Parametric EQ info

Hey all;

I have been reading about parametric EQ on this (excellent) technical site:
http://www.tonmeister.ca/main/textbook/intro_to_sound_recordingch7.html#x27-3660006.1.3

I am wondering if the parametric EQ in my Empeg is symmetric or constant-Q. The link above goes straight to the section discussing the difference. I'm imagining that as the "Q" value is set explicitly, it is constant-Q.

In practice, this means that subtractive EQ would give different results to positive EQ, as subtractive EQ gives a tighter notch than positive EQ given the same Q value.

Worth knowing if you are trying to exploit the parametric EQ

Reading on I see that there is something called "Half-gain defined Q", which means that the bandwidth calculated from Q is done using 1/2 the gain value, resulting in a symmetric filter.
http://www.tonmeister.ca/main/textbook/intro_to_sound_recordingch7.html#x27-3880006.1.6

So - is the Empegs' filter using this half-gain technique (resulting in symmetric filter) or is is regular Q (and therefore asymmetric)?

The EQ in the empeg is implemented in the Phillips SAA7705H chip. Unfortunately, I can't find details of how the Q is implemented, because Phillips expect you to use custom software to set up the profiles. Obviously, we directly configure the profiles from the empeg player. Either we reverse-engineered it, or we got hold of a non-public datasheet with the details.

For the car-player EQ, there was a non-public datasheet and an NDA. The Karma/Vibez parametric EQ is done in software, so it might be different -- but that was all done by our DSP experts, i.e. not me or Roger, and I don't know how much those guys hang out on the BBS any more.

Peter

I did find this datasheet linked on an earlier post:
http://www.classic.nxp.com/acrobat_download2/datasheets/SAA7705H_1.pdf

I couldn't find any info in that PDF, probably not the NDA document you are talking about.