Messages in this thread |  | | | From | "H. Peter Anvin" <> | | Date | Wed, 16 Sep 1998 23:49:47 -0700 | | Subject | Re: Audio tape device (was: NEWSFLASH! Linux ported to Commodore VIC-20!!) |
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This is majorly offtopic and some people have complained; therefore, I have redirected this to comp.os.linux.hardware and will post any further messages there...
Followup to: <360081B0.CB2FF0FB@his.com> By author: Feuer <feuer@his.com> In newsgroup: linux.dev.kernel > > I'm thinking. Do the data have to be stored normally? Or can they be stored as > frequency sums or something (Fourier transforms?) >
What does "normally" mean? Remember that all signalling is fundamentally analog; digital signals need some form of modulation. The "cheap" way to do this is PCM, in which one level is used for 0 and another for 1, but there are many other ways. PCM is in fact not possible at all, since a tape recorder (or a phone line) will not preserve DC, as in a long string of 0's or 1's.
Modern modems usually use Trellis-code modulation, a variation on QAM (Quadrature Amplitude Modulation), in which two carriers at the same frequency but 90 degrees out of phase are individually amplitued-modulated; if you plot the result in the (x,y) plane where x and y are the modulations of the two carriers (and the polar coordinates are the amplitude and phase of the merged signal) then you get a point-scatter plot called a constellation, where each point correspond to a combination of bits. Because audio media (including phone lines) usually have a fairly large signal/noise ratio but poor bandwidth, a fairly large number of such points provide better total data rate, but of course requires a lower noise level (less noise margin.)
Trellis differs from QAM in that whereas in QAM the two carriers are modulated using separate set of bits resulting in a constellation shaped like a square grid, Trellis uses a pre-coding phase to shape the constellation to better fit the medium. A typical Trellis constellation looks a bit like a flower trellis, hence the name, and usually has a roundish outer boundary (corresponding to a certain amplitude.)
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