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Burngate
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Re: MPEG video / audio to DTV-T encoder

Sat Oct 01, 2011 2:00 pm

Is the RasPi capable of DVB-T?
From my experience in pro-TV, the signal path goes something like this. Pictures @ 270Mb/s & AES sound @ 32/44.1/48/96 Kb/s goes to an encoder and comes out MPEG as a transprt stream at anywhere up to 10Mb/s. This data goes into a mux with others, to form a bouquet for transmission (if stat-muxing is in use, data goes back to the encoder to raise / lower the encoding compression according to available data-rate). This is sent onwards, in a terrestrial system, to where I lose focus, but which seems to have similarities with FFT. Then to the modulator / transmitter.
What we have with RasPi is a picture source (camera module) and sound source (USB mic), or what-ever we can force into it through USB. We also have some of the encoder - HDMI seems to carry some sort of data stream, albeit uncompressed, though RasPi seems not quite quick enough to convert it to the required MPEG stream. Maybe an off-board MPEG encoder exists. Or the data-stream from a DVD would work if that's your source.
But what I would like RasPi to do is the FFT-like bit. In comes MPEG, out goes ready-to-go DVB-T, put it into a UHF modulator onto a cable into the aerial socket on your TV.

chip273
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Re: MPEG video / audio to DTV-T encoder

Sun Oct 02, 2011 1:36 pm

DVB-T does not work that way. You can't simply put some generated analog signal into a UHF Modulator to go to your TV. The encoding and modulation is pretty different from an oldschool AnalogTV signal. You need to use a compatible modulator.

" to where I lose focus "
That's the actually interesting part. Also it consists of more than one or two steps until you have something that you can feed into an antennae.

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Burngate
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Re: MPEG video / audio to DTV-T encoder

Sun Oct 02, 2011 2:45 pm

I'm aware of that.
Oldschool = analogue baseband to analogue RF, 1kHz baseband signal to carrier+1kHz RF & pro rata.
Newschool = digital "baseband" to many carriers at RF. A lot of the process, though, - inner / outer Reed-Solomon , interleaving &c. - is just shuffling bits around. Producing the many carriers is the difficult bit, expensive, and not well explained by any of the sources I can find.
However, from what I can see, it is related to FFT, and there are chips out there that seem to do that, at a price. And that would be one way forward, to take an already-compressed data stream, "throw" it at the FFT chip then DAC its output into our analogue modulator.
Obviously it ain't that simple. Also, regarding the bit shuffling, beyond my pay-grade. But doing the bit-shuffling and FFT seems to me to be possible in RasPi.
Even Oldschool was too new for Aristotle, so I had to teach myself.

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