1 Reply by WhyLee

(12 replies, posted in Commodore Computers)

Would be interresting if you could measure the frequency on the clock pin on the sid-chip. (That Clock is called "Phi2" on Pin 6 on the SID-chip.
I know that this clock is different depending NTSC or PAL Version:
The Clock on PAL machines is (312 * 63 * 50) = 0,982800 MHz
The Clock on NTSC machines is (263 * 65 * 60) = 1,025700 MHz
In the Datasheet of the SID 6581 is a Table for the Clock of 1,000MHz: The value for 880HZ (note A5) is 14764 for the 16-bit value that you have to write into the frequency-register of the SID.
If you have the PAL version the chip is slightly slower, so you (or your software) have to use higher values.
If you have the NTSC-Version the SID works a little bit faster, so you (or your software) have to use lower values.
( I think there are correct formulas out there in the internet to calculate the right frequency for PAL or NTSC)
Also there different Frequencies for the notes - depending if the note scala is tempered or mathematically evenly distributet (with SQRT(12)-Steps).
Usually you use the "well tempered" (i don't know the right english term for that) frequency-scala for the notes so that combinations of them sound "right".

If you have measured the wrong clock on your SID-Chip you may change the crystal in your system to a new one and maybe also the capacitures around them to get a stable correct clock frequency that does not change over temperature.

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2 Reply by WhyLee

(16 replies, posted in Commodore Computers)

What you can do with audio-in signals in the SID:
The EXT-In Signal is mixed with the internal sound voices and through the main volume register and through the filters.

So, when you just want to use the SID with the external audio signal you can adjust the volume and filter it.
Is has a setting for high-pass, low-pass, band-pass filter and combinations of it. has a register for the resonance frequency and a register to set the q-factor of the resonance.

but that all theoretical things are maybe a little bit useless when it comes to the old generation of SID-chips. theire filters behave very weired with lots of distortion. people used it in their way with the games that it sounded good with the "originally not intended" distortion of that filters.
also different SID-chips from the same production behaved different cause of the production process that delivered not constant results for the analog parts on the chip.

so these filters are not something that made the c64 great for modifying external audio signals.
however, when you have the later models of the SID: 8580/6582 or 6581R4AR the filters work good and predictable - like the designer wanted them to behave at the first place (but did not have the time to correct it that way cause the c64 needed to be finished and sold!).

but is the SID-chip a worthy chip just for filters with a digital interface? i don't know the chips that are available for filtering, but i guess there are good chips available for that kind of task. i don't know about the ealy 80's - anyone?

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