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[quote author=”yorkie98″]Hi Stelios, I like the idea of the modulation approach as opposed to the audio level approach but the thing I don’t quite get (and I guess many don’t quite understand either) is how the modulation level can be known without the digital input level being pre-determined also.
So wether we have a digital input of -12dBfs or -3dBfs the modultion would be correct at +/-75khz? My question would be how does the difference in the digital input levels get made up for within the exciter and set accurately to a level which is the known to produce the exact required deviation?[/quote]Hi yorkie98,
There isn’t any special technology here! It’s just a different approach to make the deviation adjustment easier and safer.
Tell to the DDS the audio level you send (this idea was suggested by JesseG, thank you!). Then tell to the DDS how many kHz deviation you want for this amount of audio, and it will accurately do that.With BBP for example, when you set "-12dBFS" output, you are sure you will get -12dBFS, correct?
– With an analog exciter, you have to adjust the "audio input level" knob, relying on the exciter’s deviation meter (if any), or using a modulation measurement equipment, true?
– With this exciter, simply set "Input level = -12dBFS" and "Deviation = 75 kHz" and you are done. No measurements, not anything else. Leave it and go!In case you increase to -3dBFS without changing the "Input level = -12dBFS" at the exciter, you will overmodulate. If you think it’s necessary, I can easily add a deviation limiter – to be completely protected.
[quote author=”yorkie98″]Also, I assume that those opting to use the L/R inputs and the DSP stereo coder (or the analog MPX input) still have to use an old-fashioned audio gain approach? or is the difference taken care of in the same way as if using the digital MPX inputs?[/quote]The same way as described above. Doesn’t matter what the input source is (L/R analog or digital, MPX analog or digital, internal stereo or mono).
Did I understood your question correctly? 🙂
[quote author=”sigmacom”]In case you increase to -3dBFS without changing the "Input level = -12dBFS" at the exciter, you will overmodulate. If you think it’s necessary, I can easily add a deviation limiter – to be completely protected.[/quote]
If you do, please make it optional.. I certainly wouldn’t want it 🙂.
///Leif
Will someone "bench" this thing 🙂
Stelios, can you make some records in MpxTool from transmitter? Any test is welcome now.
[quote author=”Boki”]Will someone "bench" this thing 🙂
Stelios, can you make some records in MpxTool from transmitter? Any test is welcome now.[/quote]
There have been some benches earlier in this topic I think…
and using MPX Tool to listen to how good this sounds through a receiver is pointless. There isn’t a receiver in existence that can touch how high quality this is. Any receiver used will degrade the sound because the receiver isn’t high enough quality to capture how accurate the DDS exciter is. Not to mention an additional ADC stage.
That’s besides the point. Every step in the overall quality counts. The fact remains that it would be difficult to get near this quality even with a very well tuned "digital" (actually only part digital) exciter like the ones Harris makes. Which cost thousands upon thousands of dollars.
Getting the picture yet? 8)
[quote author=”Leif”][quote author=”sigmacom”]In case you increase to -3dBFS without changing the "Input level = -12dBFS" at the exciter, you will overmodulate. If you think it’s necessary, I can easily add a deviation limiter – to be completely protected.[/quote]
If you do, please make it optional.. I certainly wouldn’t want it 🙂.
///Leif[/quote]
i agree, … we have a bunch of then… ( limiters ) don’t add any audio clippers or limmiters.
I hope some day Sigmacom and Leif make a 2 in one box ( exciter with the " special " MPX clipper patent ..Leif made.. 🙂
Until now, an old fashioned limiter clipper…is a bad thing..for those who use breakaway becuse breakaway has eveerything.. ( exept mpx clipper..but it s ok )
I don’t think there will be any need to add a deviation limiter, it kinda goes against the whole ethos of BBP but it may be useful for those NOT using BBP as these people will be far more likely to have overshoots. I would concur with Leif, that if you do add such a limiter, make it optional and off by default.
I now understand your approach, I didn’t before but it seems a very good idea and I fully support it.
I also agree with JesseG as the default level in BBP is -3dBfs so our digital outputs are going to be running at this level too so it seems a sensible level to have as the default in the exciter. It will also give a bit more dynamic range to the output.PS, Leif, single frequency networks is a very exciting proposition also, finally with this technology, our ageing FM band can be run efficiently and would alleviate much of the overcrowding problems there are across the world.
Yorkie.
wow,
wait a moment,
if i ve got 2 of these DDS exciters can i play clear to the areas where signals from both 2 transmitters are equal ?can we make a low cost " one frequency network of transmitters " using DDS exciters without any other add ons ?
practicaly : if i have very good STLs , linear, without overshoots and 2 DDS exciters , can i make this one network frequency network without any other kinds of modification ? ( sound micro-delay maiby? GPS..bla bla..)
Camclone, the single frequency network thing will not work immediately, other work will need to be done, most importantly the transmitters will need to locked in time and in phase. The difficulty comes in getting these in sync at different locations, there would need to be some kind of reference to keep them locked. This is likely to be complex and costly to acheive, BUT in THEORY, 2 DDS exciters in sync, phase locked and with a 100% identical signal (input and output), you could have them covering overlapping areas without signal fade, much like what is already done with Digital Radio and TV transmissions.
The deviation limiter of course will be user selectable and "off" by default, don’t worry guys!
I have already done some research about SFN and I have selected the DVB-T principle.
Of course there is need for SFN adaptors with GPS reference at each TX site, plus a MIB inserter at the studio.
All these are achievable only with digital audio source (AES/EBU L/R, or D-MPX, or IP).A few minutes ago, we (John, George & me) returned from the TX mountain "Hortiatis" where we tested the DDS exciter with a 20kW amplifier.
George took videos and photos, I hope he will find some time to prepare and upload them here soon. 😉"One photo….thousand of words"
…ok, it’s not only one 😉
More pics & video coming soon.Camclone…. SMiLe
your box @ T20000 Elenos!More details by Stelios 😉
Finnaly you made it !
My RF broadcast room is waiting for that box..too!
Be prepared for live streaming video with audio from my state of the art RF broadcasting point 750 meters up 🙂
I am gonna watch this baby using a webcam and a 3G 1,8 MBps connection with dyndns… 🙂let’s put some MPX energy inside!!!
Loving the pics, especially the one with a TX with only an electricity connector and a Tos-link fiber going in, this is awesome!
I have a 12KW ERP station to try my prototype on, not quite up to the standards of your whopping 20KW transmitter but still a very good test.
What soundcard are you using there Stelios for the optical output? I see plenty with 192khz on the electrical output but very few with 192 on the optical.Keep up the great work,
Yorkie.
Hi yorkie98,
Unfortunately that day we had only Juli@ available. It was feeding the exciter with optical L/R and the internal stereo encoder was used.
Currently I’m working on those you asked here in the forum. If you have any requests for the last upcoming "mountain-test", please let me know.PS: George, we’re still waiting for videos and more photos…
So, before using this magic box , you have to make 3 !!! veeeeeeeeery important improvements ?!
1) -3 dbfs mpx imput
2) auto-reduction of RF power in case of overheating-overtemperature
3) an auto-restart in case the DSP crashes due to electricity problems ( ups and downs.)…by the way ….I use a BIG BIG BIG 20 KVA line interactive electricity UPS on my …state of the art RF ROOM 🙂
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