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davematt
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Joined: 27/09/2011
Location: Australia
Posts: 55

Posted: 12:32am 29 Sep 2020

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Morning all. I am trying to do a wave generator to test a new CRO, but the DACS on my CMM2 have me puzzled. I can brute force the DAC to produce a lovely smooth ramp wave (or whatever), but if I use Play Tone or Play Sound the output is a dreadful 5 step mess.
Can find no other way to adjust the DACs. Am I missing something?
Am using a newly flashed 5.0505. Cheers, Dave

TassyJim

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Joined: 07/08/2011
Location: Australia
Posts: 6283

Posted: 02:11am 29 Sep 2020

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Example code showing what you are trying would help.

VK7JH
MMedit

davematt
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Joined: 27/09/2011
Location: Australia
Posts: 55

Posted: 05:20am 29 Sep 2020

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Straight from the manual, P78, Play Tone, 4000, 4000 should produce a nice smooth sinewave.
But for me, I get more like a 4 bit wave, ie a curved staircase. Could someone please verify that Play Tone or Play Sound produces smooth 12 bit waves?
TIA, Dave

TassyJim

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Posted: 07:20am 29 Sep 2020

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Thanks.
I agree.
The output seems to be set at 25uS intervals, independent to the frequency.

It's a while since I have played with the audio so I am not sure when this problem appeared.

Jim

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matherp
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Joined: 11/12/2012
Location: United Kingdom
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Posted: 07:28am 29 Sep 2020

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This has always been the case. That is how tone generation works. Same as on all MMBasic versions. Can't find the reference but the technique uses a constant frequency interrupt and then picks the appropriate place in a sine table for the next sample. The interrupt frequency on the CMM2 is 44100 Hz. Because other MMBasic versions use an RC filter on a PWM output the sine wave will look smoother but the technique is the same
Edited 2020-09-29 17:30 by matherp

TassyJim

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Posted: 07:59am 29 Sep 2020

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Thanks for the info Peter, I hadn't noticed it before and the audio is OK, just looks rough on the CRO.

Jim

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davematt
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Joined: 27/09/2011
Location: Australia
Posts: 55

Posted: 11:05am 29 Sep 2020

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Many thanks Jim and Peter. If I use an array to drive the DAC as per the manual, should I be able to see a smoother wave? Can do so with a for-next loop, but haven�t yet got the array process to work ...

William Leue
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Joined: 03/07/2020
Location: United States
Posts: 405

Posted: 03:24pm 02 Oct 2020

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Just put a small capacitor between the output and ground to filter out the high-frequency components of the waveform to smooth it out. You will have to experiment to find the right value. I would start with something like 0.05 microfarad.

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