Help on MIC-Output ZX81

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zx81vga
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Help on MIC-Output ZX81

Post by zx81vga »

Hello ZX81-Friends,

need some Help on the MIC-Output (ZX81).

I want to create a Propeller-Based TZXduino-Clone with SAVE"XYZ" Capabilities. (LOAD "" works in my Prototype).

I don't have a Oszilloskope and want a Picture from the MIC-Output.

Here is an Cutout from:

http://problemkaputt.de/zxdocs.htm#zx80 ... ttesignals

Each byte consists of 8 bits (MSB first) without any start and stop bits, directly followed by the next byte. A "0" bit consists of four high pulses,
a "1" bit of nine pulses, either one followed by a silence period.

0: /\/\/\/\________
1: /\/\/\/\/\/\/\/\/\________

Each pulse is split into a 150us High period, and 150us Low period. The duration of the silence between each bit is 1300us. The baud rate is thus 400 bps (for a "0" filled area) downto 250 bps (for a "1" filled area). Average medium transfer rate is approx. 307 bps (38 bytes/sec) for files that contain 50% of "0" and "1" bits each.

I need the Amplitude of the MIC-Signal. (during SAVE"XYZ") 5 Seconds Silence, then the 4/9 Pulses.

10mV, 20mV, 50mV i don't know.

Then i will build a Circuit with OPamp/Schmitt-Triger to get 3.3V / 5V Pulses.

Any Help would be appreceated.

Thanks
Werner
Moggy
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Re: Help on MIC-Output ZX81

Post by Moggy »

I too don't possess a scope but do have audio experience.

10-20 mV is somewhat optimistic I would have thought as the MIC output was designed to feed but not overload the input of a simple cassette recorder.
Take for example a professional microphone like the Shure sm7b then you are looking at roughly 1.1 mV of output and I wouldn't have thought the cheap microphones used with such cassette recorders were more than that.

I have built a small mono amplifier for my Spectrum which is unique insofar as it requires no preamp between it and a low level output device due to its low input sensitivity. (40mV) and the Spectrum is more than adequate to power it, as it also has to power an internal speaker the level is fairly high, however I get next to nothing out of a ZX81 which has no speaker so suspect a voltage as low as 0.5-1.0 mV a possibility but no doubt someone with a scope could prove otherwise.

What ever the output I would advise against single stage preamplification, better to feed the output of one preamp set a reasonable output say (X10) into the input of another also at X10 for a good reasonably noiseless waveform of 5V p-p.

Also if using op-amps for a preamp avoid the older 741 devices and use something like an NE5534 audio grade chip.
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zx81vga
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Re: Help on MIC-Output ZX81

Post by zx81vga »

@moggy

Thanks for your Answer,

i have no Expirience with OP-Amps but i have some 741's,
and i switch to NE5534 or something...

I have only 3.3V or 5V Supply, i don't want a negative Supply.

Thanks

Werner
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1024MAK
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Re: Help on MIC-Output ZX81

Post by 1024MAK »

A few things to remember:
  1. The tape out / Mic out signal is derived from the same ULA pin as the video signal.
  2. The ULA video/Mic output pin signal goes between about 2.2V and about 4V during saving.
  3. Resistors R27, R29 and capacitors C11 and C12 form an aggressive attenuation circuit. This reduces the voltage of the signal available at the Mic connector down to less than 1mV.
Based on a crude simulation of the circuit, my guess is that the signal at the Mic socket terminal is about 1.4mV peak to peak (which is about 0.5mV RMS). So Moggy is about right :mrgreen:

This oscilloscope screenshot shows the ULA pin 16 output (blue signal) and the signal after passing through R29 (red signal).
ZX81 video and Mic signal at R29 #1
ZX81 video and Mic signal at R29 #1
This shows the same, but with a different timebase.
ZX81 video and Mic signal at R29 #2
ZX81 video and Mic signal at R29 #2
This shows the ULA pin 16 output (blue signal) and the signal at the Mic socket terminal with nothing connected to it (red signal).
ZX81 Mic output at connector
ZX81 Mic output at connector
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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Moggy
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Re: Help on MIC-Output ZX81

Post by Moggy »

Excellent information Mark much appreciated and a useful guide to what kind of pre-amp I would need to design should I ever decide to build same.. :D
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zx81vga
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Re: Help on MIC-Output ZX81

Post by zx81vga »

@1024MAK

Thanks a Lot for the Info

So i need the Circuit to push 1mV Pulses to 3.3V / 5V...

Werner
Moggy
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Re: Help on MIC-Output ZX81

Post by Moggy »

zx81vga wrote: Sat May 29, 2021 5:54 pm @moggy

Thanks for your Answer,

i have no Experience with OP-Amps but i have some 741's,
and i switch to NE5534 or something...

I have only 3.3V or 5V Supply, i don't want a negative Supply.

Thanks

Werner
In this case the 741's are no good and the minimum for the NE5534 in single supply mode is 6v. so an alternative power supply would be required as when these chips run under voltage it is not a case of they work a little bit, rather that they turn off completely.

I do believe Mrtinb has some experience with getting 5v from Arduino devices for loading/saving so if he sees this he may have some advice.
Be aware that a 5v supply voltage means the output signal maximum will be 2.5v p-p and op-amps cannot output more than the supply voltage which in this case is 5v so a supply voltage of 12v is usually preferable in these cases.

The problem is the ZX81s very low output which will need a lot pre-amp before then inputting the signal into something like a LM386 power amp chip set at its maximum gain (X200) to give the "punch" the 81 needs unlike the Spectrum that will feed into a low sensitivity power amp with no problem.
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zx81vga
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Re: Help on MIC-Output ZX81

Post by zx81vga »

@Moggy

I don't want to feed a LM386, i think on a 74LS14 Schmitt-Trigger.

I want the 1mV Pulses to be at TTL-Level,
so i can feed the Propeller-Chip to read the File from the MIC-Output.

Werner
Moggy
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Re: Help on MIC-Output ZX81

Post by Moggy »

In this case I think Mrtinb could be of help.
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Re: Help on MIC-Output ZX81

Post by 1024MAK »

In this application we mainly need voltage gain rather than power gain. But the amplifier must be a low noise type. The voltage gain has to be in the order of about 5600!

That kind of gain would get you to about 2.8V AC.
A 74LS14 input needs 2V DC to ensure it sees a logic 1 input. But it’s possible to bias the signal from the amplifier so that a 2.8V AC signal when positive goes above 2V DC, and when negative does not go below 0V.

I’m guessing that the idea is that there should be no modification to the ZX81, right?

A possible source of a suitable DC power supply would be to use a splitter cable, and use the +9V DC (nominal) supply from the Sinclair PSU. Use a 1N4001 diode (or similar) followed by a suitable electrolytic capacitor to further smooth the ripple out.

That should provide a suitable power rail for any audio amplifier chips.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
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