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Forum Index : Microcontroller and PC projects : CMM2 Audio Trouble
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j_dissuade Newbie  Joined: 24/10/2020 Location: United StatesPosts: 2 |
Hey all, the CMM2 is my first foray into the world of DIY computing. I was pretty impressed by the apparent immediacy of the device and wanted to pick it up as a hobby that could be somewhat isolated from my typical PC usage. I bought the retromax version of the CMM2 from circuitgizmos as soon as I heard about it and the unit finally arrived last week. I was going through the manual and most every function I tried checked out, but I was noticing alot of weird digital noise and distortion from the computer's audio output when I listened to mp3s and wavs through my headphones. Additionally, there was an audible tick coming from the unit's main screen, a loud hum when editing a program, and some weird noise that seems to vary with code execution. I messaged circuitgizmos to ask if they had any tips for troubleshooting, and they suggested I experiment with the power supply since these can be underpowered and compromise performance. I tried every power supply I had access to along with a few other usb-A to C cables, but no luck. I set up a little troubleshooting desk to see if I could find anything out about how to improve my experience with the device, but still nothing conclusive. Here are two videos with some additional details and sound examples. Please take a look at them if you think you might be able to help me. I also have a multimeter and am handy enough with solder if you can come up with a reason I might have to break either of those out. Part 1 Part 2 It may be the case that many of my disappointments with the unit are just part of the package when it comes to this sort of device, but there's no way for me to know that without someone explaining it to me. So if I sound like I'm whining, please excuse me. Summary of issues: How can I listen to audio through my headphones without distortion? Can I get rid of the tick? Can I edit a program without a hum? Can I improve the keyboard buffer to be more accommodating to my typing speed? From my point of view, if I can't address all of these issues, I am better off writing code on my normal computer with mmedit (free of noise, with a responsive keyboard) and then transferring it over to the CMM2. Maybe you can see why this is disappointing to me, since I originally wanted to get away from my other computer to as great an extent as possible. Thank you for taking the time to read my post. Edited 2020-11-03 16:36 by j_dissuade |
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| johnd Newbie Joined: 22/10/2020 Location: United StatesPosts: 30 |
Just got my RetroMax from circuitgizmosx. I love the unit so far, except for the audio. I'm getting the same buzzing and clicks. I watched your video, and I don't think the unit is capable of driving standard headphones -- I believe it does say that in the documentation somewhere. That is why you get better sound through amplified speakers, or maybe earbuds. I also think, because the CMM2 is kind of a niche/diy type of product, that not much design work has gone into isolating the audio output from the rest of the system. I'm thinking the leads come right out of the chip or DACs and go directly to the headset plug -- that may be a slight exaggeration, but I'm guessing it's close, and it's understandable given what the CMM2 is supposed to be. I'm not an audio engineer, so I don't have a clue on what could be done to remove the noises from the output (caps, filters, etc), but if anyone has any suggestions, I'm not against modding my unit to improve the output, and maybe it could lead to better devices in the future. Again, I'm having a blast with the device so far, but a little disappointed by the sound output. |
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| robert.rozee Guru  Joined: 31/12/2012 Location: New ZealandPosts: 2289 |
hi, the CMM2 can NOT driver a speaker or headphones directly, there needs to be an amplifier in between. also, the audio output of the CMM2 does NOT have any isolating capacitors in series with the with the processor pins that (directly) drive the audio socket, leading to distortion if the amplifier that you are feeding the audio into does not have isolating capacitors on its input. i believe that the isolating capacitors were omitted to allow using the audio output to generate DC voltages in non-audio applications. connecting speakers or headphones directly to the audio output may damage the processor pins that drive this output. cheers, rob :-) |
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| j_dissuade Newbie Joined: 24/10/2020 Location: United StatesPosts: 2 |
This is helpful. I was puzzled because my cmm2 was packaged with headphones so i expected them to work. How would I be able to guage possible damage I've done to to audio processor pins? Would you happen to know if my other issues are related to this sort of damage? @johnd I'm glad another retromax user can confirm I'm not alone. Thanks! Yes even with earbuds I get alot of clicking. Could I just ask you if you also have a different sort of humming whenn you're editing a basic program or browsing files? I suppose I will just invest in some decent computer speakers. This seems like it will alleviate most of my concerns. But I'm still wondering if the typing problem I'm having can be addressed somehow. Edited 2020-11-03 18:09 by j_dissuade |
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 5905 |
Should find a setting that suits your typing style. Re the audio, The CMM2 is line level not suitable for speakers as already pointed out. Audio transformers should do the required level shifting and have the advantage of isolation but headphone amplifiers might be better. The processor pins are reasonable robust so it is unlikely you have damaged them. A well filtered power supply helps with audio quality and most problems have ended up being in the USB cable being used. Jim VK7JH MMedit MMBasic Help |
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| johnd Newbie Joined: 22/10/2020 Location: United StatesPosts: 30 |
@j_dissuade: yes, I get buzzing when I run something like files, and then run a program (assume edit would be the same). Besides the buzzing, I get a high pitched tone that lasts about a second, then goes away, then comes back sometimes. When I run the welcome.bas from the welcome tape, I notice a different buzzing when the opening/splash screen is being drawn -- just when the letters are being drawn, but stops in between the letter drawing. I'm going to try and place an inline isolating cap to plug in between the CMM2 and my small desk speaker, since my desk speaker is cheap and may not expect to have DC coming in. But for the moment, the buzzing really is quiet compared to the normal sound effects volume, so I'm not to worried about it. Just now, I'm playing the HeartAttack by Tweaker Ray and the music sounds fine (song is awesome btw). |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 3532 |
I have not seen schematics of the retromax, but typically the audio DAC's are powered from the analog supply pins VDDA and VSSA pins. These are separate pins on the chip. If these pins are tied to the VDD and VSS (the digital voltages) then digital noise is audible in the audio output. With the classical 8bit sound this may not matter much, but if you want a good signal to noise ratio of 60dB or preferable 80dB-100dB the VDDA and VSSA must be quiet (have their own filtering or linear regulator). Volhout PicomiteVGA PETSCII ROBOTS |
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CircuitGizmos Guru Joined: 08/09/2011 Location: United StatesPosts: 1421 |
The ear buds packed with the RetroMax were supposed to be high impedance when I purchased them in bulk. I'm disappointed in my supplier as I suspect that they are not. I will likely stop supplying them. There is no capacitor inline with the audio connections. This was so that the audio could be used as PWM analog. There is a 3-pin header just behind the audio jack so that the two "audio" lines can be tapped. Making the audio lines better for audio would take away the feature of allowing the lines to be used for PWM. There is an isolation for the analog supply pins. A newer PCB layout has these lines routed away from digital lines as much as is reasonable to reduce any inductive pickup. The lines are - again as much as possible for a microcontroller with 176 pins - adjacent to ground fill. I'm keeping an eye on this and will adjust the PCB design if reasonable. Micromites and Maximites! - Beginning Maximite |
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| johnd Newbie Joined: 22/10/2020 Location: United StatesPosts: 30 |
@curcuitgizmos: I'm ultimately happy with the board and would have no problem purchasing one again. Also, it would be disappointing if we could not use the line-in as input because of an inline capacitor stripping out the DC. For music it's nice to have, but those DC filters can be cheaply purchased or made and many playback devices already have these filters in place. I agree the headphones are a little cheesy, but I didn't even expect them honestly, so hey a free gift. :) The buzzing present when running a program is only really annoying if the volume is turned up loud, and gets drowned out if music or effects are playing anyways. However, it would be nice to remove if possible to have a truly clean sound. Cheers, and now back to some happy coding. |
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| CircuitGizmos Guru Joined: 08/09/2011 Location: United StatesPosts: 1421 |
I found an alternate set of amplified speakers that I can turn up loud enough to hear a buzz when running a graphics-intensive program. The sound is in the middle audio range so bandpass filtering wouldn't help. I remember a lot of noise on '80s machines, so this is a retro "feature", right? :D I'll be trying out the alternate PCB layout in a few weeks. Micromites and Maximites! - Beginning Maximite |
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| William Leue Guru Joined: 03/07/2020 Location: United StatesPosts: 382 |
I use a cheap set of amplified speakers and they work reasonably well. -Bill |
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