Testing audio consoles

Comprehensive testing can ensure reliability and avoid costly downtime.
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The drop in the number of attendees at the recent IBC show is another reminder that it will be quite a while before the broadcast market returns to what might be called business as usual. In the meantime, it's likely that it will continue to prove difficult to make investments in new equipment and to upgrade infrastructure.

In this climate, key pieces of equipment, such as cameras, routers, switchers and sound mixers, should get comprehensive and regular testing to ensure their continued reliability as they get older. Audio consoles (mixers) in particular are complex devices to test properly due to the many possible input and output combinations and thus the many potential failure points.

Only comprehensive testing can ensure the high degree of reliability that is essential to avoid costly downtime and maintain delivery of high-quality audio. Whether all analog, analog with digital outputs or all digital, the main principles of mixer testing remain the same. Mixers with digital signal routing and processing, however, can be much faster to test.

When to perform testing

There are three basic occasions when comprehensive mixer testing should be performed. The first is as part of a regular preventative maintenance program, performed at least yearly and even more often under heavy use. Many times problems will be spotted here that haven't already been reported, due to the difficulty of identifying issues and taking adequate notes during the pressures of production.

The second case is in preparation for a special event, where reliability is imperative. This is especially important in the case of remotes, where a completely stocked service shop will not be accessible.

The third case is during repair, both before and after service is performed. The preservice test is important to quantify and verify reported problems, and to look for other problems that may not have been initially reported. This is an important cost-saving measure; having to open the mixer up again to fix problems that were missed the first time is costly and a waste of limited staff resources.

Equally important is to run a post-test after the mixer is repaired and it has been fully reassembled. This verifies proper performance before it is put back into service. There's always that chance of leaving something disconnected inside, or of having a potentiometer or switch fail during reassembly. These things will get caught during the post-test.

Mixer testing can conveniently be broken down into four separate areas: audio performance, mechanical functions, features and routing.

Audio performance

Every input channel and every output bus should be checked for good audio performance. Testing the inputs and outputs in isolation is not always possible nor the best route to take because it bypasses the mixing bus. So, first test channel one routed through the main stereo output, and verify that the combination meets the manufacturer's specifications. If it does, you can now check the audio performance of every output using channel one as your source and every input using the main stereo outputs.

A basic set of tests will characterize performance and alert you to any problems. Start with a 1kHz tone to check input gain, output level and meter calibration. Then check frequency response. Modern audio analyzers use a log swept sine wave to measure frequency response. Not only can this be extremely rapid (less than a second), but also it allows you to measure total harmonic distortion, our next test item, at the same time. Some advanced analyzers have the ability to measure individual harmonic components of the distortion as well, which can help in pinpointing faulty components in a circuit.

Noise in a mixer builds up when multiple channels are on, so when checking total harmonic distortion plus noise (THD+N) and signal-to-noise-ratio, silence all channels except the channel under test by putting their faders to zero, muting them and turning them off if possible. Noise should be checked with a line level input to measure system noise, and with a mic level input to measure equivalent input noise on the mic preamp.

Maximum overload level should be reviewed to make sure the mixer can capture the full dynamic range of the audio signal. Poor overload level could indicate failure in the audio path or in the power supply. An analyzer with a regulation feature can automatically zero in rapidly on the overload point, saving the time of finding it manually.

A common mode rejection ratio (CMRR) measurement will instantly alert you if a balanced input has one side down. Some analyzers have the IEC CMRR test built in, making it easy to add it to an automated test routine.

On a digital mixing board, do all the same tests, but also make sure the sample rate is correct and that jitter is low. You'll need a dual-domain analyzer for this — one that can test any combination of analog and digital inputs and outputs.

Mechanical functions

The most common failures on mixing boards are mechanical. Faders, potentiometers and switches can all become intermittent and dirty. The best way to test faders and level potentiometers is by listening to a 1kHz tone while sliding or rotating them through their travel. Check that there are no dropouts or crackles and that there's no sound in the full off position. While this part of the testing can't be fully automated, making it part of the test routine assures that nothing gets missed.

Having an audio analyzer that can generate a multitone stimulus signal is a great advantage for testing EQ pots. Changes in EQ can be easily heard in the multitone signal as the pots are rotated, while at the same time the audio analyzer can quickly update the on-screen frequency response graph.

Features

The features include everything else — overload indicators, phantom power, compressors, echo or reverb, and effects. Each of these should be checked as part of a systematic test routine. Phantom power should be checked on every microphone input, measuring between pin 1 and both pins 2 and 3. This will assure that all the pins are connected.

Routing

Routing includes assignment of channels to output buses, auxiliary sends, monitors, solo and tape return. On an analog mixer, every possible combination of input and output must be reviewed, because the audio for each combination goes through its own unique set of switches, connectors and circuit board traces — all of which can potentially fail. For a 16-channel board with four main outputs and four aux sends, that's 128 signal paths to check.

On a digital mixer with digital routing, only 16 input paths and eight output paths will need to be looked at. You can use a 1kHz tone to check that the level is correct and distortion is normal.

Automation and record keeping

It would be time-consuming and costly to do these comprehensive tests manually. Fortunately, it's possible with a modern high-quality audio analyzer to automate the process. This not only saves a great deal of time, but it reduces the chance of error. Time can further be saved by using a multichannel analyzer or a two-channel analyzer with auxiliary switchers. Automation features are useless, however, if they're too hard to implement.

An ideal analyzer interface allows nonprogrammers to create sophisticated automation sequences without writing any code. For more complex automation, it's important that the instrument's API is accessible through a popular platform such as .NET to ensure connectivity with other applications like LabVIEW.

Conclusions

The benefits of a comprehensive, systematic and fully documented audio mixer testing regimen cannot be overstated. More viewers than ever before are watching programs on HDTVs and listening on their home theater surround-sound systems. In light of today's tight economy, it's vital to keep existing equipment in top condition and to deliver the best sound possible at the lowest cost.

To carry out the comprehensive and rapid automated testing discussed above, you'll need to have a modern, dual domain audio analyzer with easy-to-use automation and reporting capabilities. Its expense will be returned many times over in saved downtime and reduced labor costs.

Adam Liberman is an audio engineer and service specialist, as well as a technical writer at Audio Precision.