Audio Calibration

Anthem Room Correction (ARC) System - Part 1


The Avantages of a USB Mcrophone

The PC must capture the in-coming signals to the microphone. This has been problematic in the past because the microphone needs a preamp, and the analog signal must be converted to digital with an external ADC (the sound card in a laptop is not up to the task). Since the listening seat may not be proximate to the PC, it is typical to use a microphone connected to the receiver, but this comes at a price. In many cases, the hardware is sold separately, costing hundreds of dollars. Anthem provides a USB microphone, making practical consumer-based PC-based room correction. The need for analog microphone cables, external microphone preamps, and ADCs is eliminated. The microphone preamp, ADC, and USB drivers are internal to the microphone. There is no corruption by electromagnetic coupling from power lines or other sources, because the USB cable sends the digital data.

The quality of the microphone is critical for a high quality result. Any error in the microphone's response will appear as an inverse error in the corrected response. If the microphone rolls-off at high frequencies, the room correction system will think the speakers are rolled off and will apply a boost at the top end. A microphone calibration curve has the inverse of the microphone error superimposed.

All professional quality room correction systems use calibrated microphones. In a test chamber, the microphone's response is measured. A table is made that captures the microphone's error. The microphone then ships with the error table data. This error table is entered into the PC so the room correction system is calibrated to produce a flat frequency response from the microphone. Despite the low starting price Anthem supplies a calibrated microphone with the calibration file for all products. Just saying a microphone is calibrated is not enough. The number of frequencies that the microphone is calibrated at is as critical as the procedure used to do the calibration.

I have seen a couple of products (room EQs and measurement systems) claiming a calibrated microphone, but the data file for each microphone is not supplied, and these systems cannot deal with such a file, even if available. In these situations, the calibration these companies refer to is an average error for all the microphones they ship. The variations in individual microphones is too great for this be effective. Look closely at any product's claims. You may need to download the user's manual to verify that no calibration file is supplied.

As you work your way through this review, you will see corrected plots showing the Anthem Room Correction (we will use the acronym ARC for the remainder of the review), and that the correction falls within an error bar of +/- 1.5dB. Some of this is not even ARC, but from my errors from my own calibrated measurement microphone and small differences in microphone placement.