05694: All sets in coco12.c and coco3.c: audio output level incorrect when Cassette/Cartridge selected on SC77526 chip

ID	Category [?]	Severity [?]	Reproducibility	Date Submitted	Last Update
05694	Sound	Minor	Always	Sep 8, 2014, 05:28	Nov 4, 2014, 17:06

Tester	Richard42	View Status	Public	Platform	MESS (Self-compiled)
Assigned To		Resolution	Fixed	OS	Linux
Status [?]	Resolved			Driver
Version	0.154	Fixed in Version	0.156	Build	64-bit
		Fixed in Git Commit		Github Pull Request #

Summary	05694: All sets in coco12.c and coco3.c: audio output level incorrect when Cassette/Cartridge selected on SC77526 chip
Description	The coco hardware has a fundamental design defect whereby it is impossible to read the joystick axis positions without perturbing the output audio signal, if a sound waveform is currently being played. I have written a game engine which uses the Coco's hardware in a way which minimizes the audio signal disturbance for certain cases. In order to do this, it relies on the specific behavior of the Coco's SC77256 DAC chip, but this behavior is not emulated correctly in MESS. As such, my software produces a moderately loud buzzing sound in MESS which is much quieter when run on a real Coco computer. Page 42 and 43 of the Tandy Color Computer 3 service manual state that the SC77526 chip internally biases the Sound 1 and 2 inputs (from the Cassette and Cartridge sound lines) at the midpoint of the 3.9v output range. I tested a real Coco 3 machine using a DVM to verify that this is the behavior of the real chip. However, the logic in the MESS emulator brings the output audio value to 0 whenever the Cassette or Cartridge audio lines are selected, and they are playing no sound. I have written a patch to correctly model the behavior of a real Coco for the Cassette input. When this patch is applied to the latest SVN version of MESS (I am using r31981), the emulator correctly attenuates the buzzing sound when running my game engine. I have attached the patch file to this bug report.
Steps To Reproduce
Additional Information
Github Commit

Flags
Regression Version
Affected Sets / Systems	All sets in coco12.c and coco3.c

Attached Files	coco3-cassette-sound-fix.diff (1,593 bytes) Sep 8, 2014, 05:28 Uploaded by Richard42 patch to fix minor sound defect [Show Content]

No.10981 Tafoid Administrator Sep 17, 2014, 10:18	If you have a submission you want to include in main source, this is not the correct place to submit it for review/addition. Visit http://www.mamedev.org/devwiki/index.php?title=Submitting_Source_Code
No.11226 Richard42 Tester Nov 4, 2014, 15:39	It looks like my fix has been merged into the source repo: https://github.com/mamedev/mame/commit/7b7f2456b90847187707b2447b4143d73837a4b2 Thanks!