Tape Loading

Discussions about Sinclair ZX80 and ZX81 Hardware
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Danjovic
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Tape Loading

Post by Danjovic » Mon Jul 29, 2019 4:13 pm

Hi folks. I am routing a custom version of ZX97 board and after reading some material about tape loading I got concerned about what should I use as a front end on the input buffer pin (ear). I haven't found answers on the plenty articles and posts I have researched, not even on the ROM lisiting available online.
Then I have analyzed the code in detail and came to conclusion that a low pass filter will be fine, as the GET-BIT routine only cares wheter the bit coming is 1 or 0 and instead of counting transitions (like MSX for instance).
Another issue that might be relevant is that if a TTL chip is used as a input buffer it should have a pull down on the (ear) input pin as the ZX81 expects the silence (no audio-in signal) to be at LOW level.

Code: Select all

|----- "0" bit ----|
/\/\/\/\____________  as coming from ear
________
        \___________ as should be seen by GET-BIT
 1200us    1300us        
Anybody else that have studied tha matter have come to the same conclusions?


[]s

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1024MAK
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Re: Tape Loading

Post by 1024MAK » Mon Jul 29, 2019 5:45 pm

Have you studied the ZX80 schematic?

Mark

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Danjovic
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Re: Tape Loading

Post by Danjovic » Tue Jul 30, 2019 6:11 am

1024MAK wrote:
Mon Jul 29, 2019 5:45 pm
Have you studied the ZX80 schematic?

Mark
Yes, I have studied the zx80 and some zx81 clones, and all of 'em use a capacitor in series thus forming a high pass filter but then again a low pass filter would make more sense.

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1024MAK
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Re: Tape Loading

Post by 1024MAK » Tue Jul 30, 2019 7:50 am

The series capacitor also provides isolation of any DC offset voltage that may be present on the input.

The capacitor and the resistors are also needed to provide some protection in case of a very high volume level, and will provide some protection against static discharge.

If the value of the capacitor is increased, this should increase sensitivity.

Mark

albertusj
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Re: Tape Loading

Post by albertusj » Tue Aug 06, 2019 12:09 pm

I did an analysis of tape data storage here:

http://www.myprius.co.za/tape_storage.htm
ZX80/ZX81/ZX Spectrum

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