09302: mbeepp,mbee256,mbee128p: continual tone at start

ID	Category [?]	Severity [?]	Reproducibility	Date Submitted	Last Update
09302	Sound	Minor	Always	Nov 14, 2025, 03:55	Nov 20, 2025, 05:52

Tester	Robbbert	View Status	Public	Platform	MAME (Self-compiled)
Assigned To		Resolution	Open	OS	Windows (x64)
Status [?]	Acknowledged			Driver	ausnz/mbee.cpp
Version	0.282	Fixed in Version		Build	x64
		Fixed in Git Commit		Github Pull Request #

Summary	09302: mbeepp, mbee256, mbee128p: continual tone at start
Description	When started, there's a continual tone that never goes away. After investigation, I found it was caused by a change to the SN76489 device. This device used to initialise to 0 volume, but that was changed, causing this issue. The device is optional in a real Microbee, and the bios does not write to it at all. It's up to user programs if they want to make use of it.
Steps To Reproduce	>mbee mbeepp Ignore the red screen, that's only for a possible bug in the ram banking. The unwanted tone will start up straight away and never stops.
Additional Information	The suspected commit that caused this issue is aa5150448a063fe77c500677c81daf282f797a5d. There was another adjustment at d986538dcfd1410c994bc8bea7d4d6351de1c7b3. If a compromise cannot be reached, it would be better if the device was completely removed from the microbee driver, as there are only one or two pieces of software that use it. In the meantime, in my build, I've set the volume in the machine config to zero until a solution is reached.
Github Commit

Flags
Regression Version
Affected Sets / Systems	mbeepp,mbee256,mbee128p

Attached Files