The Sinclair QL Forum

I need to get sound on my vDriveQL so was going to duplicate what i did for my vDriveZX, except I didn't want to use the hacky vero board and socket this time so I bit the bullet and created a PCB in KiCAD, now that I have the skills, and I've ordered 5 from PCBWay. Hopefully I'll have it installed before the Retro Computer Festival a week on Saturday.

I've added the KiCAD project and ATTiny85 code to Github: https://github.com/stephen-usher/vDrive-Sound-Board

Nice, I have a few ATtiny85 in a drawer doing nothing

Perhaps add another on a 7 second delay to give an authentic 'splice crunch through tape head'

I had a play with this some time back and reduced the space used by the sketch so that even more sound data could be used. Dunno if anyone is interested? If so, it's on the forum at viewtopic.php?f=2&t=3708&p=40401#p40401 -- I'd be interested if it works, given that my only 8 ohm speaker was dead and a 4 ohm one seemed awfully quiet. The voltage on the output isn't, as far as I'm aware, ok for the line in inputs on an amp?

Cheers,
Norm.

And I need educating here please, I have two questions:

1. Given that the Attiny85 is outputting 5V and the speaker is 8 ohms, does this mean that the ATtiny85 is pumping out 5V / 8 ohm ==> 625 mA of current? That's a lot of mA for a wee ATtiny. The Absolute maximum on a pin is 40 mA. (DC) and typical should be around 60 mA per entire port (and the ATtiny only has PORTB). Yes, I'm aware that this PWM stuff is not a constant DC current, but even so.

2. Shouldn't the speaker be DC Blocked with a capacitor (or two?) between PB1->GND and PB4->GND? I realise that dramatically reduces the voltage spikes across the speaker, which are quite large according to my scope (Labrador - https://espotek.com/labrador/product/es ... dor-board/) without the caps, and might seriously reduce the speaker volume.

I tried to find the original article on http://www.technoblogy.com from 23rd October 2017 as per the source code, but I don't see anything with PB1 and PB4 together driving a speaker, only PB1 and GND connections, and with a DC blocking capacitor installed.

I'm not a hardware guy, nor do I play one on TV, be gentle!

Thanks.


Cheers,
Norm.

I looked up original, from 2014 : http://www.technoblogy.com/show?QBB

I agree, I would use higher impedance speaker and/or resistor in series with electrolytic capacitor.

If I do another iteration I'll add a cap. But it's easy enough to fit one on the wire to the speaker in this case. I must admit that the original prototype worked fine and the ATTiny85 seemed none the worse for wear. Maybe it's because the currents are so transient and it's not running for very long that it's coping. With a constant sign wave the effective current would be the root mean square of the peak. Actually I'm not sure the QL (ZX Interface 1) would be able to throw ~600mA down the bus.

Of course, you have to remember that the voltage difference is actually +-2.5V between the two outputs as it's the differential between the two ports which is driving the speaker.

Admittedly I've never put an oscilloscope on that circuit to see what the voltages are. Maybe I'll try next week when the boards arrive.

From the link posted by Sylvester, I see this:

To avoid the need for a coupling capacitor between the ATtiny85 outputs and the loudspeaker, the loudspeaker is directly coupled to two outputs, which are fed complementary versions of the audio to avoid any DC being fed to the speaker. Note that if you want to feed the output to an audio amplifier you should include a low-pass filter; otherwise the 250kHz carrier could damage the amplifier.

I'll take that.

But, the voltages did seem a tad high to me. Previous disclaimer applies!

Cheers,
Norm.

Yeah, but I think the chip keeps going because it runs for a short period of time and hence never gets a chance to overheat. It is literally only powered when the "drive is running", and the current isn't flowing constantly. A 0.1uF ceramic capacitor between GND and the speaker wires should act as a rough and ready low pass filter I imagine.

The boards arrived today, at last. They spent four days bouncing between "Processed" and "On hold" at DHL's Shenzen facility.

I managed to make one up this evening and it works: https://youtu.be/A9s6XKbfVUY

Unfortunately the Dupont connectors are too tall to fit into the case so I'm going to have to solder the wires onto the board and the double sided tape I was attempting to use to glue the speaker to the inside of the case is so weak it can't even hold the speaker flat when the whole lot is laying down!