Help on MIC-Output ZX81
Help on MIC-Output ZX81
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
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
Re: Help on MIC-Output ZX81
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.
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.
Re: Help on MIC-Output ZX81
@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
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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Re: Help on MIC-Output ZX81
A few things to remember:
This oscilloscope screenshot shows the ULA pin 16 output (blue signal) and the signal after passing through R29 (red signal). This shows the same, but with a different timebase. This shows the ULA pin 16 output (blue signal) and the signal at the Mic socket terminal with nothing connected to it (red signal). Mark
- The tape out / Mic out signal is derived from the same ULA pin as the video signal.
- The ULA video/Mic output pin signal goes between about 2.2V and about 4V during saving.
- 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.
This oscilloscope screenshot shows the ULA pin 16 output (blue signal) and the signal after passing through R29 (red signal). This shows the same, but with a different timebase. This shows the ULA pin 16 output (blue signal) and the signal at the Mic socket terminal with nothing connected to it (red signal). Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
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ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
Re: Help on MIC-Output ZX81
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..
Re: Help on MIC-Output ZX81
@1024MAK
Thanks a Lot for the Info
So i need the Circuit to push 1mV Pulses to 3.3V / 5V...
Werner
Thanks a Lot for the Info
So i need the Circuit to push 1mV Pulses to 3.3V / 5V...
Werner
Re: Help on MIC-Output ZX81
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.
Re: Help on MIC-Output ZX81
@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
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
Re: Help on MIC-Output ZX81
In this case I think Mrtinb could be of help.
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Re: Help on MIC-Output ZX81
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
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
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.