Live Sound University Article Sat, October 11, 2008

It’s a Dual Channel FFT analyzer
Recall that Smaart requires two channels in order to compare one audio signal to another. For now, we will work with the output of a console and a measurement microphone. The console output is the reference channel and the measurement microphone output is the measurement channel. Smaart compares the two in order to yield the transfer function, which displays the difference between the two signals.

So, we need to get two channels of audio into the computer. The simplest way to accomplish this is via the line inputs of the sound card. Note very carefully: not all laptops have line inputs, and those that do often have only mono inputs. A stereo line input is required, unless you choose to get audio into the computer via a USB device or other external hardware. For now we will assume a laptop with a stereo line input. Don’t try to use the microphone input, it is mono and has a difficult time with line level signals.

Why can Smaart use that inexpensive sound card chip found in laptops? Smaart takes the audio from the analog to digital converter at the hardware level and does not rely on the chip to do all the work. Interestingly, almost all laptops use the same chip at the hardware level. Because of this Smaart supports virtually all laptop audio line inputs.

By convention, Smaart assumes the left channel of the laptop sound card to be the measurement signal and the right channel the reference signal. We need to get audio into the sound card, so let us examine a simple connection scheme using a small mixer:

Simple Interconnect
Figure 1 illustrates a simple interconnect scheme for Smaart, taking console output and the measurement microphone to a small mixer, which feeds the computer sound card:

So we see that in order to “tap” the console output in this example we need a “Y” cable to split the output. There are some issues with this arrangement. Certainly you would not want your console output to “see” the sound card, electronically. Using the measurement mixer solves this problem. In the event one chooses to take console output directly to the sound card without the benefit of the measurement mixer, it is necessary to use an appropriate transformer. Use a measurement mixer of some sort: this is generally agreed upon as the best solution.

Another method which works for larger consoles is to use a matrix output, with the desired channel (house left or right) assigned to the matrix.

The “laundry list”:

1. Take console output (left or right) into a channel of the measurement mixer as your reference channel
2. Send measurement microphone output into another channel of the measurement mixer as your measurement channel
3. Pan the measurement channel left
4. Pan the reference channel right
5. Ensure all channel EQ is flat or bypassed
6. Send the measurement mixer left output to the left channel of the sound card line input
7. Send the measurement mixer right output to the right channel of the sound card line input

Now we have signal properly routed and present at the computer sound card input. The next step is to use the Windows audio mixer to manipulate the sound card line input.

The Windows Audio Mixer
Unquestionably, the most common problem encountered by Smaart novices is failing to set up the Windows Audio Mixer properly.

Access the Windows mixer via the Control Panel. Go to Multimedia and in the audio portion of this control check the box labeled “Show Volume Control on Taskbar”. Apply this change and you should see the loudspeaker icon in the system tray.

Double click the Volume Control icon and the Windows Audio Mixer is displayed.

Your computer may/will display a slightly different window, depending upon the sound card capabilities.

Select Line In, and lower the input level using the fader. Due to the fashion in which some sound cards implement gain controls, it is possible to lose bit resolution when increasing the gain using the Windows mixer. Set this control to a lower position and get appropriate gain by increasing it with your measurement mixer.

After starting Smaart, the signal level on the Smaart meters should read about –12 dB. This provides the optimum operating level for Smaart. Note that after starting the Smaart Live program it is necessary to push the “On” button to actually start it and display audio input levels.

Microphones and Mixers
It is tempting to use a condenser microphone you already own, but most likely it is inappropriate for measurement use. There a good chance the frequency response is not flat enough to take a respectable measurement. Additionally, a measurement microphone should be omnidirectional. Why? The construction techniques used to create non-omnidirectional pickup patterns (cardioid, hyper-cardioid, or super-cardioid) cause the phase response of the microphone to become skewed.

Use a microphone built for the task. There are several relatively inexpensive measurement microphones which work fine for basic measurements. After you get some experience and want to move on to more serious measurement work consider one of the “better” microphones. It is not necessary to spend thousands of dollars on a laboratory grade microphone. Suitable microphones are available for under $400 U.S.

Inexpensive measurement microphones are manufactured by Audio Control Industrial, Audix, Behringer, and others. “Pro” models are manufactured by Earthworks, Josephson, and others with the Earthworks model currently the preferred microphone in Smaart Live circles.

You may use any mixer which has phantom power for your measurement microphone and the ability to pan left and right. In fact, in many applications it is possible to do a basic system measurement without a “measurement mixer”.

1. Use a matrix output or Y-Cable to get one channel of the console output into the computer sound card. If you use a Y-Cable consider using a high quality line level 1:1 isolation transformer in order to keep the console output from “seeing” the sound card input.
2. Use a channel on the console for your measurement microphone. Engage phantom power and send that input channel to the computer sound card’s left channel by:
a. Using the channel direct output
b. Assigning the channel to a subgroup, taking the subgroup output to the computer
c. Using an aux send, taking the aux output to the computer

You may now use the direct output level, subgroup fader, or aux send to control the measurement microphone level. Controlling the console (reference signal) level is a bit trickier if you do not have matrix outputs. You may have to use the Windows mixer to control the line input level to get the reference (console) signal within a reasonable operating range (-12 dB is optimum). Once this level is established for the reference channel you may use the direct output, aux send, or subgroup to match the measurement level.

CAUTION: be absolutely positive your microphone channel is not assigned to the mix bus lest you get a huge surprise.

Starter Kits
Audio Control Industrial manufactures an excellent starter kit for Smaart, the MP-200. The MP-200 kit contains a calibrated measurement microphone and a small mixer, powered with either a “wall wart” or internal battery power. The mixer provides both microphone and line inputs, plus an internal pink noise generator. If you choose to use pink noise for measurements the MP-200 (and its big brother MP-400) can internally monitor the pink noise it sends to the system. This means “no Y-Cable required”, and for quick one-offs this can be very handy.

Smaart Live for Beginners: Part 1
Smaart Live for Beginners: Part 3