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#1 2023-01-06 17:44:19

PixelDeco
New member
Registered: 2023-01-06
Posts: 4

GB Sound Wave Table Registers, Question

So,

As I understand it, there are 16 registers for the different wave forms of the wave sound channel, and each register is divided into 4-bit nibbles making a total of 32 sound wave variations for the 'wave table'.

My question is, how do these 4 bits represent wave patterns and how are there 32 variations instead 256?  How does the system interpret them?
I've tried figuring out the math, but as far as I can tell you'd need more than 16 8-bit registers for all the sound waves since it seems like you'd need 8 bits for each 4-beat wave (the minimum points for a wave pattern).  Even if each wave has only 4 points, it seems like each point would need 2 bits to show where they appear on the graph; 00=off, 01=low, 10=mid, 11=high.
So what is the algorithm here?

I clearly don't how this is working, but given what I've described above, here's how I thought it would be coded:

Waveform (1 byte):
-point 1: 2 bits
-point 2: 2 bits
-point 3: 2 bits
-point 4: 2 bits

4 possible places for 4 wavepoints = 4^4 = 256 waveforms (which include all wave variations that include 'turned off' notes)


Thank you

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#2 2023-01-06 17:49:03

PixelDeco
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Registered: 2023-01-06
Posts: 4

Re: GB Sound Wave Table Registers, Question

Obviously you could cut out duplicates, but I didn't check if this was exactly half of the variations.
Maybe it is?
That would solve the problem because then you'd only have 2^4 variations = 16.

Seems like something is off here though. 
A flip register?

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#3 2023-01-06 19:36:44

nitro2k01
Administrator
Registered: 2008-02-22
Posts: 237

Re: GB Sound Wave Table Registers, Question

It's so simple that you're probably going to say "d'oh!" when you hear it. It's simply a single looping waveform with 32 samples. Each byte contains two 4-bit samples, one in each nibble. So for example a sine wave might look something like this:

Code:

    __---__
  _-       -_
 _           -
-             -_               _
                _             _
                 -           -
                  -_       _-
                    --___--
9ACDEEFFFEEDCA986532110001123568

In this case, [$FF30]=$9A, [$FF31]=$9CD, [$FF32]=$EE and so on.
If you want to play a new waveform you need to stop the channel (to be able to safely access the wave buffer) and load the new waveform.


Blog: Gameboy Genius
"A journey of a thousand miles begins with one small step"
Old Chinese Proverb

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#4 2023-01-07 05:49:30

PixelDeco
New member
Registered: 2023-01-06
Posts: 4

Re: GB Sound Wave Table Registers, Question

nitro2k01 wrote:

It's so simple that you're probably going to say "d'oh!" when you hear it. It's simply a single looping waveform with 32 samples. Each byte contains two 4-bit samples, one in each nibble. So for example a sine wave might look something like this:

Code:

    __---__
  _-       -_
 _           -
-             -_               _
                _             _
                 -           -
                  -_       _-
                    --___--
9ACDEEFFFEEDCA986532110001123568

In this case, [$FF30]=$9A, [$FF31]=$9CD, [$FF32]=$EE and so on.
If you want to play a new waveform you need to stop the channel (to be able to safely access the wave buffer) and load the new waveform.

D'OH!

Haha, seriously - THANK YOU!
That is so awesomely simple I can't believe I didn't see it.  I was trying to make charts figuring out 16 variations of tiny waveforms.  It never occurred to me it would be a big 32-part wave that you could take parts out of and duplicate to form new 32-part waveforms.  I'm an amateur, so I was expecting the waveform to be 16 bits at most due to the addressing.  I guess my next obvious question then is, why is it 16 bytes (although I definitely get why you might break those up into 32 parts)?  Also, does it take longer to listen to a full sine wave than one that uses fewer registers?

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#5 2023-01-07 10:18:53

nitro2k01
Administrator
Registered: 2008-02-22
Posts: 237

Re: GB Sound Wave Table Registers, Question

It's 16 bytes because that fits 32 4-bit samples. Why 32 samples? Ask Nintendo. I'm guessing it's a sweet spot, not too short, not too long.

The wave channel always loops all 32 samples. There's no way to make it use fewer of the registers/samples.


Blog: Gameboy Genius
"A journey of a thousand miles begins with one small step"
Old Chinese Proverb

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#6 2023-01-07 18:50:14

PixelDeco
New member
Registered: 2023-01-06
Posts: 4

Re: GB Sound Wave Table Registers, Question

nitro2k01 wrote:

It's 16 bytes because that fits 32 4-bit samples. Why 32 samples? Ask Nintendo. I'm guessing it's a sweet spot, not too short, not too long.

The wave channel always loops all 32 samples. There's no way to make it use fewer of the registers/samples.

I thought that might be the case.
Thank you again for answering.

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