Quote:
Originally Posted by radiobob
Bill:
There's weirdness afoot! Suddenly I can get AC3 audio on PBS via my optical (toslink) cable where I couldn't before.
Today I was messing around with AMC3, scanning for additional channels and decided (why?) to try one more time to see if I could get AC3 audio. The optical input to the surround receiver was already selected since regular (non-Dolby) sound comes through fine, just as you describe with your Mercury. I hit the Dolby button on the surround unit's remote and switched to "Auto" mode and--VOILA!-- for the first time, all the PBS channels now have sound! I'm pretty sure I've tried the "Auto" mode on my surround receiver before, but maybe... 
To compound the weirdness, I still don't get AC3 from the "silent Spanish" channels on G10R. I haven't tried Pentagon Dolby on AMC1 yet, but I will after I finish this post.
Since Fortec touts the Mercury as having Dolby (as does Viewsat their box), I'm wondering what's going on. Even though I claim to be an audio engineer, I really know nothing about the actual nature of the AC3 stream. Is it possible that there's something new in the coding that your Mercury/Extigy combo doesn't "like", but my Viewsat/Sherwood combo suddenly does? I'm going to have to ask around on this one...
Meantime...hang in! |
That's interesting. AC3 is usually associated with Dolby 5.1, and I have heard about something called Dolby 7.1, but don't know what the difference is. Whatever the difference is, I don't think it should matter for the Fortec, since I don't think it's supposed to decode the Dolby, just pass it through, but perhaps it is reformatting the headers or something.
I hooked up the
coaxial signal from my new optical-->coaxial converter to my oscilloscope. The trace from Montanna PBS looked like this...
I have the scope on 0.5 uS/div and it looks like about 2/3 cycle/div, so I think this looks like about 1.3 cycles/uS, or 1.3 MHz (if I'm thinking straight this morning). But you see the first 2 (actually 1) cycles are solid, but the cycles after that are kind of scrambled, and jump around with the sound, so that must be where the data is.
Compare that to the signal I got from one of the PBS AC3 channels, ie:
You see that there just doesn't seem to be any data in this signal, or if it is there, it is very low intensity relative to the main carrier.
I don't understand at all how the Dolby digital works, but I think it's looking for phase modulation of this carrier. This is all very interesting, and I'm learing a lot, but I just wish I could hear the audio, because 95% of my satellite viewing is PBS, so I need my AC3.
