> > Let's consider the most basic function we'd be using in this, which is > a digital FM encoder. Using lookup tables and some other logic, it > would be easy to generate a sine wave of any frequency. Then to > encode an FM signal, we just modulate the freqency. To ensure there > are no discontinuities, we need to ensure that the input to the > modulator does not change too fast. Oh, and doing amplitude > modulation is trivial.
For modulating an ATSC signal the basic modulation is a single side band AM modulation using a 8VSB modulation scheme. For generating the frequency I concur that creating the signal is easy a DDS do that function by doing a basic phase accumulation and using a lookup table having the sinus value. The problem present is having enough definition in the signal generated so the signal constellation would be well defined. Also There are some encoding step to be perform to obtain the base band 8VSB modulation with the synchronization signal. If we want to generate a TV signal, to begin with, we need the carrier > wave, so we have a sine generator that runs at that frequency. > Channel 2 for NTSC is like 54MHz, which we MAY be able to approximate > and modulate well enough through a 350MHz DAC. (If not, we can > probably add different delay lines to each of the R, G, and B channels > of our DAC so that when they're added, we get like GHz temporal > resolution). In this, we can just use amplitude modulation to encode > the syncs and video. For color, we generate a 3.58MHz subcarrier (for > NTSC I anyhow), apply amplitude and phase modulation to that to encode > the color signal, and apply that to modulate the B&W signal. Finally, > we need a low-pass filter on the output to filter out the spikes > caused by the DAC making discrete steps. Generating an NTSC signal the carrier wave at 54MHz digitally is possible but there could be some jitter present in the fundamental of the signal generated, because of the intrinsic phase error present by generating the sinus digitally. Encoding the information on the 3.58MHz sub-carrier if the generator is functioning at the same speed than the DAC wouldn't be a problem there would be enough sample and the phase error of the sinus generator would be hidden. The problem is that we would need to reproduce the analog encoding pipeline in digital, that part could be more troublesome. But I didn't look to much what would be the requirement. A first approximation the requirement would be using 2 DDS 1 for the carrier and 1 for the sub-carrier, some mixing stage for combining the 2, also adding the AM modulation part is easy there would be a need for some digital filter before outputting the signal to cut the image frequency generated to a minimum. We would also need a analog cut band filter to respect the 6Mhz channel available for the output. Being sadly ignorant of RF technology, what am I missing? Not that much I think, mostly the problem relied to dealing with a software radio architecture and the quirky behavior it can have in some case. I'm probably missing some part in the requirement for doing ATSC or NTSC modulation I just look at it rather fast for the modulation scheme.
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