AP - The Recognized Standard In Audio Test

Mixing console tests don't present all the power and heat issues that can make power amplifier testing exciting, but they do present their own challenges–hundreds of knobs and settings, analog and digital inputs and outputs and a multitude of channels. Mixing consoles are often used in situations demanding extreme reliability–from a six channel unit on a film set, where down-time can cost hundreds of thousands of dollars, to a 128 channel unit in a televised concert being beamed live to millions across the world.

Multi-channel capability is essential for the fastest testing. The APx585 and APx586 are perfect for this application, with 8 outputs and inputs, and 8 outputs and 16 inputs, respectively. For higher channel counts, APx and 2700 Series both support up to 16 switchers for a total of 192 channels.

Because of the potentially overwhelming count of signal paths and channels, mixing console test benefits greatly from automation: a comprehensive set of measurements can be run quickly and efficiently, minimizing exposure to the human error factor common in highly repetitive procedures.

Audio Precision offers several alternatives for automated test. With its multiple channels plus the ability to make automated projects without writing any code (thanks to the Measurement Sequencer) and extensive LabVIEW and VB .NET support, the APx585 or APx586 is probably the AP instrument best suited to high channel count automation. Users requiring performance better than -103 dB THD+N, or needing to make electrical tests of digital interfaces, will be well served by the 2700 Series.

Even with automation, testing of analog mixing consoles demands a large degree of manual interaction and listening. Does the high EQ knob sound scratchy, or cut out at full rotation? Does the gain control have a dead spot? Does the limiter switch pop when it is engaged? Proper testing involves prompts to set EQ and other controls to various positions while listening for problems, and then running automated sequences to measure and graph the results. Both the APx Series and the 2700 Series let you define high and low limit curves, giving you both pass/fail reports and visual results. On boards with multiple main and auxiliary outputs, every combination of input and output channel must be tested.

Common Challenges

• Don't miss anything! Make sure that every knob, switch, and signal path combination is tested. Use a manual chart, or automate the testing. If the mixer is being serviced, test both before and after servicing.
• Control the mixer with automation. If the mixer has digital eq, limiting, and gain control, with an external control or computer interface, try to tap into its API to further automate testing. If you are designing a mixer, keep automated control of the mixer in mind. Reducing human interaction will greatly speed testing and reduce associated costs.

Tips for Optimum Testing

• Measure one channel at a time. For testing of noise and distortion, it's necessary to mute or turn off all channels except for the one being measured. For frequency response, it shouldn't matter.
• One-to-many and many-to-one crosstalk measurements are a special capability of the APx585 and 586 which can't be done on any two channel analyzer.
• Verify headroom. Set the channel and master faders at their nominal positions, adjust gain for zero VU, and then increase the signal generator to find the headroom (use regulation on the 2700 Series, or the Maximum Output measurement on the APx Series). If you expect to be able to feed a digital recorder at -20dBFS, make sure you really have 20dB of headroom.
• Analog mixing boards get a lot of wear on the controls. Scratchy or intermittent knobs and switches may not get detected when taking measurements. You need to listen while you are testing and rotating the controls. Listen to a multitone for a smooth spectrum change while rotating the EQ, then plot the response curve at full rotation.