4 ms·

Cool. Kind of unrelated, but is there anything stopping someone from just generating a sawtooth DPCM sample at runtime and using the channel as normal without h

Cool. Kind of unrelated, but is there anything stopping someone from just generating a sawtooth DPCM sample at runtime and using the channel as normal without having to use crazy timing stuff?

zeta0134 4y ago
Yes, it comes down to how the DPCM channel encodes sound data. A sawtooth gets most it's energy and harmonics from the sudden jump at one end. This impulse is impossible to encode in 1-bit delta encoding, the closest you can get is a slightly steeper slope. The audible effect is similar to a low pass, with the strength dependent on the intended amplitude, so instead of a nice sharp saw, you get a muddy not quite triangle. A few commercial games did use the DPCM channel melodically, but the main drawback is the large size of the samples relative to the commonly available ROM chips of the era. Price is a big factor when you need to put one chip in every game. Here's how that sounded in practice: https://www.youtube.com/watch?v=LEgoYUzwabI https://www.youtube.com/watch?v=LEgoYUzwabI
- superdisk 4y ago
  Ah sure, but how about just generating a triangle using a small block of code in RAM? Then you're not wasting any storage.
  - zeta0134 4y ago
    Further problems! The DPCM channel can only address memory from the region 0xC000 - 0xFFFF, due to how its "address" byte is interpreted. Most cartridges cannot place RAM in this memory region, so you're stuck baking the samples into ROM space. Many cartridges cannot even bank switch this region, so the samples additionally cut into your "fixed" ROM that is usually precious space for interrupt service routines and common framework code.
    - superdisk 4y ago
      Ah very interesting, thanks for the explanation! I'll definitely check out your lib if I ever get around to writing a homebrew NES game, I love quirky audio tricks :)

4 ms·

Cool. Kind of unrelated, but is there anything stopping someone from just generating a sawtooth DPCM sample at runtime and using the channel as normal without h

by superdisk 4y ago

Cool. Kind of unrelated, but is there anything stopping someone from just generating a sawtooth DPCM sample at runtime and using the channel as normal without having to use crazy timing stuff?

zeta0134 4y ago
Yes, it comes down to how the DPCM channel encodes sound data. A sawtooth gets most it's energy and harmonics from the sudden jump at one end. This impulse is impossible to encode in 1-bit delta encoding, the closest you can get is a slightly steeper slope. The audible effect is similar to a low pass, with the strength dependent on the intended amplitude, so instead of a nice sharp saw, you get a muddy not quite triangle. A few commercial games did use the DPCM channel melodically, but the main drawback is the large size of the samples relative to the commonly available ROM chips of the era. Price is a big factor when you need to put one chip in every game. Here's how that sounded in practice: https://www.youtube.com/watch?v=LEgoYUzwabI https://www.youtube.com/watch?v=LEgoYUzwabI
- superdisk 4y ago
  Ah sure, but how about just generating a triangle using a small block of code in RAM? Then you're not wasting any storage.
  - zeta0134 4y ago
    Further problems! The DPCM channel can only address memory from the region 0xC000 - 0xFFFF, due to how its "address" byte is interpreted. Most cartridges cannot place RAM in this memory region, so you're stuck baking the samples into ROM space. Many cartridges cannot even bank switch this region, so the samples additionally cut into your "fixed" ROM that is usually precious space for interrupt service routines and common framework code.
    - superdisk 4y ago
      Ah very interesting, thanks for the explanation! I'll definitely check out your lib if I ever get around to writing a homebrew NES game, I love quirky audio tricks :)