High frequency sound in lower quality DPCM samples

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DRW
Posts: 66
Joined: Mon Oct 19, 2015 8:24 pm

High frequency sound in lower quality DPCM samples

Postby DRW » Wed Jun 15, 2016 11:02 pm

When I import a wave file as a DPCM sample and use a quality lower than the maximum, I always get a file that has a high frequency noisy sound in the background.

Originally, I thought that this might have to do with whatever data is in the wave file itself. But that's not the case.

Because even if I record one second of silence (i.e. I record a file without a microphone plugged in and without Windows making any sound) and I import it into FamiTracker, I still hear that noise, unless I use quality 15. The lower the quality, the louder the noise.

You can see my results in the attachments.

The example shows a silent file, but the same issue of course happens in a file where you hear something: A voice sample, for example, is accompanied by a high frequency noise.

My question: Is there anything that I can do to the converted DMC file to remove that noise?

I want to use a voice sample in a game, so I need to keep the quality low. And since the noise doesn't come from any actual sound that the wave file makes, tampering with the wave file probably won't help. So, I need to find a way to edit the DMC file to remove the background noise.
Attachments
Silence.zip
(42.43 KiB) Downloaded 44 times
Quality 15.ftm
(4.7 KiB) Downloaded 47 times
Quality 8.ftm
(1.87 KiB) Downloaded 44 times
Quality 0.ftm
(1.23 KiB) Downloaded 46 times

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HertzDevil
Posts: 475
Joined: Thu Apr 23, 2015 7:39 pm
Location: Hong Kong SAR
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Re: High frequency sound in lower quality DPCM samples

Postby HertzDevil » Wed Jun 15, 2016 11:25 pm

This is why they are called delta PCM: at each sample, the wave must go either up or down, never retaining the same value as the previous sample. Therefore, it must make some sound unless the DPCM sample moves towards zero bias or maximum bias (0x7F). This square wave is the least amount of noise you could get in any DPCM sample that is not tilted.
refactoring 0cc-famitracker

DRW
Posts: 66
Joined: Mon Oct 19, 2015 8:24 pm

Re: High frequency sound in lower quality DPCM samples

Postby DRW » Wed Jun 15, 2016 11:27 pm

So, is there anything that can be done here? Any trick or workaround to still remain in the sound format's rules, but to effectively remove or at least reduce the noise?

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za909
Posts: 79
Joined: Tue May 12, 2015 12:10 pm
Location: Hungary

Re: High frequency sound in lower quality DPCM samples

Postby za909 » Thu Jun 16, 2016 11:02 am

Ths sound is there all the time, it's just too high-freq to hear when you use a higher playback speed. There's nothing you can do other than perhaps giving it more gain which will distort the sound into a more triangle-shaped sequence, or add very quiet noise from the noise channel to mask it up at least a little bit. But both of these might not be enough.

DRW
Posts: 66
Joined: Mon Oct 19, 2015 8:24 pm

Re: High frequency sound in lower quality DPCM samples

Postby DRW » Thu Jun 16, 2016 12:13 pm

O.k., looks like I have to live with it.


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