The RioCar parametric EQ's frequency response does not reflect settings in all cases.

I have conducted series of 1/3 octave RTA tests on the low frequency settings of the RioCar's parametric EQ. The results are disappointing.

Here is a short summary:

1). AttenuationProblem#1 "the 39Hz effect": While attenuation results in the expected Band Reject response at center frequencies above 38Hz, when center frequencies below 39Hz are selected, the EQ's response changes to that of a Low Pass filter.

2). AttenuationProblem#2: While the EQ is stable below -20dB, each reduction in gain, does not result in a reduction of output at the center frequency. For instance -26dB results in more attenuation than -27dB.

3). BoostProblem#1 "flat response": Between 18Hz and 59 Hz, all positive gain values reach a point above which the EQ shuts off. For instance, 42Hz at +9dB results in the expected Band Pass response, however at +10dB, the EQ no longer effects the signal, eg. flat response. The gain at which this behavior accurs is frequency dependant. For instance, at 54Hz the response is BP at +11dB, but at +12dB the response is flat. This problem does not occur above 58Hz.

4). BoostProblem#2 "the 39Hz effect": At gains where the EQ does work (see BoostProblem#1 above), the EQ also suffers from "the 39 Hz effect". While above and below 39Hz, the response is Band Pass, at frequencies below 39 Hz, the center frequency drops to somewhere between 27 and 73 Hz, dependant on gain. For instance, +8dB at 39Hz results in a +7dB boost at a center frequency of 43Hz (about what you would expect from this DSP chip). However, +8dB at 38Hz results in a drastic center frequency drop to 27Hz with only a +5dB gain. The gain dependancy refered to previously is as follows. As the gain is decreased, the center frequency rises. For instance, from the previous example - +8dB at 38Hz results in a 27Hz center frequency, but reducing the gain to +1dB results in a 73Hz center frequency.

With frequent reboots, these results were consistantly duplicatable. Though I will say I have seen some adjustments result in totaly wacked response curves...that if played through drivers would have had disatrous results. This OS is very buggy!

All the above analysis was done using the locked Q settings mode. In this mode, the Q value automatically changes based on the gain. Another factor I have noticed, but have yet to explore fully, is the effect of adjusting Q values. All 4 problems described above are effected by, and can be caused by, adjusting the Q value manually. For instance, while 42Hz at +9dB results in the expected Band Pass response (see BoostProblem#1 above), by increasing the Q above its automatic setting of 1.56, eventually at value is reached where the EQ shuts off.

I would be happy to send the spreadsheet that ducuments this behavior and the details of the test conditions. It is much easier to see this behavior visually rather than try and understand my written description.

I understand that some of this behavior may be by design. For instance, preventing people from blowing drivers with a gain of +12db at 18Hz may explain BoostProblem#1. A better approach would be to simply prevent setting the EQ beyond those design limits, rather than letting the user fall off a cliff.

Knowing some of this Non-Linear behavior I have measured, I may have a slightly better chance of achieving the response I desire in my system. But not knowing when the EQ's response is going to deviate from settings, does not make having this parametric EQ very useful for the rest of you.

Can the responsible party please post the logic behind this non-linear behavior?

Better yet, please let me know who can re-write the DSP programming routines so the response is linear across all gain, frequency, and Q settings.