PSG Sound Update
Added 2019-11-06 18:34:38 +0000 UTCThe old AY-3-8910 3-channel programmable sound generator (PSG), used in the Atari ST and Amstrad CPC computers along many arcades had a second life inside the FM chips YM2203 and YM2610. You may recognize them as the sound synthesizers used in NeoGeo and many arcades from the 80's.
This PSG chip seemed to have an output stage less than ideal. A good amplifier produces an output voltage rather independent from its load. This doesn't seem to be the case for this chip. It seems that the higher the load resistor connected to it, the less the dynamic range available at the output. I have found some measurements in the internet and I have setup a small board to make my own. This chip has a number of variants and it will take a while until I get enough data about it.
MAME seems to do a great job with this. It simulates a small electronic circuit in many games using this chip in order to reproduce this effects. How faithful it actually is is hard to tell, though. But it does sound quite different from what you'd expect from just looking at the data sheet.
For the time being, I am going to emulate the load effect by reducing the dynamic range in my JT49 sound core. For 1942 and Vulgus, I have gone for an aggressive reduction to just 21dB (it used to be 40dB). For the JT03 sound core (YM2203, used in CPS0 games) I have chosen a more conservative value of 29dB. With this change some sounds that were a bit too faint before have come to light.
I cannot release an update today because I'm traveling this week. But I hope to be able to provide a general core update (as JT49 affects all my arcade cores except Bionic Commando) soon.
For developers, you can grab the latest JT49 from the repository and try the different compression outputs.
