Freshly produced azimuth alignment tape for your VIC anyone?

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eslapion
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Re: Freshly produced azimuth alignment tape for your VIC anyone?

Post by eslapion »

pixel wrote:And you want regular alignment if you intend to make tapes for others. But then again I wouldn't use the datassette for that anymore in order to get it right down to the CPU cycle and to get a cleaner signal and a head alignment that is probably OK.
A real datasette isn't that accurate.

The motor speed varies slightly from machine to machine by about 5% and the low frequency hum of the motor introduces irregularities in the signal even if the speed adjustment is close to perfect. Even if you're using a tape generated by a studio quality tape deck as reference, the drive you want to align can only play it back with a certain level of accuracy.

Going to CPU cycles accuracy is probably overkill.
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Re: Freshly produced azimuth alignment tape for your VIC anyone?

Post by pixel »

beamrider wrote:I'm assuming the .tap file you provided has the correct volume (assuming .tap files describe volume)?
No, TAP files don't do that. :) They're just lists of pulse lengths.
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Re: Freshly produced azimuth alignment tape for your VIC anyone?

Post by pixel »

eslapion wrote:Going to CPU cycles accuracy is probably overkill.
Oh dear. Let me rephrase: you get the highest accuracy there's to have. The irregularities of datassettes didn't exactly escape my attention over the last couple of months. But I didn't really care about measuring them.
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Re: Freshly produced azimuth alignment tape for your VIC anyone?

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pixel wrote:Oh dear. Let me rephrase: you get the highest accuracy there's to have. The irregularities of datassettes didn't exactly escape my attention over the last couple of months. But I didn't really care about measuring them.
They are significant.

On another note: A few months ago, I entertained myself to build a test bench for datasettes which allows me to directly attach the digital input/output lines to a function generator/oscilloscope. I used that to create a reference tape with a 2.5kHz signal. I also generated another tape with a range of different frequencies to study the transfer function of the built-in filter-integrator of the datasette.

See: http://sleepingelephant.com/ipw-web/bul ... =datasette

I also used it to convert the playbacks from a datasette to a signal that can be captured by a PC's sound card and generate .TAP files. The reverse also works.

A modified version of this test bench could effectively be used to generate perfect reference tapes assuming you know exactly the type of signal you want to put on there and I would be happy to use it to help you with your project.

The funny thing I discovered about the datasette is that because of the playback filter, if you try to play music with it you'll get junk but the opposite is completely untrue; you could convert your music with a high speed 1 bit DAC and you'll get top quality recordings albeit in mono.

IMHO, one of the worst problems of the datasette is head magnetisation and it is getting more and more difficult to obtain good demagnetisation tools. Magnetised heads weaken the recorded signal with each playback of a given tape.
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Re: Freshly produced azimuth alignment tape for your VIC anyone?

Post by pixel »

eslapion wrote:The funny thing I discovered about the datasette is that because of the playback filter, if you try to play music with it you'll get junk but the opposite is completely untrue; you could convert your music with a high speed 1 bit DAC and you'll get top quality recordings albeit in mono.
Sounds severely unreasonable and border–lining porn. I'm in.
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Re: Freshly produced azimuth alignment tape for your VIC anyone?

Post by eslapion »

pixel wrote:
eslapion wrote:The funny thing I discovered about the datasette is that because of the playback filter, if you try to play music with it you'll get junk but the opposite is completely untrue; you could convert your music with a high speed 1 bit DAC and you'll get top quality recordings albeit in mono.
Sounds severely unreasonable and border–lining porn. I'm in.
This tells me you could use a VIC-20/C64 to record some sort of PWM on a datasette. On playback, the datasette's playback filter will turn these PWM pulses into triangular waves of varying amplitude which will result in pulses of progressive durations.

That could allow for the creation of a tape specifically created for the purpose of alignment.
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Re: Freshly produced azimuth alignment tape for your VIC anyone?

Post by pixel »

Bare statistics with varying pulse lengths certainly do. The more misaligned the datassette is while playing PWM audio the noisier it gets.
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Re: Freshly produced azimuth alignment tape for your VIC anyone?

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pixel wrote:Bare statistics with varying pulse lengths certainly do. The more misaligned the datassette is while playing PWM audio the noisier it gets.
Noisier where exactly?

What I see on a scope tapping at the input of the Schmitt-Trigger is the more misaligned the datasette is, the weaker the amplitude of the triangle wave coming out of the integrator and so the shorter the resulting pulses.
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Re: Freshly produced azimuth alignment tape for your VIC anyone?

Post by pixel »

eslapion wrote:
pixel wrote:Bare statistics with varying pulse lengths certainly do. The more misaligned the datassette is while playing PWM audio the noisier it gets.
Noisier where exactly?
In my freakin' ear canal. Now in my freakin' whole skull. The Australians make great Merlot. %p

No, seriously, if the cassette doesn't sit in right: noise.
What I see on a scope tapping at the input of the Schmitt-Trigger is the more misaligned the datasette is, the weaker the amplitude of the triangle wave coming out of the integrator and so the shorter the resulting pulses.
I don't have tools like that. :( Don't you think sines come close at the Schmitt–trigger frequency limits. Flippin' heck, I'll do triangles as long as there's human right for plain water.
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Re: Freshly produced azimuth alignment tape for your VIC anyone?

Post by eslapion »

@Pixel

In the datasette, on the playback circuitry is a pair of cascaded integrators with gain control.

They are described here:
http://www.circuitstoday.com/integrator ... sing-opamp

http://www.electronicshub.org/operation ... ntegrator/

http://www.electronics-tutorials.ws/opamp/opamp_6.html

If you send something that's too much out of the expected frequencies range then you get junk... or nothing at all.

What comes out is then passed to a Schmitt-Trigger which in turns sends it to the VIC-20/C64/C128
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