Changing audio GPIO pins?

I have tried searching and was probably using the wrong magic words.
Does RISC OS support changing the pins that the PWM output for audio comes from?
My Pi Zero is a good fit for the OS but occasionally I think it would be nice to have sound. Building the filter is trivial but it’s pointless if there is nothing to output to it.

My plan here is to buy an Orange Pi lite or something to free up my Zero from media server / NAS duty so the Zero can be used for RISC OS any time. It seems to be happier than the Pi3 with it at this point.
Im not saying I will abandon it on the 3, but the crashes can get frustrating when I’m trying to do something.

Does RISC OS support changing the pins that the PWM output for audio comes from?

I think the first question is – what does the PWM audio? RISC OS/Linux or thé firmware? If thé latter, is it possible on the Zero?

Does RISC OS support changing the pins that the PWM output for audio comes from?

AIUI RISC OS doesn’t alter any of the GPIO settings, so the default will be set at whatever the firmware leaves it as (I’m not sure what that would be for the Zero). Presumably you could fairly easily put together some code to reconfigure the pins using e.g. the GPIO module and then put it in your boot sequence.

I think the first question is – what does the PWM audio? RISC OS/Linux or thé firmware? If thé latter, is it possible on the Zero?

The firmware handles PWM audio. It is supported on the Zero, you just need to build your own output filter and (I think) fiddle with the GPIO configuration.

The firmware handles PWM audio. It is supported on the Zero, you just need to build your own output filter and (I think) fiddle with the GPIO configuration.

Great! I seem to recall I saw something that says something can be added to config.txt to specify a DTO for changing the PWM audio pins of a Pi Zero without extra resources, however I haven’t tried it. Perhaps I’m just a dummy but everything I have read on DTOs has never said explicitly if / how they interact with an OS (probably Raspbian). I was hoping for a little enlightenment here.

When I have some time I’ll give it a go. I’ll just chuck a multimeter on one of the pins to see if something happens before getting out the soldering iron.

Well, now I’ve finally worked out how to talk to the IO controller I can change the pin modes if necessary.

It’s a little bit of an aside, and I’m not sure if it’s my imagination but I was trying out audio using different combinations of things on Raspbian and RISC OS on the Pi 3, but I swear in RISC OS it’s louder and has less hiss. Sadly they are both beaten by the cheap 7.1 channel USB dongle. We actually bought it a while back because it was recommended on some site for use with the mic. It worked really well for that. I never even bothered setting up the phantom power supply to drive the mic. The level was a touch low but not terrible. Amazing for a PC mic connector though.
I only tested it today in Raspbian. I was going to grab it’s info from Linux but didn’t get around to it. I want to find out if it’s one of the chipsets that’s been played with on RISC OS.

I ordered an Orange Pi PC to take over the role of NAS / UPnP server from the Zero. It will hopefully be here in a month or two :( I also ordered a neat little thingy for the Zero. It’s a 3 port USB hub with an Ethernet adaptor with a RISC OS supported chipset and a Micro USB OTG connector. It also has a 5v power connector. Talk about lots of birds with one stone.
This should be fun. I’ve got a preamp kit coming that I’d like to use with it too.

It’ll probably be a couple of days before I can spare time to check whether just changing config.txt works but I’ll let you all know when I do.

I believe we had to set up GPIO appropriately but I can confirm that when we connected up a Pi Zero’s PWM Audio pins to a circuit copied from the B+ schematic it worked fine under RISC OS!

One of great things about the Pi is that most hardware aspects are set up by start.elf and can then be controlled by config.txt or GPIO settings.

n.b. In this case config.txt isn’t used to control things, but for the record an authoritative list of what config.txt can do, is now on the official site at:https://www.raspberrypi.org/documentation/configuration/config-txt.md

Are these the same for all models of Pi?

It’s a little bit of an aside, and I’m not sure if it’s my imagination but I was trying out audio using different combinations of things on Raspbian and RISC OS on the Pi 3, but I swear in RISC OS it’s louder and has less hiss.

There are at least a couple of config.txt settings that can influence PWM audio quality (avoid_pwm_pll and disable_audio_dither), so there’s a good chance it’s one of those which is set differently.

Are these the same for all models of Pi?

Yes, the config.txt settings listed on that page should apply to all models of Pi.

Hi! I stole some time to do a little test. Honestly I should be sleeping but I really wanted to test it.

I snatched this line from AdaFruit to go in config.txt
dtoverlay=pwm-2chan,pin=18,func=2,pin2=13,func2=4

As it was pointed out to me, it won’t work out of the box. GPIO 13 had to be changed to ALT 0 and GPIO 18 to ALT 5. After changing my all-elbows GPIO program to do this, I fired up an MP3 in DigitalCD to test. It appears to work fine.
Please note I don’t have any speakers attached nor do I have the filter. I’m just using a 3 colour LED module that I use for testing things. Because it has the resistors built into the module it’s plug and play. What I saw was consistent with audio, and the song I had playing. Both channels appeared to be working because of the bicolour light show.

Jeffrey, I did a little reading and what you said makes sense about the audio options.

I’m looking forward to building a filter, and when the preamp arrives, building and connecting it too.

addendum:
There is a detail last night I noted however forgot to mention. After the audio test, the pins park in a logic high for some reason. It may be the same for the normal PWM audio setup, I don’t know. It just strikes me as odd is all.

Time’s a bit limited so I haven’t had a chance to make a filter board, but my preamp arrived and I built it. Even though it’s a cheap Chinese valve preamp and playing through my TDK noise cancelling earbuds (also inexpensive), the Pi 3 internal audio sounds really good.
I’ve got a Pi specific prototype board I’m going to chuck the filter on. I might see if slightly larger caps will help with the hiss at the cost of a slightly lower cutoff. I can live with slightly dulled high frequencies.

Yes, it’s going to be a monstrosity of a thing using a Pi Zero, PWM audio, a valve preamp and RISC OS. Not quite an Archimedes but still something fun.
The filter circuit will be based directly off the Pi blueprints. No sense messing with it. Seems to work fine.

Again, time issues. However I did manage to scavenge up the right caps and resistors to test the PWM output on a breadboard with a speaker from a broken earbud plugged in. High tech, I know. I couldn’t find a jack. As it is I’m pretty sure all the components I used were 30 years old minimum but they all check out.

I have a suspicion that with the correct incantation to config.txt the GPIO pins could be initialised correctly for output without the need to fiddle them after boot. What exactly is a bit more of a mystery. Info on config.txt is pretty scattered and incomplete with varying levels of age and obsolescence.

I thought somebody might appreciate this. It’s version 1 of the audio for my Pi Zero. There was however an issue with the 3.5mm plug sticking out a fraction of a mm too far and preventing it from seating correctly. It’s been rearranged so it’s connected in a rather fragile manner to a 4 pin connector. It’s only temporary. I’ve ordered some board mount 3.5mm stereo jacks. They’ll be here any month!

It works fine. Sounds good. I can confirm it can talk to the RTC and the bonus EEPROM in Raspbian, and the Ethernet is unconfirmed.
The RTC and EEPROM give me a basis to start working on i2c. I can also use the Zero to listen to tunes now too so it’s all good!
One thing I’m a little dissapointed about was I couldn’t find any suitable diodes to protect the PWM pins.

I can’t seem to work out inline pics. Sorry.
http://i.imgur.com/qWloQUWl.jpg

Info on config.txt is pretty scattered and incomplete with varying levels of age and obsolescence.

The link I posted https://www.raspberrypi.org/documentation/configuration/config-txt.md is on the official RPF site so should be authoritative. If you think anything is missing, confusing or wrong, I’m sure they would love feedback via the forums! Or it does say “View/Edit this page on GitHub”

Chris, It seems to be a bit more comprehensive now. The dtoverlay link needs some work though I think.
I was asking about the overlays in my GPIO thread. I’m a little thrown that the dtoverlay that I mentioned above seems to work. I guess some are lower level. I still needed to set the pin modes manually, but IIRC just doing that without the config.txt entry did nothing.