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Sound Blaster 16 CT1740 how to switch between OPL3 and CQM?

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First post, by Dusko

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Hello all!

I’m trying to understand how I can switch between the OPL3 and the CQM on this SB16 CT1740.
From what I’ve read, the jumpers (IFSD and APSD) that activate the CQM should be open, which they are. In fact, the pins weren’t even installed from the factory.
Now, I’m almost certain that the FM sound I’m hearing is coming from the OPL3. I’m tempted to install jumper pins and close them just to see what happens, but I wanted to ask here first.
What also confuses me is this: if the jumper pins weren’t installed from the factory, forcing the use of the CQM, then why bother including an OPL3 chip in the first place?

Could it be that it's actually using the CQM? In that case, it doesn't sound bad after all, somewhat dull, but that may be due to the old caps.

Any thoughts?
Thanks!

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Electronics, mods and tools channel: https://www.youtube.com/@RetroRust-75

Reply 1 of 5, by AlaricD

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did you mean the ASP/CSP?
Advanced/Creative Signal Processor

That isn't Creative Quadrature Modulation.

Reply 2 of 5, by ux-3

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AFAIK, this card does not do any CQM. If you get FM, it will be OPL.

But where did you get the bios for bug free midi? My CT1740 has 4.04, which is supposed to be bug free too.

How is the noise on this card?

Retro PC warning: The things you own end up owning you.

Reply 3 of 5, by Dusko

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AlaricD wrote on 2025-10-29, 04:45:

did you mean the ASP/CSP?
Advanced/Creative Signal Processor

That isn't Creative Quadrature Modulation.

My bad, you are correct, it's the ASP/CSP chip. (CT1748).
Does my question still makes sense? If not, what is the chip doing?

Retro PC games channel: https://www.youtube.com/@myOldPC-75
Electronics, mods and tools channel: https://www.youtube.com/@RetroRust-75

Reply 4 of 5, by Dusko

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ux-3 wrote on 2025-10-29, 06:27:

AFAIK, this card does not do any CQM. If you get FM, it will be OPL.

But where did you get the bios for bug free midi? My CT1740 has 4.04, which is supposed to be bug free too.

How is the noise on this card?

It's the ASP/CSP chip, sorry.

Regarding the DSP chip, I replaced it with a new firmware programmed in a Atmel AT89C52. You can find the details here: https://youtu.be/Sx1fnBFUht0
Noise level it's not that bad, considering it has a CT1701 DAC. I'll make a follow up video in the near future regarding noise levels (and compare the old caps with new ones)

Retro PC games channel: https://www.youtube.com/@myOldPC-75
Electronics, mods and tools channel: https://www.youtube.com/@RetroRust-75

Reply 5 of 5, by NeoG_

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Dusko wrote on 2025-10-29, 23:47:
AlaricD wrote on 2025-10-29, 04:45:

did you mean the ASP/CSP?
Advanced/Creative Signal Processor

That isn't Creative Quadrature Modulation.

My bad, you are correct, it's the ASP/CSP chip. (CT1748).
Does my question still makes sense? If not, what is the chip doing?

The question is moot as SB16 cards shipped with OPL or CQM, not both- the ASP/CSP chip is a feature expansion processor that adds hardware ADPCM decoding, speech synthesis and pseudo surround sound.

Retro Rig: BabyAT AladdinV, K6-2+/550, V3 2000, 128MB PC100, 20GB HDD, 128GB SD2IDE, SB Live!, ES1868F, PicoGUS, WP32 McCake, iNFRA 6000 CD

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