Re: How is the IIgs beep generated?

[vladitx <vladitx@nucleusys.com>]

On Dec 23, 4:58 am, "Michael J. Mahon" <mjma...@aol.com> wrote:

> I'm guessing that the primary difference is that on the IIgs, the
> speaker is driven by an audio amplifier from the actual speaker toggle
> logic level, while on all other Apple II's, the speaker is driven by
> a Darlington driver with a peculiar AC-coupled, DC-clamped input.

I never investigated this, by there is very audible asymmetry in each
toggle direction. Some day ... yep, definitely.

> The PWM approach assumes that the Apple II clock is constant
> frequency, but that is not strictly true, since there is a stretched
> cycle every horizontal line, or 62 cycles.

65, I know it's a typo. :-)

> This introduces a 15kHz
> sub-cycle timing variation, causing an audible beat which is most
> apparent under conditions of very low modulation (near silence).

Surely you have tried PWM at 15.7 kHz (65 cycles), where no modulation
occurs, just slightly inaccurate PWM. Is the carrier that noticeable?