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Hello again!
@Boki:
– Yes! and by software programming perspective, it is much easier than constructing the MPX inside the mCPU.
– About the RVR DDS, it creates the modulated carrier at 10 (or 12?) MHz. Then, using a PLL, upconverts it to 70 MHz. Then, with another PLL, upconverts it finally to FM band.
I strongly disagree using PLL oscillators, because they insert noise. If they were using XTAL oscillators, would be better.@yorkie98:
– If you are willing and can test it in actual environment, let me make one prototype and send it to you as sample.
I will inform you here in this topic for my progress.@Leif:
– Sorry, I didn’t describe the details… My omission! 😳
The 66MHz mCPU drives another chip that does the DDS, which runs @ 300 MHz.Hello everybody!
This is my first post here and I would like to congratulate the creators of BBP and this community!About me and this topic:
Recently I designed and manufactured a digital FM modulator. It accepts 16…24bit digital L/R audio (AES/EBU coax or optical), and produces the FM stereo carrier.
There is no PLL/VCO, or any other ordinary analog modulation concept, like almost everybody is using for decades now (and I HATE IT!). The carrier is genarated by DDS (Direct Digital Synthesis) straight in 87.5-108 MHz band, without upconversions – thus no phase noise. A 32-bit mCPU running at 66MHz, accepts the digital audio and does all the math (MPX matrix, pilot, baseband, etc). It decides what frequency should produce at the output, 540.000 times per second.
So rocco123456, I can tell you it is possible to make a DDS modulator that accepts MPX. Actually, it is more simple than creating internally the stereo coder, RDS, etc. Unfortunately, there is an issue: There are no radio links capable to carry 192kHz digital audio. Even if they exist, they will require a lot of RF spectrum (thus more fees to Authorities). By implementing the stereo and RDS inside the DDS modulator, you need less STL bandwidth to do the same job.
On the other hand, you can put BBP at the transmitter site and use ordinary digital STL equipment. BUT, in case you are operating a network of transmitters, you need to put one BBP at each site…After all this, isn’t it better to let the exciter do the MPX work?… 🙂
(*) Gossip:
Yesterday, with some audio-expertised friends, we evaluated the performance of my DDS modulator, among RVR PTX-30UHT, and Rohde & Schwarz SU125. For the two analog FM exciters, we used the internal stereo encoder of BBP to create the MPX via 192kHz soundcard output (Julia). For the DDS, we used plain L/R via AES/EBU optical cable. Well, the DDS was the winner for it’s clarity and high frequency response. The SU125 came in second place, and PTX-30 in third place (which, by the way, had very bad sound compared to DDS…). The stereo separation ("BBP coder" versus "DDS coder") was the same, +/-2dB’s.
