On Fri, Apr 8, 2016 at 10:48 AM, Andy Walls <a...@silverblocksystems.net> wrote:
> > > The audio codec is proprietary and not documented anywhere AFAIK > so > > even if you demod the bitstream, you won't be able to do much > with it. > > > Thank you all for the conversation, it is pretty interesting. That > makes sense > > that they have a proprietary protocol, it's a shame though. Is the text > that > > comes through encoded in a proprietary way? > > > > Does anyone know what the frequency of channel 1 is on (I'd like to see > if I > > can even see the signal popping up above the noise)? > > > > ~Jason > > The XM center freqs are shown here: > > > https://www.rohde-schwarz.com/us/technologies/satellite-broadcast/xm-satellite/xm-satellite-technology/xm-satellite-technology_55613.html > > The Sats are QPSK, the terrestrial is COFDM. > > That page also states the audio codec for voice programming is AMBE, and > AAC+ for all other programming. > > > The XM radio patents give you a lot of clues: > > https://www.google.com/search?tbo=p&tbm=pts&hl=en&q=inassignee:%22Xm+Satellite+Radio+Inc.%22 > > Images 4 and 6 of this one: > > https://www.google.com/patents/US7020217?dq=inassignee:%22Xm+Satellite+Radio+Inc.%22&hl=en&sa=X&ved=0ahUKEwj5ycbeoP_LAhXEXh4KHWSdDeA4HhDoAQgwMAM > Image 4 of this one: > > https://www.google.com/patents/US6510317?dq=inassignee:%22Xm+Satellite+Radio+Inc.%22&hl=en&sa=X&ved=0ahUKEwikn4Ogof_LAhVIJx4KHYaYCt44KBDoAQhTMAg > > And maybe the most useful I've skimmed: > > https://www.google.com/patents/US7123875?dq=inassignee:%22Xm+Satellite+Radio+Inc.%22+MCM&hl=en&sa=X&ved=0ahUKEwidqpT1of_LAhUFXh4KHXqIDuEQ6AEIKjAC > Images 1, 5, and 6. > > This particular patent also indicates that after the demod, RS decoder, > viterbi decoder, and deinterleaver, that an MPEG TS *might* be the > transport multiplex. > > The images show that the decryption happens after all the FEC and > deinterleaving is handled and also after SL (service layer) decoding. > The I2C connected external NVRAM is likely where the program decryption > key is stored (inside of a tamper boundary), according to other internet > pages from 7 years ago. > > So I'm guessing you have to totally demodulate and decode one of the SAT > or terrestrial channels, and then dig around in a (hopefully MPEG) > transport stream to find your unencrypted "Channel 1" > > The STA400A datasheet gives a lot of good information about the XM signal characteristics, bit rate, RCC filter bandwidth, code rates and puncturing, proper order of FEC blocks, etc.: http://www.chipfind.net/datasheet/pdf/stmicroelectronics/sta400a.pdf Enough info to give one half a chance of getting the demodulation correct. It also talks a little bit about the frame structure. This MAXIM 2140 App Note has a yet another picture of the spectrum allocation, that also shows Sirius' and WCS' allocations in context: http://pdfserv.maximintegrated.com/en/an/AN3851.pdf I also found a Sirius patent that talks about an overlay modulation, but it wasn't clear to me if it was being applied to the XM satellite signals. Regards, Andy
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