USING A DIGITAL CROSSOVER WITH THE AUDIGO SYSTEM (June 1994)

The current trend of using a "DAW" (Digital Audio Workstation or hard disk recording systems) for audio production offers some interesting side effects if you spend some time realizing what you really have. All of these systems have some form of "DSP" chipset available for processing all the work that analog effects equipment used to do. As a result if you have good software control of these effects processes, a "Digital Crossover" can be easily setup provided your DAW provides "REAL TIME" processing of the signal that is on the mixing buss and very flexible "Multiband Parametric EQ" is available.

We have been using Waves Q10, a "Multiband Parametric EQ", with Digidesigns Sound Designer since it first came out however, it was not possible to do "REAL TIME" processing of anything other than short preview samples untill last year. The entrance of Digidesigns ProTools III with TDM coupled with "Waves Q10 TDM PlugIN" and more powerful computers is changing all this. For about the cost of a high quality external digital delay line and analog crossover, you can move your monitoring system into the "Digital Domain". The following article describes how we have done this with our AUDIGO Near Field Monitors.

EQUIPMENT LAYOUT

Our test system consists of a Digidesign Pro Tools III system with two DSP farms. We have found that a digital crossover system like the one outlined below will use 3 of the DSP farms 8 chips. Monitoring of the systems playback results is done using listening tests and measurements. In order to maintain constant level and instantaneous switching between the test systems two infrared controlled DBX 5bx companders (in the pass-through mode) are used and programmed to adjacent memory positions. When unit 1 is selected unit 2 is shut off instantly since they are both receiving the same signal at there sensor. The crossover was created using several bands from the "Waves Q10 TDM PlugIN". Levels are adjusted using pink noise from a "TEF20" analyzer at the listeners position and EQ is applied in the digital domain using several more bands of the "Waves Q10 TDM PlugIN" eliminating the phase error that analog devices add. The memory positions in the 5bx units make it easy to program several test monitor systems so A-B comparisons can be performed between different sets. In our tests we used 3 systems the AUDIGO with and without a digital crossover and the Genelec 1031a. As our measurements progressed we decided to use a the Pro Tools III Delay Line PlugIN to adjust phase error between the low frequency driver and the mid range. This was accomplished by monitoring the output of the test systems with a "TEF20" analyzer. The end result sonically was far better than we could have imagined and is outline in the following graphs. One of my main concerns was the possibility of turn on/off transients when dealing with direct connection to the 3000 watts of amplification the AUDIGO’S require. This proved so reliable and quiet during accidental power failures that I have never lost a driver since I connected to the outputs of the Pro Tools III system. The system is truly transparent to the operator.

SETTINGS FOR THE DIGITAL CROSSOVER

The following two illustrations show the settings that we used to tune the listeners position response to the pink noise response and achieve the best overall system performance. The Wave’s Q10 plugin makes it very easy to change and experiment in real time , while monitoring the changes visually with the TEF20 and acoustically by ear. The results can be saved and loaded in real time to make comparisons at a latter date when changes are made in the system . (Irregularities are adjustments for room modes)

The upper graph is the low frequency settings as stored. The lower graph represents the high frequency settings.

The previous two crossover settings result in the graph as seen below. The combination of the high frequency and the low frequency settings result in the red curve. As can be noted the cross over point is at 1506hz.

One of the most difficult aspects to achieve in speaker design is maintaining linear phase response between the drivers. Using a digital delay line to compensate for time delay errors can be very useful in correcting these problems . The following four images give you a before and after look at the effects of time delay correction.

This graph represents an ETC (energy time curve) of the first sound received by the test microphone on the TEF20 from the high frequency drivers (pink) and low frequency drivers (yellow) on our AUDIGO test system. There is about a .310ms time delay error between these drivers.

We now make a .31ms time delay adjustment using the Pro Tools III time delay module.

Now we take another ETC curve for the high frequency drivers (white) and low frequency drivers (yellow). The delay error is corrected. What does this mean? Take a look at he phase graphs below.

The cyan curve represents the corrected phase response of the system as compared to the red curve without. The clarity and imaging was definitely improved when our listening tests continued.

- ©1995 alienconcepts inc - for information call 708.686.7285 or email audigo@alienc.com
- updated 04/21/05 11:28:08 AM
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