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Forums > C64 Coding > scope view of some tape signals
2018-12-26 13:29
Hoogo

Registered: Jun 2002
Posts: 105
scope view of some tape signals



I've connected the scope to the r/w head of the Datasette and made some tests. The test signals were coming from the C64 and recorded on a Datasette with a normal Type I tape.

Most test signals were made of "bits" of a length of 22 cycles. That length come close to the usual sample rate of 44100Hz, might be handy some day to create files with a sound card.
Also the test signals were really simple until now, always the same peak and gap lengths repeated.

------------------------------
Upper row:
-Noise: I just pressed the scope to the head without pressing play. I assume that the signal comes from the monitor, so one wave should be 63 cycles. I don't see that noise in the other signals, so I will better check again what I did there.

-44+44: 11.025 Hz, somewhere near the maximum frequency of a tape. The signal is not stronger than the noise, imho not usable for any turbo tape.
-55+55 can't be created that symmetric with a sound card, . Seems clear enough for a tape.

-The last 3 in the row were tests with an assymmetric signal, smaller peaks and longer gaps. Surprisingly it looks just like the normal sinus of weaker level.

------------------------------
Lower row:
-Nothing (=the noise of the scope?)
-The normal tape format, just saved something with the save command.
-Something very long. I just added a loop from $40.
-Something VERY long, loop from $00. The scope was adjusted to the longer signals. Apparently the signal falls to the base line when it gets too long.
-And finally: My finger pressed to the head. As this is perfectly one pulse per line, I'm confident that this signal comes from the monitor.

The signal should be steep when it crosses the baseline, so the long flattening signals should be worse than the fast signals.
------------------------------
Coming next, maybe this weekend:
-Check the noise again.
-See the writing signal at the head.
-Compare the input received from the r/w head to the resulting signal on the tape port. Unfortunately there's only one sensor attached to the scope.
-See more complicated signals made from different peaks and gaps
-Write quick changes at the center of a peak
-See a chrome tape
-See something written by Hifi equipment.
 
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2019-02-06 14:22
chatGPZ

Registered: Dec 2001
Posts: 11523
oh, yes, of course. it also helps to understand what precompensation does, and why.
2019-02-06 14:33
tlr

Registered: Sep 2003
Posts: 1814
IIRC the effect that needs precompensation only occurs when the information density gets sufficiently close to the particle size of the medium like in a HDD or floppy. I wouldn't think it applies to the rates applied to a typical tape, especially since the recorded bit rate is far below the audio bandwidth of a typical cassette tape.

What I think could apply though is the "memory" effect of the simple drive circuit, being a coil driven by a square wave through a resistor. Maybe that is what hoogo was refering to with the graph?

I don't know the inductance of the recording head so I can't calculate the time constant here, but the drive resistor is 10k in the c2n schematic.
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