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    SubjectRe: Audio tape device (was: NEWSFLASH! Linux ported to Commodore VIC-20!!)
    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.)

    -hpa
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