On Sat, 9 Oct 1999 g.j.p.a.a.baltissen@kader.hobby.nl wrote:
> Hallo Marko,
>
> > > Could anybody explain me shortly two things:
> > > 1) Is the data send to the recorder the same as the real data to be saved?
> >
> > In principle yes, I think. I haven't played much with the tape, but I
> > think that -FLAG is an edge-sensitive input, while the processor line you
> > use to control WRITE is level-sensitive.
> >
> > > 2) How is the data translated to "music"?
> >
> > The data is stored as a square wave (okay, it is more sinusoidal, but an
> > op-amp and a cutter in the read circuitry of the datassette make it look
> > like a square wave again).
>
> I understand the part of translating a sinus to square; that is purely an
> electronic thing.
>
> Let's ask a simple question: 1) how are the bytes $FF and $00 put on tape?
>
> The thing I can add is that with the KIM a byte is split up in two nibbles.
> Then each nibble is translated to a character ranging from ??? to ???. Each bit
> of these bytes is translated in a two series of sinuswaves of two different
> frequencies. The length of each of the two blocks determines differs for bit 0
> or 1.
>
> 0 = |AAA|BBBBBB| 1 = |AAAAAA|BBB|
>
> I cannot give more detailed info as I cannot get to it. If you look at the time
> the mail is generated (05.15), I writing during some pauses. Somebody messed up
> things at work and I was phoned out of bed to clear it up :(
>
> I can imagine that with the PET they skipped the splitting of the byte but
> translating a bit to a bit pattern still must nescesary.
>
> Groetjes, Ruud
>
I have document from the early 1980's that describes the tape signal. It
looks like part of a newsletter, but it has been copied and recopied so
many times I have no idea where it originated. It is several pages long.
so I will summarize it.
The cassette uses an unequalized constant current recording method. It
uses pairs of pulses to record a bit. A long pulse followed by a short
pulse is a one. A short pulse followed by a long pulse is a zero. A mark
pulse is used to separate bytes.
long pulse = 262 microsec. half cycle 1.91 KHz
short pulse = 182 microsec. half cycle 2.75 KHz
marker pulse = 342 microsec. half cycle 1.46 KHz
Each byte consists of a marker, 8 bits, and a parity bit(odd).
Data is output to the cassette on the positive transitions and input on
the negative transitions, so a phase shift can make the signals
unreadable. High frequency emphasis on audio cassette decks can shift the
phase as much as 90 degrees. Tape duplication usually produces a small
phase shift. Better results can be obtained by setting high frequency
response to nearly a minimum.
The cassette deck records at around 500 nanowebers per meter, just under
saturation on low coercivity tapes. When duplicating tapes, setting the
volume high enough to fully saturate the tape gets good results.
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