ADSR and Yamaha Cost Saving (Patreon)

This has been the FM week. I have fixed some NeoGeo sound issues, which have been released on MiSTer today and continued work on the YM3526 OPL sound chip. Let me talk about FM sound a bit.

NeoGeo, Megadrive and Ghosts'n Goblins have something in common: FM sound chips built around the OPN Yamaha operator. These FM chips were capable of producing realistic sounds. A key aspect of that feeling was the envelope of each sound.

The envelope is how the volume of a note changes over time. Each instrument has a different profile, which Yamaha originally defined with five parameters:

* Attack rate: how fast the sound went up, exponential in dB
* Decay rate: how fast the sound faded away once reached the maximum, linear in dB
* Sustain level: some instruments will change to a different decay at this level
* Decay 2 rate: how fast the sound faded after the sustain level had been met, linear in dB
* Release rate: how fast it faded once the "piano key" had been lifted, linear in dB

You can imagine, the more the parameters, the larger the memory needed to store them and the more complex the circuit used to process them. YM2203 and successors (OPN) used 5 bits for each attack and decay rates, then 4 bits for sustain level and release rate. Making it a total of 23 bits per FM operator.

Although OPN was very successful, after some years of use it was clear that things could be simplified. Most instruments actually fall in one of two categories: 

* Percussive: sound cannot be sustained, like an acoustic guitar
* Non percussive: sound can be sustained, like a flute

So the need for a different decay rate beyond sustain level was not all that clear. More bits, more circuits, more money... and for little value. So Yamaha removed it for YM3526 (OPL operator). Instead of that, it created a new option: one bit to choose between percussive and non percussive. The difference being that non percussive sounds will stay at sustain level until the "piano key" was released. See attached figure.

Another change was to reduce the resolution of attack and decay rates. Instead of using 5 bits to program each rate, it was reduced to 4 bits. With these 4 bits rates could be selected to last anywhere from 39s to just 0.002s. As you see, a very wide range.

These were small cut downs in the flexibility of the sound generator that seemed small but that formed the basis for the consumer market. The YM3526 is the origin of the OPL series of Yamaha devices that were used on MSX and PC computers for many years. During the same time the more complex OPN and OPM (YM2151) dominated the professional market (including arcade games).

SEGA Megadrive had still a different approach to cost savings: it kept the OPN operator but integrated the DAC (digital to analog converter) chip into the FM chip, effectively cutting the resolution from 16 to just 9 bits. What does that mean? It means that the difference from a loud sound to a quiet one could be 40dB larger on a NeoGeo -with its full DAC- than on a Megadrive.

And why the interest in YM3526 and OPL? Because of Bubble Bobble music, which you should hear soon...