Re: LZ: cassette tapes
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Re: LZ: cassette tapes
On Fri, 13 Sep 1996 23:58:27 -0500 (CDT), you wrote:
>woah, woah.. hld up here.
>You guys have it all wrong.. I haven't been following this thread, but =
the
>ti-85 doesn't accept sound, it'd be accepting electronic pulses, you =
guys
>have the wrong idea about all of this.. It's not like the tape player
>would be emitting sound waves over a wire, that's impossible.. The
>sound occurs when a speaker is hooked up to that wire. Essentially,
>all that is being sent to the microphone is 1's and 0's.. On's and
>off's telling the microphone to contract and such (I believe it's done=20
>w/ a magnet on most?).. The problem is, those 1's and 0's are entirely=20
>too fast for the 85 to handle. It's not like a modem, where it
>would take sound and demodulate it into pulses. That's not how it's =
done.
>The only way you could do what you guys are trying to do is to slow the
>player down somehow..
>
>The other problem I see is converting a binary file to a wav.. uhm =
=3D)..
>quite frankly that's not gonna work. =3D)
>That's all =3D)
>
Frankly, using a tape recorder is not the ideal way to store external
data. I do believe, however, that it's possible. =20
What you would need to do is set up what is known as the Kansas City
standard for recording data onto magnetic tape. You cannot simply
write 1's and 0's into the recorder and then play it back into the
link. That method is just too eroneous for the TI to handle. Even
under ideal conditions it would still be too unstable. The way to do
it is to convert each bit to a frequency, and then write a subroutine
that "plays" the frequency out to the link.
For example, you can define a logic "1" as, for example 1200 Hz. This
frequency is used because even crappy recorders record well at this
frequency. Therefore, to write a binary "1" to the tape, you would
send maybe 4 or 5 cycles of 1200 Hz (which would be equivalent to
writing 1's and 0's repeatedly at a specified rate to replicate a 1200
Hz sound source).
The Kansas city standard also defines a logic "0" as a few cycles of a
2400 Hz tone, and the process repeats. The way to read the data,
however, requires a special trick. As you read the data coming in
from the tape recorder, you observe the timing that the 1's and 0's
come in and from that you determine the frequency, which will tell you
the value of the bit.
Keep in mind that this gives you a MAXIMUM transferr rate of around
300 bps, which is fine if you've got about 30 minutes to backup your
calculator's memory! There really isn't a simpler way to do this.
You would also need to hastle with level converters, since the output
of a walkman varies. It won't simply give you TTL levels that the
calculator can read, it would need to be converted (which, in itself,
is also a difficult task). =20
This is the way it was done about 10-15 years ago in early tape backup
systems (i.e. before the commodore 64). Modern tape backup units DO
NOT operate in this way. They will write data directly to the tape
and have special circuitry to keep everything timed correctly. We
don't have that luxury, however, since a tape recorder isn't a
precision digital instrument and it's quality is too poor to be very
reliable.
In conclusion, the tape recorder method has promise, but it's
extremely impractical and not worth the trouble.
-Mel
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