Sierra AGI Ports to the CoCo 3, Any Sound Improvement?

[Great Hierophant]

The Tandy Color Computer 3 received official releases of King's Quest III and Leisure Suit Larry, and they look and run almost identically to their IBM PC counterparts. Because the CoCo series only have a 6-bit DAC, the music output from a CoCo is PC-speaker quality on the official ports. Also, outputting sound to the DAC is rather CPU intensive so the official AGI engine for the CoCo never plays sound while animation occurs on the screen. Even an IBM PC can manage animation and PC speaker sound at the same time.

While Sierra only officially ported these two games, the CoCo community has ported the rest of them and made them all friendlier to larger storage devices than the official 157.5KiB CoCo disk format. A cursory review of these ports suggest that they simply used the official engine and tweaked it to work with the data files from the other games' PC versions and called it a day.  They appear to have the same musical limitations.

But has there been any more ambition than this? Tandy released the Speech/Sound cartridge which contains an AY-3-8913 sound chip. The 8913 is quite capable of doing justice to the three voice music and sound effects that the AGI engine supported. The music and sound effects were originally designed for the SN76496, which the 8913 can eclipse in almost every way. Unfortunately, Sierra could not use it because of a design flaw : the cartridge fails to work in the CoCo's 3's high speed mode, being designed for the CoCo 2 which only supported low speed mode. However, the Speech/Sound cartridge can be modified without too much difficulty to work with the CoCo 3's high speed mode.

The Tandy CoCo 3 would make a great, fairly compact AGI adventure game playing device if this one remaining flaw was addressed. Has there been any effort to do so?

[JamesD]

The sound and speech cart doesn't allow the CPU to have direct access to the sound chip.
From BASIC it can be great since it can play music while the computer is doing other things, but it makes programming it a bit more difficult for regular music players. 
 

[turboxray]

If you ran FIRQ TIMER for 6khz, you could dedicate 15% cpu resource to outputting to the DAC. 6 bits is more than enough to easily mix four 4bit square-wave PCM channels together (about 1% cpu resource for mixing with 8.8bit PHA counter per channel), and 6khz is decent enough for pure square waves for frequency responses. 6khz with a four sample square-waveform (to emulate duty cycles), has a max frequency of 1500hz which puts it at G octave 6. Considering those AGI games use low res 160px mode, it shouldn't be eating up that much cpu resource for blitting, but I haven't looked at the interpreter (I'm sure it can be optimized). Either way, the CoCo3 is definitely capable to outputting multiple channel sounds while doing animation. 

 

 That gets you pretty close to popular SN/AY chip. Plus it allows more effects than those chips are capable of. You can do more synth-y stuff like hard-sync, waveform blending/morphing, larger sample wave-forms for lower octaves notes (bass instruments - triangle, sawtooth, etc), etc. 

Edited October 21, 2020 by turboxray