09310: ibmpcjr: Loud buzz from the SN76496 until something plays sound on it

ID	Category [?]	Severity [?]	Reproducibility	Date Submitted	Last Update
09310	Sound	Major	Always	13 hours ago	9 hours ago

Tester	cuavas	View Status	Public	Platform	MAME (Official Binary)
Assigned To	AJR	Resolution	Fixed	OS	Windows (Arm)
Status [?]	Resolved			Driver	pc/ibmpcjr.cpp
Version	0.283	Fixed in Version	0.284GIT	Build	Arm64
		Fixed in Git Commit	a0c64b9	Github Pull Request #

Summary	09310: ibmpcjr: Loud buzz from the SN76496 until something plays sound on it
Description	When you start the PCjr, it produces a continuous buzz from the SN76496 until an application plays a sound using it. This does not happen on a real PCjr, the SN76496 is silent from power-on reset. The change to the SN76496 reset state feels like a "tail wags the dog" situation, where a workaround for one system breaks numerous other emulated systems.
Steps To Reproduce
Additional Information
Github Commit

Flags
Regression Version
Affected Sets / Systems	ibmpcjr

Attached Files

No.23851 hap Developer 13 hours ago edited on: 12 hours ago	Is it a different chip, or an official SN chip? The BTANB note I wrote in the sn device source is from multiple references on youtube, from arcade games doing a cold boot. And the BBC Master what Nigel did this change for in the 1st place. edit Or maybe it only applies to SN7648x and not SN7649x? (all BTANB references have SN7648x)
No.23852 cuavas Administrator 12 hours ago	IBM PCjr used a Texas Instruments SN76496N (16-pin PDIP package, manufactured in Malaysia). It wasn’t a second-sourced chip. You can see it near the audio output section in the corner of the motherboard, sort of near the MC6845P.
No.23853 cuavas Administrator 12 hours ago	There's a decent photo on Wikimedia Commons: https://commons.wikimedia.org/wiki/File:IBM_PCB_Jr_Motherboard.jpg The sound section is at the top left corner with the board in that orientation. The SN76496N is almost on the left edge of the board.
No.23854 Robbbert Moderator 11 hours ago	Same problem as MT 09302, which I logged a week ago and was completely ignored.
No.23855 AJR Developer 10 hours ago edited on: 9 hours ago	Page 2-89 of the PCjr Technical Reference shows an analog multiplexer selecting one of four sources (8253, cassette, "external channel", 76496) based on PB5 & PB6 from the 8255. This is not emulated in the driver, which allows the three sources actually emulated to be active at the same time. The MC14529B Dual 4-Channel Analog Data Selector can be seen on the Wikipedia PCB photo to the left of the white connector. Sheet 10 of the schematics in the same reference shows the sources: X0 = TIMER AUDIO, X1 = CASS AUDIO TO MUX, X2 = AUDIO INPUT, X3 = SN76496N AUDIO. (Y0-Y3 are tied to GND and unused.)