GSC Sound Engine Part 2 - The Pulse Channels

The Pulse channels are the most reliable and predictable of all the channels, and they make what is the "classic" chiptune sound, because their basic concept was used in a lot of old PSGs. There are two Pulse channels, 1 and 2, and they are nearly identical to each other (save for a single difference which I'll explain later).

Each Pulse channel can produce what are called "pulses", which are probably the simplest way possible to make sound. You just increase the air pressure for a given amount of time ("on" state), then make it 0 for another given amount of time ("off" state). Rinse and repeat, and you have sound! The ratio of the amount of time the impulse stays "on" compared to "off" is called the dutycycle. Unfortunately, the dutycycle of the GameBoy Pulse channels isn't programmable: there are only 4 to choose from: 12.5%, 25%, 50% and 75% (1/8th, 2/8th, 4/8th, and 6/8th, respectively). Think of them as different instruments the "band member" can play. Here they are in order, playing Totaka's Song:

There's a parallel to how they sound vs. their shape on the oscilloscope. The 12.5% is a very thin, piercing sound, and rather aggressive on the ears. The 25% is a bit thicker and more "neutral" sound. The 50% is a rather dull sound.

And the 75%? Well, this is one of my biggest pet peeves with the GameBoy: it sounds identical to the 25%! This is due to how it something sounds identical if the "reverse" wave is played. The NES works the exact same way, so I can only assume they made it like this because the GameBoy's PSG was based on the NES'. Why they did it like this in the first place on the NES I have no idea, but as an arranger I'd kill to have another "instrument" at my disposable. The obvious contender is the 37.5% (or 3/8th), which sounds something in between the 25% and the 50%.

You can hear how the fabled 37.5% sounds here (played on the Wave channel, more on that later).

Sadly, this isn't to be, so let's move on.

You can change the envelope or volume of the notes (15 steps), their fade-in (7 steps) and their fade-out (7 steps). The only difference between the Pulse 1 and Pulse 2 channels is that the Pulse 1 can do an hardware sweep, which is basically a quick pitch change from one note to the next. Otherwise they are identical.

In terms of usage on actual songs, they can be used for anything: melody, bass and accompaniment, you name it, the Pulse channels can do it. They are easily the most versatile of all the channels. One fun fact is that the GSC sound engine always uses the Pulse 2 for the melody (when it is on the Pulse channels, that is) because Pulse 1 is used for sound effects. So next time you're banging your head against the wall in a Gen 2 game, you might notice the music cuts off for a bit to "make way" for the sound effect: that's the Pulse 1 channel being utilized!

With the Pulse channels out of the way, let's move to the spicy member of the channel family: the Wave channel.