Audio input suggestion

[badrianiulian]

Since I figure that the ultimate goal is to recreate all functionality of the original ZX Spectrum, I am thinking that by using some modified hardware (or some new one even) from this microphone library, this goal can be achieved.
By adding a 100uF for isolation purposes as the guidelines for the analog module say maybe we can reach the goal.
I may be able (and willing) to test some hardware... I'm no audiophile but I got the tools to make what I usually need so that the analog "Line In" function can be available for the old cassette players out there... maybe even a save to ".tap" directly to the SD Card at the end... for comparison purposes since I still remember how error prone was this process in the old days.
I can try some different ways with the hardware part (will research more this part so we don't fry the pico) but since all this work is your baby, only you know if the analog part of the library can be adjusted to work with all the different layouts and if one input pin is available.

[fruit-bat]

This may be of interest :
http://www.zxdesign.info/cassette.shtml

Might be fun to be able to load from a real audio source 🙂.

The software side will be tricky. There are moments when the z80 is paused to read the keyboard etc. It catches up afterwards but any audio input will need to be kept in sync.... I think it will require the CPU speed moderation to be rewritten and based on the depth of an audio queue.

[badrianiulian]

From the above, I extracted this:

As for the loading part, how about adding an Cassette mode - F2 key is free - that sets the CPU to 3,5MHz and monitors only the F2 key... just a thought...

[fruit-bat]

I'm not 100% sure but I think Vcc in their design is 5v... So we need to adjust R42 & R41 so that the gate bias is correct for 3.3v. Also, the author says his audio signal goes from -2.3 to +2.6 which seems high to me, I would expect more like -1.1 to +1.1v. The threshold voltage on the pico is about 1.8 volts, rather than 3.7 for the 74hc gate.

[fruit-bat]

Hmm, also this circuit has no gain, which we may want a little of if I am right about the voltage range on the input pin... May need to give this a little thought. If you can take some measurements of actual voltage ranges into a 1k load that would really help. E.g a pc audio jack, a phone, an old tape player.

[fruit-bat]

...Maybe the higher voltage is to able to drive headphones?

[fruit-bat]

On the pico...

"Normally the voltage threshold is about 1.8V, but it isn't guaranteed; it can be anywhere between the maximum input low and minimum input high, that is, between about 0.8 and 2.0V."

[fruit-bat]

Could just try it with 39k for R42. Please correct my junior school maths if I have got it wrong.

Centre voltage...
(2.0v - 0.8v) / 2 + 0.8v = 1.4v

Add in the BE voltage...
1.4v + 0.6v = 2v

So...
2 = 3.3 R42 / (R42 + 27)
=> R42 ~ 40kohm

And pick one that is easy to buy...
39kohms!

[badrianiulian]

Been on the road all day and will be the same for the next two days... we had a planned family trip. As soon as I'll be home, I can take some measurements with actual components and different audio signals.

[fruit-bat]

I hope you have a nice time with your family.

[badrianiulian]

Thank you! I had a nice time with the family ^_^
As for the tests, today I did some measurements and here they are:
I set volume to max from PC and had playing sound from Fuse (game menu - ADayInTheLife 1985 - has some really loud sounds):

• PC headphone source:

1k resistor = maximum of 290mV AC and 7mV DC
output voltage ~= 1,48V-1,5V DC

• TV line out source (PC HDMI audio through TV):

1k resistor = maximum of 650mV AC and 3mV DC
output voltage ~= 1,53-1,55V DC

As for actual components, for the 27K I used 22K+4,7K (~26,3K measured), for the 39K I used 33K+3,9K+3,9K (~39,4K measured) and for the 56K I used 47K+10K (~54K measured). I used a 2N2222A since it was already in my components stack and it's an equivalent for BC548.
For the VCC (3,3V) I used a USB-TTL adapter.

[fruit-bat]

Thanks for those measurements. It looks like the voltage swing is not really big enough 🙁. It's a shame the adc doesn't work, it would be ideal for this. Maybe we need an amplifier (rather than an emitter-follower) or a Schmitt trigger? Don't want it to get too complicated but I've seen some 2 transistor designs for a Schmitt trigger.

[badrianiulian]

At first glance, an amplifier may be too much (on the other hand it's late and I might not have all the marbles in place right now).
I did found some explained details about a Schmitt trigger here and also here.
Now regarding the details, we could learn from some actual builds, as this was another find.
What should the actual voltage swing need to be?
Also as I seem to understand from an alternative design there might be also a need to invert or non-invert the signal... (?)

[fruit-bat]

Well, if the 0.8 to 2.0 volts thing is correct, at least 1.2v.I
I don't think it matters if it is inverting or not. Thanks for the links I will read through them when I get a mo.

[fruit-bat]

I've done some more reading and found the rp2040 inputs have their own hysteresis...

https://raspberrypi.github.io/pico-sdk-doxygen/group__hardware__gpio.html#ga1f52eecc12d4348ef8bd1e17bdb4b107

I probably need to find out more about that.

[badrianiulian]

I found an example with hysteresis experimentation on the raspberry pi forums.
Unfortunately I don't have an oscilloscope to play with.
If it takes too much of the CPU though, the hardware way is the way to go.

[fruit-bat]

Been reading this...
https://en.m.wikipedia.org/wiki/Line_level
... which suggests 10k (rather than the 1k) load resistance.
Perhaps that would give a bit more of a voltage swing.

[badrianiulian]

As I should have expected, when using the 10K, with the PC headphone source (it was the only test I did), the AC voltage is the only difference encountered:

10k resistor = maximum of 65mV AC and 7mV DC
output voltage ~= 1,5V DC

Also the same AC voltage seems to be present at the output line (same as before witch I failed to mention above)... so this is definitely not the way to go.
I'll try to find components for the Schmitt trigger schematic... something equivalent to what the 74C/HC14 does.

[fruit-bat]

They are easy to make with an op amp. I've ordered some that will run at 3v to have a play with. We can compare notes then 🙂. I'll post the circuit when I get a mo.

[fruit-bat]

This site is good...

https://www.random-science-tools.com/electronics/inverting-schmitt-trigger-calculator.htm

[fruit-bat]

Something like this...

The pot is there to adjust the threshold, which we could fix if we find a good value. I'm not sure about the lm224n, just need something that will work at 3.3v.

[fruit-bat]

...but it does turn out the op-amps I have ordered are going to take a long time to get to me!

[badrianiulian]

I dug and found some schematics from different clones of the ZX:
HC-2000

CIP-01

After that I changed the testing procedure:
I set volume to max from PC and had playing sound from PlayTZX inside DosBox (same game - ADayInTheLife 1985):
With the first schematic (transistors) the test had the same result as before.
For the op-amp schematic, I used a βM358N instead of a LM224N and also changed the 10k+100kVR to 100k+100kVR(with the actual value set at about 150k).
Results:

• PC headphone source:

output voltage in idle mode ~= 1.71V DC and 0V AC
output voltage in play mode ~= 1.06-1.08V DC and 1.15-1.27V AC (1.23V mostly)

• TV line out source (PC HDMI audio through TV and TV volume set to at least 30):

output voltage in idle mode ~= 1.71V DC and 0V AC
output voltage in play mode ~= 1.01-1.08V DC and 1.17-1.23V AC (1.20V mostly) | I'm not sure but I think I noticed DC voltage going down when TV volume goes up 

[fruit-bat]

Hi, sorry for the slow reply, 'real-life' has been distracting me!

The 1st circuit looks like a non-inverting comparator, without hysteresis. I like the way the bias is arranged; the single (rather than dual) voltage divider means you don't have worry about tolerance on the resistor values.

The 2nd circuit looks like a high gain inverting amp... but I'm not sure what R30, R70, D9 & D10 are up to! Do you know what VMED gets connected to?

[badrianiulian]

I know what you mean about 'real life'. I've been pinned to the bed for the last two days due to a nasty cold... better now.
I changed the above labels for the schematics since I mislabeled them. I only just noticed by searching through the schematic for VMED witch goes through at least three pages (pages 2,3 and 4).
There are other schematics on that site that could prove to be useful.

[fruit-bat]

Thanks for finding those... I will get back to this at some point!

[fruit-bat]

I've added a little test program which reads GP11 at 44Khz and sends 1s and 0s to the USB uart. Not as good as a dual-channel scope but it's something!

cp uf2/ear_in_test.uf2 /media/pi/RPI-RP2/
tio /dev/ttyACM0 

[fruit-bat]

See https://github.com/fruit-bat/pico-zxspectrum/pull/54/files

There is even a small chance this may work! It samples GP11 once a microsecond using the PIO and feeds it into the emulator.
Should cope with 32x32 microsecond delay in instruction emulation.

I'ts going to take a bit of work to fix all of the targets so only ZxSpectrumBreadboardHdmi4PinAudio is working at the moment.
Let me know if you want a binary to play with.

[badrianiulian]

Been a bit busy digging the net for anything Spectrum for a while now (I lost at least four hours tonight just to find this in the end). Also I've been checking some wav dumps of some tapes since I'll surely need them to test the audio input. I will have to put together another Pico and test it with the breadboard since it's easier in the long run (I ordered a while back one piece of Wren's module and a micro sd card module).
For now I'm off to bed (huge headache) but a binary and some instructions to test in a couple of days would be nice. As soon as I can test it (using the components with βM358N), I'll be back with feedback.
By the way... when I tested the op amp circuit, I used 3.3V. Since the 3.3V has to be stable for the SD Card, maybe I should test the circuit beforehand using 5V from VSYS just like I used for the HXJ8002 amplifier circuit?

[fruit-bat]

Hope your headache is better. Stick to 3.3v on the opamp. I think 5v will damage the pico.

[fruit-bat]

I've just loaded Frogger from a WAV file 🙂
The circuit I used is a mess but it proves that the software sort of works!!!

[badrianiulian]

Back with news:
I tested the last schematic and got no results: no sound and no data whatsoever on the screen.
Then I went to the last working schematic and added a 5k variable resistor in series with a 22k resistor. I changed some components inspired by the last schematic and measured all the components.
This is the schematic with real live values of the components:

circuit-20220924-1538.circuitjs.txt

I tested wav files with both the LM358N and the LM224N using this schematic and this components:

• the two diodes connected to the signal input : BAT54S SMD Schottky diodes (changed its place accordingly)
• 25.7kΩ : 22kΩ resistor + 5kΩ variable resistor
• 21.8kΩ : 22kΩ resistor
• 87.8nF : 100nF capacitor
• 100KΩ: 100kΩ resistor
• 987Ω : 1kΩ resistor
• 46.4kΩ : 47kΩ resistor
• 4.44uF : 4.7uF capacitor
• 1.012MΩ : 1MΩ resistor
• 99.7kΩ : 100kΩ resistor

I took in consideration this with the LM224N:

On any unused op-amps I connect the output to the - input, and leave the + input floating.

All wavs loaded fine with a little less or a little more volume level from the phone/pc.
By the way: using a larger capacitor like 47uF gives in a little lag... it takes a while till the capacitor is charged.
A 10uF capacitor has issues with the phone signal.
A 2.2uF capacitor has issues with the PC signal (a ripple can be heard once in a while when no signal is present)
That's why I settled with the 4.7uF... it's in-between.

Now for DC voltages: LM224N | LM385N

• 0.0V | 0.09V - Output without GPIO11 connection while signal/data is not received
• 0.55V | 0.63V - Output with GPIO11 connection while signal/data is not received
• 0.86V | 0.93V - Output without GPIO11 connection while signal/data is received
• 1.24V | 1.05V - Output with GPIO11 connection while signal/data is received
• 1.44V | 1.47V - Output of the first op-amp
• 1.47V | 1.47V - Input '+' of the first op-amp

Also, in the process, I found that the audio phone splitter, when hold in hand and no connection to the phone, generates a lot of noise. The 2m shielded audio cable (witch connects to the audio splitter) does no such thing when I hold it in my hand and isn't connected to any signal source. Maybe I should have made my own splitter after all.
Noise is gone with this setup but high quality cables have to be used.
I did the same 'shielding' of the wire connecting the circuit '+' input to the connector/Schottky diodes as you can see:

And unrelated, as a last thing I did, I changed the input resistor to the amplifier from 56kΩ to 100kΩ since the wife was complaining about the 'noise' :)) . She did not grow up with this 'noise'.
In the end I will have to add a pot for volume control on the audio output.

[fruit-bat]

Thanks, your resistor ladder values work much better for me too :-) I will try the other values as well but got a bit side tracked (can you believe it).

I've been playing with trying to mix the L&R and been experimenting with decoupling both the signal and shield.

Using the bias voltage as the shield voltage gives me better noise rejection and I pick up very little noise from picking up my
cheap audio lead. With this arrangement the capacitors have very little initial 'charging' to do and I can use anything from 100nf to 47uf and it still works reasonably... 4.7uf may indeed be the sweet spot - but I don't have any at the mo... I will get some ordered. Doing this may introduce a whole raft of other problems but its interesting anyhow.

If I plug my phone into a charger the amp becomes swamped with noise. This may be the same problem you were having if you were sending audio from your p.c. and powering the pico from it; possibly the decoupling capacitor will sort it... but probably not! I'm not sure what to do about it... I think I need a lot of coffee.

Don't feel you have to do anything with the following (unless you want to). It's just what I have been experimenting with:

circuit-20220924-2051.circuitjs.txt

[badrianiulian]

So... tried the above and not doing wonders for me... lots of R-Type errors... it seems very unstable.
I'll remain at this schematic that worked every time (with both op-amps LM224N and LM358N):

On the plus side, this version works well with the ESD protection on the input. And if we replace the 25.7kΩ with a 24kΩ and a 5kΩ small potentiometer, we could give more control for testing loading from tapes witch come in 'all size and shapes' sort of speak.

As for mixing of the two channels, I think in this case it's not a good idea: one channel might have a distortion when playing from tape players. If the two channels overlap, the distortion might give out an R-Type error.
As I found in some tutorials for dumping tapes and converting them to tzx files, if one of the channels has a problem converting, then the second channel can be used since it might not have the same distortion.

So, as I've seen from my tests and I think you saw in your tests, in my opinion we should warn people that proper grounding or a stabilised power source on the signal source is a must if they don't want to hear the background noise in the pauses, because I think we've done all that we could do about filtering it.

I'll wait for the tapes in the meanwhile but, as I remember, the walkman had batteries witch is a more stable power source than a smps power supply (PC/Charger) so I think my only concerns will be about the tapes... if they have demagnetised over time or not.

[badrianiulian]

So, some tapes came today but unlucky thing about the walkman... It does not output sounds from any tape... now I have to figure out what's wrong with it.

[fruit-bat]

On no! Hope you can get it to work. It will be quite an achievement to read a real tape. Also, Walkmans are still cool.

[badrianiulian]

Back again... good news and bad news:
Found the problem in the walkman: replaced the volume potentiometer. I even had to open up one tape to fix the pressure pad and also stick the magnetic tape to the non-magnetic tape at the beginning of the tape... it broke off somehow.
Started testing and this were the first results:

After those few games, the walkman went haywire... it outputs sounds but I get volume spikes and messes up everything. So the walkman's busted... again... while it worked fine for a few minutes, I don't expect it to satisfy my needs anymore (old stuff will stay old).
I'll have to find a normal cassette player to proper test the rest of the tapes.

[badrianiulian]

I couldn't help myself and tried again to fix the walkman and replaced the electrolytic capacitors.
So now, the volume stays at a constant level and music cassettes are hearable but the games won't load since the sound is distorted.
Conclusion: walkman is not a good audio source (at least the cheap ones like mine aren't).
I'm already in the process of finding a cassette deck or something similar.

[fruit-bat]

Nice to see at least a few loaded first 🎉🙂

[fruit-bat]

From messing about, having some resistance between the amp and the pico really helps stability. Doesn't seem to need much 10 to 100 ohms seems enough.

[badrianiulian]

Hello!
Been busy this couple of days... late nights while I tried lots of repairs with a tape player I got from a coworker:

In the end, the above tape player has too many issues, but the main issue has to do with it's motor. When I first started it, the motor started rumbling with noise from spinning. I oiled it, cleaned it, changed capacitors but in the end I gave up on it... it just doesn't work as it should.

Then I went to this:

This had some problems a while back (2-3 years ago). It wasn't starting and all it did was blinking the power status led. I had it laying around doing nothing since then and today I opened it up and fiddled with some gears in the CD sections. The tape part was intact. In the end this one started to work.

I added an 100 ohm resistor between the output of the op-amp and off I went with the demo tape from the old HC-91:

With volume level 25 of 30, I loaded the first 6 parts and with volume level 20 of 30 I loaded the last part:

Maybe I overdid it but I loaded all the programs on the tape.

[fruit-bat]

Amazing! I'm pretty sure that can be called successful... and thanks for posting all the pictures :-)

I've been doing some messing around but not had time to write it up nicely... I have a nice working transformer coupled input stage. It has the advantage of no direct d.c. coupling and is wonderfully low noise (as the transformer acts as a filter). I'm not sure how practical it is as transformers are more expensive than a capacitor! I just wondered if it would work as the transformer will be reacting to the slope of the input rather than its amplitude.

Also, and this is a bit embarrassing, I've found out why the input goes crazy when my phone is connected to a charger... it's because the audio output of the phone goes crazy!! So yes, I should have checked that first before assuming I had some complicated weird ground loop problem (grr).

[badrianiulian]

Show some pictures of the transformer... I do believe I bought some audio transformers a while back for another project involving a theremin... 5 pcs Audio Transformers 600:600 Ohm from one seller and another 2 pcs from another seller...

[fruit-bat]

Hyuduo Audio Transformer 1300 : 8

[fruit-bat]

Just tried it with a 600:600 and it seems to work with that too.

...well actually works much better with the 8:1300 might need to tweak the design for the 600:600

[fruit-bat]

Probably very badly designed....

• sorry about the zenner diodes, I just happen to have loads of them!

It's likely the circuit you are testing with is probably more sensible!

[badrianiulian]

I tried with the 600:600 audio transformer.
It picks up static even when I'm not near the plug and the cable is unplugged.

I'll stick to this one:

With this schematic, the only static I hear is when I'm near the plug at about 1cm.

By the way I just loaded another cassette on both sides with this schematic.
I actually extracted a good section of another tape that I have a dump of (from the RHC Group). The original section from the dump had some CRC errors so I rebuilt it using the good section I just extracted.

Did you get the 4.7uF capacitors?
Zenner diodes cut down the signal so that's why I'm using the Schottky diodes. They do help in calming the noise along with the 4.7uF cap.

[fruit-bat]

Yes, and I get a bit more signal through them than the 1uf capacitors.
I am still having trouble with the d.c. coupled circuit, I think the problems may be related to my monitor.

[fruit-bat]

When I get a mo, I will add you design to the 'readme' so others can have a play, as it is the best so far. Thanks for your tireless help and testing.

[badrianiulian]

In my opinion, you can add it to the readme and also you should add a note near the schematic: If someone is willing to improve it then we're willing to test it. If other people can improve it, then thats for the best for all of us.

By the way, it's your design and I just tweaked it a little... I always understand and learn things by experimenting on stuff.

I found browsing online that we could have better results if the input capacitor is as close as possible to the circuit... just a thought of what we could try until somebody can improve it.

Now, if things are done with the design (until someone comes along and improves it)... how hard would it be to add tzx support? I'm thinking of the SpeedLock-ed apps that have pulses and stuff. I should add another issue for that... right?

[fruit-bat]

I've added a new issue for TZX support here #58
Add a comment on if you want just support for read or write as well.

[fruit-bat]

I've put up the diagram on the readme of the version that you have tested the most. I probably need to tidy it up at some point but its there now :-)

...because I clearly obsessive...

I've done a version with the 600:600 audio transformer. Works rather nicely for me, including the degraded wav files, although they need more volume than PlayZX.

I resorted to asymmetry rather than hysteresis to keep it quiet with no signal present... and it is absolutely silent, even with a dangling lead.

[badrianiulian]

As soon as I can, I'll test the last schematic.

For now I've been sorting out through the tapes I got last week... Some of them are either really badly recorded or really badly degraded.
One thing is for sure: 3 original tapes work amazingly in any condition... the copies, not so much. Whoever made the copies, definitely had problems with their tape recorders.

[badrianiulian]

I'm back with an update.
I tested the last schematic and reached this conclusion:

So it seems that the transformers on the left are very noisy (or bad for audio projects in general?).

When I tested the last schematic

with the yellow transformer, the results weren't that great. I got some R Type errors when using a clean working tape (one of the original tapes).

The it got me thinking... since the yellow transformer doesn't output the noise then maybe it works as it should in the previous schematic.
So I tested the clean tape with both schematics:

and

I used 1N4148 (witch I got lots of) instead of those Zenner diodes. Also the 25k resistor had a value of about 25.4k for both setups.
For both schematics the tape player volume was set at 23 out of 30.
All I did was change the components from one schematic to the other and ran through the tape on both setups.
And go figure: No more noise with the second schematic due to the yellow transformer.
And with both setups all data was read (both sides of the tape).

So... what do we do now?

[fruit-bat]

Do you know what the yellow transformers are, winding/impedance wise?

[badrianiulian]

I ordered them the same time I ordered the red ones.
They are 600:600 isolation transformers. I guess that the yellow ones are of higher quality than the red ones.
This is where I got the yellow ones and this is where I got the red ones.

[fruit-bat]

Thanks, I'll get a couple of the yellows so I can compare... and at some point I will build up the circuit that I believe you had some success with. It would be nice if there was a circuit that worked the same for both of us!

[badrianiulian]

I'll have to buy some yellow ones too (again) since I'll be using the two I got for this project and the red ones don't seem so reliable anymore.
I think that the two above schematics (two op-amps with hysteresis), one with the Schottky diodes and the other with the yellow transformer are both good designs. The transformer has to be good quality in order for the second one to work as it should (no noise I mean). The difference between them seem to be that the Schottky diodes circuit is more sensitive on the input if the power supply is from a PC/Charger (SMPS supply). Isolating the input with the yellow transformer seems to work magic with the noise (that is definitely generated by the pico's power supply).
I'll be waiting for your transformers to arrive so you can do the tests.

Meanwhile, I made a promise to sort some roms from some sites, while I wait for the next batch of tapes (witch I'll dump and check against the TOSEC for ZX Spectrum). I'm mostly searching for roms made in my country but whatever I can find and hasn't been dumped, it's a plus for everyone. After I'm done I'll be sending them upstream.

[fruit-bat]

Yes, I will also see if I can source some BAT54S diodes. You probably know this already, but the pins on transformers can take some cleaning; don't know why but they always seem to have gunk on them.

[badrianiulian]

I've seen tape glue on resistor pins but on transformers it would be a first.
So I checked the pins on the yellow transformer and the ones on the red and they had both clean conducting pins.
Then I figured to check with the ohmmeter the pin resistance: red transformer has 134:139 while the yellow transformer has 74:90(44+47).
While using the yellow transformer, I used the 74 part for audio signal jack and the 90 part for the op-amp input.
Maybe i should give it another try with the red transformer while using the 134 for audio jack and 139 for op-amp input. The red transformer doesn't have any markings on it so the first time around maybe it was wrongly connected. Also, I do recall that when using transformers, winding direction tends to be important as well.
But for now, off to sleep I go.