on 1255 MHz
Tutioune RX freq asked is 1255000 kHz

I switch Off the Derotator:

Derotator Off.jpg
Derotator Off.jpg (99.87 Kio) Vu 6060 fois


I switch ON the Derotator:

Derotator ON freq 1255.jpg
Derotator ON freq 1255.jpg (110.79 Kio) Vu 6060 fois


We can why we have chosen this name "Derotator", when it works, constellation rotation stop.

Now I change the TX DATV modulator frequency : 1255.2 MHz
Tutioune RX freq asked remain 1255 MHz

RX freq 1255 MHz TX freq 1255_2MHz .jpg
RX freq 1255 MHz TX freq 1255_2MHz .jpg (38.01 Kio) Vu 6059 fois


The derotator has shift to find the new offset and finf the new frequency to be 1255.247 MHz

Maximum offset S2-3200 can find is + or - 600 kHz
Maximum offset S2-1600 can find is + or - 2.8 MHz


But derotator works to find and lock on a frequency. When it has found the offset a first time, with some newest receiver,
the derotator algorithm suppose that the freq will not change all the time we don't ask for a new frequency.
So with an offset changing - due to Doppler effect - in this case some derotator algorithm will fail to follow Doppler shift.
I have for example modified the setup made by Technotrend for S2-1600 to allow Derotator to follow.
If you use S2-1600 with original Technotrend driver and standard very good software as DVBviewer, derotator will fail if I give a new TX freq offset (1255.2) after a first lock( on 1255 MHz)
some Set to Boxes /FTA boxes with recent chip could have this issue.


Jean Pierre F6DZP
F6DZP
 
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Re: Why Doppler Effect is not an issue

Messagede VE4NSA » Mer 30 Oct 2013 02:17

Thanks for bringing this issue up.

While I understand that doppler effect might not be an issue with the Tutioune software, it still might be an issue for 90% of the DVB-S receivers that cannot be used with the software. What we really need is a test with a DVB-S transmitter relatively rapidly changing the frequency +-60KHz at 2.4GHz and see how the DVB-S receiver respond tho that without the use of software.

Stefan, VE4NSA
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Re: Why Doppler Effect is not an issue

Messagede F6DZP » Mer 30 Oct 2013 10:04

No I don't think so.
I think that a very few % of DVB-S receivers will not be able to follow the Doppler frequency shift.
It will be an issue only for some DVB-S receivers that use last generation of demodulator chip with a software that lock too much derotator after first acquisition.( I think it's near to be a bug for them because we know that even using broadcast "fixed" frequency we can have shift due to temperature changing or other factors.)
Most of DVB-S receivers you will test will be good.

Jean Pierre F6DZP
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Re: Why Doppler Effect is not an issue

Messagede VE4NSA » Mer 30 Oct 2013 22:42

Thanks!

I have a DVB-S transmitter on order and will test some of the units. Will let you know how it goes.

Stefan
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