Re: Changing the clock speed

David Koogler wrote:
>
> Funny, I have been considering making the same change. The 18.2 ticks per
> second of the IBM PC is rather coarse. It was acceptable for slow machines
> like 8088's and 80286's but is not acceptable for today's higher speeds. In
> fact almost every machine I worked with since the early 1970's used either 60HZ
> or 100HZ as the basic interval.

Ouch 60 Hz - I'd rather have my mains frequency of 50 Hz :-) FWIW we are
currently using 20 Hz as it's more accurate than the 18.2 we originally had
(that was my first patch for VSTa in fact :-))

> For accurate performance measurements it is also important to calibrate the
> interval timer. I am thinking along the lines of using the Real Time Clock as
> a standard and then measuring the interval against the RTC. On most machines,
> the RTC drift is on the order of a few seconds per day or about a minute per
> month.

I think you'll find an awful lot of machines that are *much* less accurate
than that - typically 250 ppm (about 8 mins per month). The other thing is
that unfortunately this is usually *very* temperature dependent.

I think if I wanted really accurate time info I'd go for NTP (when we have
the tcp server) or some sort of radio standard synchronisation instead
(although this tends to be rather country specific unfortunately).

In reality I'm not sure that an error of < 0.1% is really too important for
basic performance measurements - anything else and I'd prefer to use an
independent calibrated frequency reference instead.

                                Regards,
                                Dave
Received on Tue Feb 14 07:42:21 1995