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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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