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Old 11-06-2009, 21:56   #1
DRM-OM
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To illustrate what I ment there:
http://www.drmrx.org/forum/showpost....&postcount=619 and a few posts before that.

This is a recording of 1296kHz in AM - same equipment, same location, same time of day.
To listen, just remove the .txt ending of the filename.
And that's why I say DRM reduces the target area.
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Old 13-06-2009, 11:00   #2
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Quote:
Originally Posted by DRM-OM
To illustrate what I ment there:
...
And that's why I say DRM reduces the target area.

Hi!

No, that can't be said in general. In this case itsright. But this is because BBC decided to transmit in a higher bitrate for better soundquality instead of extending the range of the signal.
If they would use bitrates lower than 20 kbit (16 kBit for example) in Mode A instead of 23 or 26 kBit you would be able to listen without problems even at your place.

Stephan
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Old 14-06-2009, 15:31   #3
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I doubt that because according to my measurements in case of co-channel interference (which you also can hear in my example) anything less than 15dB ratio wanted/unwanted station will cause total audio failure whereas AM-co-channel with this same ratio is noticeable but will not harm the possibility to follow the contents.
Sound Example for this
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Old 14-06-2009, 21:00   #4
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In Mode A with 14.76 kBit you need something about 10 dB of SNR for dropout free audio. At groundwave reception even 9 is more than enough for dropout free listening.
With the 12 kBit that Rai uses sometimes, even less is enough.

Co- or On Channel QRM should be included in the SNR mesurements in Dream. So SNR goes down if theres QRM, but if some XX SNR is required for correct decoding it shouldnt matter if theres QRM or not. If SNR is high enough for decoding, its simply enough for decoding.

Listen to HCJBs 15280 kHy programming. They use Mode A 14 kBit, too. For example: http://www.drmrx.org/forum/attachmen...7&d=1243205264

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Old 14-06-2009, 22:13   #5
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Lab measurements show that a 64QAM signal needs around 16dB SNR, a 16QAM signal needs around 12dB SNR for a BER of 10-4
Measurements also show that even a single carrier around 5Hz off center frequency (which is common for "co-Channel" Situations) kills the signal when stronger than above values.

And if it's argued that DRM is good for ground wave you cannot stop skywave AM interferers from reducing your target area anyway.

These are simply facts.

Testsignal 1 ("16QAM" bzw. "B 4/16QAM"):
RobustnessMode: B
Kanal-Bandbreite: 10kHz
SDC QAM Mode: 4
MSC QAM Mode: 16
Interleaver: long
Hierarch. Mode: std
Coderate: 0/0.5/0.62

Testsignal 2 ("64QAM" bzw. "B 16/64QAM"):
RobustnessMode: B
Kanal-Bandbreite: 10kHz
SDC QAM Mode: 16
MSC QAM Mode: 64
Interleaver: long
Hierarch. Mode: std
Coderate: 0/0.5/0.60

btw.: Mode A or B doesn't matter in this respect as this only differs in the length of the guard interval (important for multipath skywave propagation)
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Old 15-06-2009, 11:19   #6
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Quote:
Originally Posted by DRM-OM
Lab measurements show that a 64QAM signal needs around 16dB SNR, a 16QAM signal needs around 12dB SNR for a BER of 10-2

I've been in Quito for the last weeks and got dropout free audio at groundwave reception from Pifo even under 10 dB avarage SNR without any problems. 16 QAM, Mode A, 14,76 kBit. The enineers at HCJB approved my observations. Try 12 or 14 kBit Mode A in your lab as well and find out.

Quote:
Originally Posted by DRM-OM

Mode A or B doesn't matter (...) as this only differs in the length of the guard interval

At Mode A you've about 10% more carriers available that transport the data. So with the same audiobitrate you can achive much better error correction which stablizizes the signal. This allows audio beeing decoded even at lower SNR.

Btw: Did you test HCJBs transmissions on shortwave 15280 kHz or the RAI programming on 846kHz? The should work with SNR < 11 dB -using bitrates lower than 15 kBit - without any problems even at your place. Please take a look at the SNR over Carriers index tab.

Please try out the software spark as DRM Transmitter as well. If I remember right I could "receive" dropout free audio down to about 7 or 8 dB lowering the signal in "lab environment"



Quote:
Originally Posted by DRM-OM
Measurements also show that even a single carrier around 5Hz off center frequency (which is common for "co-Channel" Situations) kills the signal when stronger than above values.

Single carrier? Without any modulation? Wow, what kind of TX has that been? A SparkTX?

A standard TX - with carrier only - somwhere near the DRM Center frequency wouldn't rot the complete signal in normal operation in real world!
The signal you measured should
a) have been very noisy or
b) very - veeeerrryyy - strong?!


Interesting thing: Upper side Co-Channel QRM (for example 5 kHz up) seems to be more a problem then lower side. Thats maybe because of the pilot carriers that shouldn't be disturbed.

Dreams mesurements of the SNR are not very accurate at some situations. They are always estimations. But in gerneral they work out quite well.

Btw: How would the BBC @ 1296khz perform using digital Modulation at 16 QAM? Should work out very well at your location, even with some QRM, doesn't? At that operation Modes DRM can achieve at least the same coverage than AM (AM quality that listeners a willing to accept for listening). And if you take the lower output power (1296 is operated in DRM with 35 kW only) in consideration as well...


Greetings,
Stephan
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Old 15-06-2009, 13:52   #7
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Quote:
Originally Posted by maxpower
Single carrier? Without any modulation? Wow, what kind of TX has that been? A SparkTX?
Just a Rohde and Schwarz signal generator (take old SMG or newer SMV, results are the same) in CW mode
My good old RE signal generator cannot tune in 1Hz-steps.
But, off course, you can also use a DT230 test equipment for that purpose ...

Quote:
Originally Posted by maxpower
Upper side Co-Channel QRM (for example 5 kHz up)
I spoke of 5Hz, not 5kHz, that's about 10% of inter-carrier-spacing of the OFDM signal.

Quote:
Originally Posted by maxpower
take the lower output power (1296 is operated in DRM with 35 kW only)
That makes me wonder as a standard AM-TX of 500kW can operate in 150-200kW in DRM - or is that British TX a real post war oldtimer?

Quote:
Originally Posted by maxpower
A standard TX - with carrier only - somwhere near the DRM Center frequency wouldn't rot the complete signal in normal operation in real world!
As I told you.
Carrier within clearly less than 5Hz off center may be as much as 15dB stronger than the DRM power, but with 5Hz and more offset it has to be reduced to -15dB compared to the DRM-signal not to harm decoding.
And for these cases it doesn't matter if you have carrier, AM, multitone or whatever interferer signal, even a second DRM-station as co-channel interferer, the result is always the same: you need - roughly - 15dB of signal-to-interferer ratio (I have to check for exact results with my report, but that doesn't really matter since natural fluctuations of field strength are far worse than that.
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Last edited by DRM-OM : 15-06-2009 at 14:57. Reason: added last paragraph
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Old 15-06-2009, 15:33   #8
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Quote:
Originally Posted by DRM-OM
I spoke of 5Hz, not 5kHz, that's about 10% of inter-carrier-spacing of the OFDM signal.

I know. :-)
I wrote about the typicle interferer at shortwave. Its mostly 5 khz down, up (and makes QRM with one of the sidebands...) or at the center frequency.

Quote:
Originally Posted by DRM-OM
That makes me wonder as a standard AM-TX of 500kW can operate in 150-200kW in DRM - or is that British TX a real post war oldtimer?

I don't know, but it's probably a linearized MW Txer which is capable up to broadcast with about 10% AM Power (50 kW max) or it's simply not working at maximum power for bringing down power bills/tube cost.

Quote:
Originally Posted by DRM-OM
As I told you.
Carrier within clearly less than 5Hz off center may be as much as 15dB stronger than the DRM power, but with 5Hz and more offset it has to be reduced to -15dB compared to the DRM-signal not to harm decoding.
And for these cases it doesn't matter if you have carrier, AM, multitone or whatever interferer signal, even a second DRM-station as co-channel interferer, the result is always the same: you need - roughly - 15dB of signal-to-interferer ratio (I have to check for exact results with my report, but that doesn't really matter since natural fluctuations of field strength are far worse than that.

Maybe you can post a picture of SNR over Carriers from this case?!

Maybe I understand wrong what you try to tell me (still jet-lagging). One carrier without modulation more than 5 Hz away from the center ruins the complete DRM Signal?

Stephan
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Old 15-06-2009, 15:48   #9
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I don't know, but it's probably a linearized MW Txer which is capable up to broadcast with about 10% AM Power (50 kW max) or it's simply not working at maximum power for bringing down power bills/tube cost.

The transmitter is only a few years old it is a Nautel NA200 capable of 200 kW AM or 80 kW DRM, so if its being run at 35 kW that is reduced power.

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Old 15-06-2009, 16:03   #10
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Quote:
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capable of 200 kW AM or 80 kW DRM

That corresponds exactly to a crest factor of 10dB - what I supposed above
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Old 15-06-2009, 16:08   #11
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Quote:
Originally Posted by maxpower
I wrote about the typicle interferer at shortwave. Its mostly 5 khz down, up (and makes QRM with one of the sidebands...)

But that's "adjacent channel interference"

Quote:
Originally Posted by maxpower
Maybe I understand wrong what you try to tell me (still jet-lagging). One carrier without modulation more than 5 Hz away from the center ruins the complete DRM Signal?

Maybe you have a look at this
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Old 15-06-2009, 18:25   #12
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Quote:
Originally Posted by DRM-OM
But that's "adjacent channel interference"

Depends on definition. I would presume that every signal, thats direct on the same frequency-range is On Channel.
If you've got an AM TX transmitting 5 khz up from the center of the DRM TX, it's lower side band (and a little bit of the carrier) is "on channel interferrence, because its interfering the upper half of the 10 kHz DRM Signal. Same thing at the other side.

With my PC/Soundcard SDR I have very few problems with adjectant channel interference, because the "digital filter" work very nice.
It's the AM TXers that transmit much wider spectrum than 10 kHz that interfere into DRM (or AM) transmissions and cause Problems. (Look at 6075 that interferes sometimes with 6085 kHz because of the use of more than 10 kHz Spectrum by DW Relais-Partners). But anyway.



Quote:
Originally Posted by DRM-OM
Maybe you have a look at this

Thanks for that Information and pictures! I'll take a closer look at 1296 this evening. (Hmm, its, only working at the morning this summer season...)

It's maybe interesting for you that Transradio has build a Transmitter for AM & DRM thats able to achive up to 73% of the AM Power (with up to 125% AM Modulation).

So the Transradio 100 kW AM Transmitter can transmit as much as 73 kW DRM Power. http://www.transradio.de/DRM-AM-TRANSRADIO.pdf (Page 9). I think thats quite an impressive development and you loose someting about 2 dB Power switching to DRM.


Quote:
Originally Posted by jbriggs
The transmitter is only a few years old it is a Nautel NA200 capable of 200 kW AM or 80 kW DRM, so if its being run at 35 kW that is reduced power.

Thanks, James (upps, sorry, said Jeff ) for the information and pictures provided. I hoped that you or one of your collegues would add some info here. Solid state Modules?

Maybe it's possible to make some tests with lower bitrates and higher protection in Mode A @ 1296???

Stephan
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Old 16-06-2009, 00:43   #13
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Quote:
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It's maybe interesting for you that Transradio has build a Transmitter for AM & DRM thats able to achive up to 73% of the AM Power (with up to 125% AM Modulation).

The latter is a feature for the US market where 125% of (positive) AM modulation is known (and also regulated by the FCC) for a long time. It only means that the transmitter has to have some more linear headroom (so that it is able to run 2.25x instead of 2.0x the carrier voltage as a linear peak voltage).

Before FCC rules went into effect even 140 or more % of modulation were common.

And by extending linear response of the amplifier/modulator even further you also can get 73% in DRM mode.
Dissipation loss is not the big problem since peak power is reached only for a small fraction of time.
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Old 16-06-2009, 05:25   #14
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Quote:
Originally Posted by DRM-OM
The latter is a feature for the US market where 125% of (positive) AM modulation is known (and also regulated by the FCC) for a long time. It only means that the transmitter has to have some more linear headroom (so that it is able to run 2.25x instead of 2.0x the carrier voltage as a linear peak voltage).

Before FCC rules went into effect even 140 or more % of modulation were common.

And by extending linear response of the amplifier/modulator even further you also can get 73% in DRM mode.
Dissipation loss is not the big problem since peak power is reached only for a small fraction of time.

Actually what Stephan refers to is not just the overmodulation capability, the main improvement is the implementation of DPE algorithms in the DMOD3 which reduce the crest factor of the COFDM signal by about 3 dB, I think in tests developers from Transradio achieved more than 80% of the AM power, still within the specification (especially concerning MER and out of band emissions).

@Stephan:The Nautel NA 200 at Orford Ness is similar to the transmitters described in this Brochure from Nautel
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Old 16-06-2009, 09:26   #15
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the main improvement is the implementation of DPE algorithms in the DMOD3 which reduce the crest factor of the COFDM signal by about 3 dB

Just calculate 125% AM modulation and a crest factor of 7 instead of conventional 10dB and you get ... roundabout 72% ... ah, yes.
So it's a combination of both effects.
For comparison: Without overmod cap. it would be 56%, without overmod and DMOD3 the standard value is around 40%.
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