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Would that allow for better sound samples or double/triple the number of sprites & colours on screen or would that put too much pressure on the CPU or cause bottlenecks, maybe requiring 2 CPUs for such a set up?

Say for example you had a 68000, paired up with 2 PPU (SNES) & 2 Yamaha 3812 sound chips. Or any combination of 8/16bit CPU with appropriate era graphics & sound hardware.

I know that some arcade machines had 2 CPUs & 2 or more sound chips (can't think of any off the top of my head) & the Sega Saturn which was a bit later than the 16bit era had 2 graphics cards but I can't think of any 8/16 bit machines that had multiple VDPs.

Are there any historical examples of using multiple VDPs / sound chips in a single product, or any persuasive arguments for doing so in a modern retro-build?

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    This does not seem to be a question about any historic system or it's usage, but rather a questionnaire for some kind of new build, isn't it? Beside, for CPU load, simply add the needed memory bandwith. – Raffzahn Feb 7 at 1:48
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    It wasn't unknown to use multiple sound chips to provide more audio channels, often one for the left and another for the right. Most CRTC and other video driver chips assume they are solely responsible for the video output, generate it fully internally, and so would not be nearly as easy to just scale out like that. You could run a second display, I suppose. – RETRAC Feb 7 at 1:51
  • Seems related: retrocomputing.meta.stackexchange.com/q/981. So far the only answer leans towards accepting such questions. – user3840170 Feb 7 at 10:46
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    An Amiga with two Denise chips genlock'd, that might be interesting to build. – Simon Richter Feb 7 at 21:05
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    @RETRAC - the output of most video processor chips / CRTCs (at least up until the 90s) was either just a sequence of addresses and timing signals (eg 6845) -- in which case there is little point using multiple chips as the support circuitry does everything interesting -- or are separate colour channels (eg TMS99x8) which can easily then be merged using chroma keying, thus allowing the use of additional sprites and/or extra colours in the tile map. – occipita Feb 12 at 4:40
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Double-SID mod for Commodore 64 is quite popular. Simultaneous work with CGA and MDA, albeit on different monitors, was also quite popular. Sharp X68000 deserves special attention, taking into account its popular hardware extensions. A classic example of doubling VDP is PC Engine SuperGrafx.

Considering the principles of operation of the graphic subsystems of the type mentioned in the start topic, the main problem is not with the performance of the processor, but with the speed of the buses, RAM and DMA channels.

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Having multiple graphics or sound chips in a system has exactly the effects you would expect. It would allow the composite system to perform multiple processing tasks at once, with the results of each contributing processor being combined into the final output (or spread across multiple outputs), producing more sophisticated visuals or sounds.

Graphics:

Multiple video processors provide the ability to compose an image with multiple independent display layers, whether they be bitmaps composed of bitplanes or chunky layers, sprites or tilemaps.

An example: The 32X attachment for the Mega Drive provides a separate video chip independent of the one in the base system. They both draw their own set of tilemaps and sprites simultaneously, with the output from the Mega Drive being routed into the 32X with a video cable, and then output to the TV afterwards. The result is additional independent layers of graphics, and additional video RAM for storing tiles and sprites (in that both units are independently storing their own resources).

A similar system is used when a 2D graphics adapter is used with an early 3D graphics accelerator in a Win 98-era PC. Like the 32X, you plug the output of one into the other. The 3dfx Voodoo accelerator provides 3D graphics rendering, which is mixed with the 2D image buffer provided by the 2D card.

Later, a similar card-linking system for the PC would be developed (with various names, such as like SLI) where two cards work in unison to provide a single output image by processing partial parts of the output simultaneously.

Sound:

A system comprised of multiple independent sound chips can use all the different voices and modes of the base chips mixed in any way the system designer has allowed for. If the outputs of the chips are mixed together, this allows all the independent waveform/PSG/FM voices to be used at once, or assigned to different stereo channels. It could also be used to provide a higher bit-resolution output.

Examples:

The original Sound Blaster PC sound card included a single OPL2 FM synthesizer chip for AdLib music card compatibility. This chip provides mono music output. The Sound Blaster Pro 1 improves on this by including a second OPL2 alongside the first. The two chips are linked to the left and right channels to provide stereo musical output; as each chip is programmed independently, complex panning and echo effects are possible. Many Sierra DOS games provided special support for the Dual OPL2 version of the Sound Blaster Pro in their music driver.

(For backwards compatibility, attempted direct hardware access to the original single OPL2 affects both the chips in the Pro 1 equally, resulting in a mono sound. Later SB Pros used an OPL3 chip instead which uses a different interface - still single OPL2 compatible, but not with dual OPL2.)

Later Sound Blaster cards and clones combine even more multiple independent sound interfaces together! Final generation ISA Sound Blaster AWE cards contain:

  • OPL3 FM synthesized sound (backwards compatible with OPL2),
  • a typical SB series DMA-driven PCM stereo waveform sound stream,
  • an EMU8000 wavetable sound processor,
  • a feature connector for an add-on wavetable sound processor with its own ROM sounds, (distinct from the EMU)
  • CD-ROM interface Redbook audio input,
  • and a line-in input (which may be connected to an external MIDI device through the MPU-401-compatible gameport for high-quality sampler MIDI music)!

DOS and Windows games may use any combination of these to provide sound and music output.

And, yes, I believe that if you put multiple sound cards in a PC and there were no resource conflicts (manual settings for DOS, maybe a bit of tweaking for Windows), you could combine multiple sound cards together in that fashion as well. It's possible to install a Gravis Ultrasound wavetable-based card alongside a Sound Blaster PCM-waveform-based card, though very very few pieces of software would attempt to use them both simultaneously!

The Mega Drive contains both the YM2612 FM chip and the earlier SN76489 PSG chip from the Master System. Games use both together in various ways to provide different musical timbres - the FM chip produces bassy sounds, and complex effects; the PSG chip produces square waves. Instrument patches can use any number of voices from either chips to create complex output sounds one chip alone could not produce.

The CPU overhead of controlling a synthesizer chip is not high. The whole purpose of using chips like these is to offload waveform calculation to the chips themselves rather than the CPU. In the simplest case, a small routine updates the register state of each chip at regular intervals. The most complex thing a synthesizer chip control routine would do is to constantly alter the parameters of the synthesized tone by calculating or stepping through an ADSR envelope to provide a more pleasing sound than the straight square-wave tone of a PSG chip (such as vibrato effects in Game Boy music), and even that is very low-overhead.

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    Apparently the SNES has a video system split into two different chips, where one generates backgrounds and the other applies visual effects on top. – user253751 Feb 11 at 11:56
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    The earlier Sound Blaster cards had a pair of SAA1099 chips (or the sockets to add them) to provide Creative Music System compatibility. The card's I/O map suggests you could use both the CMS and Adlib functionality at the same time on an SB 1.0, 1.5, or 2.0 card, provided you had the chips installed, of course. – Rohan Feb 24 at 6:12
  • Yes that's true Rohan. The advertising posters for SB 1.0 describe it as a "24-VOICE" card due to the 12 voice Game Blaster, 11 voice Adlib and 1 voice mono PCM output hardzone.es/app/uploads-hardzone.es/2021/01/… . I don't know of any software that uses AdLib and Game Blaster simultaneously, but it almost certainly could be used in either of the same ways (additional voices, or higher complexity voices) as I describe in Mega Drive example. – knol Feb 24 at 17:33

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