We all need to make measurements on our audio gear at some time or another. It could be as simple as testing for a properly grounded power outlet, checking to see that the sound level at a concert is within the legal limit, or as complex as analyzing a new design or equipment modification. In this overview, I'll look at several tools that cover a wide range of measurements.
What Do You Need to Do?
For the purpose of this article, I've chosen to group test and measurement gear into some broad categories:
- Utility tools for making quick checks
- Bench or shop equipment for troubleshooting, confirming basic performance haracteristics or interface analysis
- Lab or production equipment suitable for design or quality control
- Software that works in conjunction with other hardware, yours or the software vendor's
The golden rule of metrology is that your test equipment must be at least an order of magnitude greater in accuracy and resolution than what you want to measure. If you're checking out a mic preamp that claims 0.001% THD, you need a distortion analyzer that can measure to 0.0001% (that's –120 dB, a formidable task). Accuracy, resolution, and measurement range, as well as the cost associated with those characteristics, are what most often differentiate lab quality equipment from shop grade equipment that makes similar measurements.
Before you choose a piece of test equipment, you must assess your own needs so you don't over or under buy. Do you really need all the accuracy that the manufacturer (presumably) had when he verified the specs? Do you need the capability to supply and measure digital signals as well as analog? Test equipment usually lasts a long time, so as long as it doesn't become obsolete, it's a good investment.
Studio tools such as level and phase meters could be included here, but in the interest of brevity, I'll leave them for another article. So on with the show.
In the Workshop and On the Road
The Gold Line (formerly Loft) TS1, having been around for 25 years or more, is the great granddaddy of modern audio test sets. It combines a sine wave generator, frequency counter and level meter in a single compact box. While not up to the standards of today's very low distortion gear (the generator is rated at a modest <0.3% THD, though mine clocks in at just over 0.1%) the TS1 is great for routine testing and setup. The level display reads in dB and the reference level can be set with a rear-mounted trim pot. Most will keep it set for 0 dBu (+4 dB on the meter equals +4 dBu) as it comes from the factory. A rackmount version, the TS1RMX packs a bit more output juice than the TS1 (+24 dBu vs. +18 dBu maximum for the tabletop version), sufficient to kick most A/D converters up to full scale.
NTI, the test equipment division of Neutrik (the connector people), offers the MR1 Minirator, ML1 Minilyzer and DL1 Digilyzer, hand-sized Minstruments ideal for general audio testing. The Minirator is a multiple waveform function generator providing low distortion sine wave, square wave, white and pink noise, a polarity test signal, and a stepped sweep over the audio range of 20 Hz to 20 kHz. The Minilyzer measures RMS audio level in dBu, dBV or volts, total harmonic distortion, offers a 1/3 octave spectrum display with numeric readout of the amplitude in each frequency band, an indicator of signal polarity when used with the Minirator's polarity test signal, and an oscilloscope-like waveform display. It also measures the pin 2/3 differential of a balanced output, particularly useful for checking electronically balanced sources. Level measurements can be made either relative to a standard reference (dBu, dBV, or volts) or relative to another measured level (for example, the input source) for convenient gain or frequency response measurement. A combination PPM and VU bar graph display is handy for real time monitoring.
The MR1 and ML1 pair can replace a whole shelf full of traditional test equipment. While they don't measure with sufficient accuracy and resolution to validate specifications of some of today's gear, they're ideal for analyzing problems, showing you why something doesn't sound right. Frequency range is 20 Hz to 20 kHz. The Minirator's maximum output level is +6 dBu which won't quite push most A/D converters to full scale but is useful for general testing. With an optional software package and a calibrated measurement microphone, the ML1 can also make acoustic measurements (SPL, RT60, and speech intelligibility). Alternately, an acoustical measurement version can be purchased as the AL1 Acoustilyzer.
The NTI Digilyzer makes similar measurements to the Minilyzer except that its input is digital rather than analog, a capability that's becoming increasingly important in today's working environment. Inputs are AES/EBU, S/PDIF coax and optical, and ADAT optical selectable in channel pairs. A built-in D/A converter provides an audio output so you can hear what you're measuring. Cleverly, the XLR input connector will accept an analog input and pass it to the audio monitor, and let you know that you plugged the wrong cable into the AES/EBU input. A very powerful feature for digital interface troubleshooting is a display of the complete set of channel flag and status bits. The MiniLink option adds a USB port to the NTI analyzers for computer control, uploading of new software, and as a means of storing measurements and transferring stored data to the computer.
The Phonic PAA3 Personal Audio Assistant is similar in form factor to the NTI Minstruments but it's more oriented toward acoustic measurement and analysis. SPL and RT60 measurement, polarity test, and 1/3 octave spectrum analysis can be performed using either the built-in mic or via the line level input. The built-in test generator provides either a 1 kHz sine wave, pink noise, or a polarity test signal. Measurements can be averaged if desired, and stored for later display and analysis. A USB port allows transfer of stored data to a computer.
Sencore's DA795 DigiPro Digital Audio Analyzer packs a comprehensive set of digital tests similar to the NTI Digilyzer into a unit that's small enough to take into the field but powerful enough to use on the test bench. Digital I/O includes AES/EBU, S/PDIF optical and coax, and ADAT optical, with dual digital inputs for checking channel synchronization. Tests include level, THD, jitter, latency, channel status and bitstream display, and a unique “transparency” test that measures bit differences between input and output. The DA795 is powered by an internal rechargeable battery, and the signal generator and analyzer functions are controlled by a single turn-and-push knob with menu selection and display on a good sized LCD.
A sound pressure level (SPL) meter is a very basic measuring tool that tells you how loud something is. In addition to keeping your concert sound level legal, the SPL meter is handy for balancing speakers and making rough acoustical measurements. These meters are available with either an analog or digital display, of which I find the analog meter to be the most useful since when measuring SPL, you often have to take an “eyeball average,” Inexpensive units such as the Radio Shack model 33-4050 or ATI SLM-100 (analog) or Galaxy CM130 (digital) will keep you out of trouble for under $50. With 50 dB SPL being the minimum value that can be read on these meters, they don't, however, have the sensitivity necessary to measure low-level ambient noise. In order to determine the effectiveness of that new fan that you put in your DAW computer, you'll need a more sensitive meter such as the BK Precision Model 732 ($230) which measures down to 30 dB SPL, or a lab-grade analyzer such as those made by Brüel & Kjær, a different company with similar initials.
A cable tester is a continuity tester that's equipped with common audio connectors so you can simply plug in both ends of a cable, even odd cables such as XLR-to-TRS. The tester indicates continuity of each conductor including the shield, shorts between conductors (which sometimes isn't a fault, for example with a balanced-to-unbalanced cable), and displays wiring errors such as reversed tip and ring. While you can test any cable with a multimeter as long as you know how it's supposed to be wired, a purpose-built cable tester is handy, particularly in the field when you're in a hurry and you suspect that you have a faulty cable.
Top of the heap is probably the Ebtech Swizz Army six-way tester ($180) with connectors for XLR, 1/4-inch, 1/8-inch, RCA, TT (bantam), and MIDI cables. In addition to testing cables, it generates 1 kHz and 440 Hz tones at +4 dBu, -10 dBV, and mic level and it indicates the presence of phantom power. The $30 Behringer CT100 is remarkably close in function to the Swizz Army tester, and for $50, the Inspector from the makers of Cascade Microphones adds Speakon connectors and a pair of test probes for making continuity checks on anything you can reach.
Last but not least, don't underestimate the value of a simple line voltage tester. It will tell you if you have voltage at a socket, and it also displays wiring errors such as a missing ground or hot/neutral reversal that can cause problems ranging from electrical shock hazard to hum. Five dollars at practically any hardware store can help you diagnose ground hum problems and it might even keep you or your clients alive longer.
Audio Precision has long been the leader in modern audio test systems. Their top of the line 2700 series is found in engineering labs and quality control stations worldwide. The 2700 is a self-contained two-channel signal generator and signal analyzer, available in four basic configurations – analog in and out with analog hardware signal generation and measurement, analog in and out with DSP signal generation and measurement, digital in and out, and the fully dual-domain model with both DSP and analog generation and analysis of analog signals and DSP generation and analysis of digital signals. Other options allow computer control and logging for production QA and long term monitoring, measurement of intermodulation distortion (IM) using alternate standard measurement methods, sine wave burst analysis, tape flutter measurements, and Dolby Digital (AC3) encoding and decoding. Frequency response, THD, noise, spectrum analysis – you name it, the 2700 measures it, with digital I/O up to 192 kHz sample rate.
The new APx585 multichannel analyzer (see review on page 32) is an eight-channel analyzer with similar functions to the 2700. Inputs are analog or digital, with sample rates up to 192 kHz. With applications for this test set leaning more toward the production environment or for field test and setup during installations, Audio Precision has put a major effort into streamlining the process of taking measurements by means of preprogrammed test sequences and computer control and automated logging and report generation.
The Audio Precision Portable One is a self-contained analyzer designed for field measurements or for the lab with limited space and budget. The basic model has analog I/O only, but there's a dual domain version adding digital I/O up to 96 kHz. Everything is controlled from the front panel and displayed on a built-in LCD – no computer required. The measurement suite is comprehensive and includes level, noise, THD+Noise, channel balance and crosstalk, wow and flutter, input-to-output phase and SINAD. In the digital domain, measurements include jitter, data integrity, and channel status bits.
While Audio Precision can hardly be accused of building “budget priced” test equipment, the ATS-2 analyzer saves costs by generating and analyzing signals digitally and using high linearity A/D and D/A converters for analog input and output. The ATS-2 connects to a computer through a supplied PCI card or PCMCIA card. All control and display is handled by the computer.
The Prism Sound dScope Series III is a Windows PC application with a dedicated hardware I/O interface that connects to a computer via USB. More than a fancy sound card, the I/O box utilizes internal gain ranging to provide a measurement input range of approximately 140 dB. Certain real-time measurements are performed by onboard DSP in the interface, while control, display, and heavy number-crunching for FFT analysis and multitone testing are performed by the PC.
The Prism DSA-1 digital audio signal analyzer appears on the surface to be somewhat similar to the NTI Digilyzer but it's really a whole different instrument. The DSA-1 provides detailed analysis of the digital interface rather than the audio passing through it. It includes two test generators, one to generate standard audio test signals in the digital domain, the other to generate a test signal with known amounts of jitter for analyzing a system's susceptibility to jitter and other cable-based transmission defects. Channel status bits of the test signals are fully programmable, and incoming channel status data can be modified on the fly to verify the response of the receiving device.
Basic Tools and Shop Equipment You Need
Nobody in this business should be without a multimeter. The “multi” in multimeter is AC and DC voltage and current and resistance, though most multimeters also provide a beeper for eyes-off continuity testing, and some also offer capacitance, frequency, and temperature measurements. The Fluke model 77 (now discontinued) was the shop standard for over 20 years. Its replacement, the 177, is becoming today's go-to professional digital multimeter. It measures voltage down to 0.1 mV (-78 dBu), frequency up to 100 kHz, and measures capacitance as well as.
There are, of course, less expensive alternatives. Radio Shack offers multimeters for as little as $20 with adequate sensitivity and accuracy to check power supply voltages, resistance, and continuity. Whatever your budget, a multimeter is a tool that you shouldn't be without.
An oscilloscope can be mighty handy for tracking down where a signal has gone astray, seeing at what stage and level clipping occurs, and for looking for oddities such as a stray high frequency oscillation or ringing, or the digital clock riding on an analog signal. There are many inexpensive or free software programs that turn a computer with a sound card into a sort of oscilloscope but I've found that because of latency and bandwidth limited by the sample rate, these are really not very satisfactory for anything other than visualizing a waveform. You don't need a very sophisticated scope for audio work, but if you're inclined to get one, it's worth getting a real one.
Even low-end new scopes can cost several hundred dollars. At the risk of having to write a follow-up rticle on how to repair old test equipment, I'll suggest that your best bet is to look at the used market. There are many reputable dealers who offer refurbished and guaranteed Tektronix or Hewlett-Packard scopes for a few hundred dollars. These scopes, originally selling for several thousand dollars, are very well designed and built, and almost never wear out or fail, even after dozens of years. A Tektronix 465 at today's used price is a great buy and a very useful tool in the audio shop.
With just about any modern computer having plenty of number crunching horsepower, it's tempting to use it as a signal generator and analyzer. There has been quite a bit of audio test software developed in recent years that utilize a Mac or PC, and a conventional sound card for I/O. When configuring a computer as a test instrument, remember the golden rule – your software-based test equipment will only be as good as the I/O hardware you're using. While there's still room for computer-based test equipment with specialized high quality I/O such as the Prism and Audio Precision products, for routine shop work or making acoustic measurements (which by nature, are fairly low resolution and quite variable) you can get a lot of mileage out of a computer, a respectable sound card, and an application program. This is often a good way to put your last generation's laptop computer to use.
SmAArt Live from SIA Software (a division of LOUD Technologies) is a popular acoustics analysis tool for touring sound engineers, but it's also becoming popular for setting up and tuning an installed sound system or a studio. In addition to the real time spectrum analysis and energy-time curves, SmAArt is also capable of measuring the system transfer function in real time using any source. This allows you to continuously monitor the frequency response of a hall during a concert by using the PA mix as the source. In addition, SmAArt has a built-in interface capable of controlling most of the popular remote-controllable signal processing devices such as equalizers or loudspeaker managers. This allows you to adjust, for example, crossover frequency or speaker delay right from the SmAARt program while watching the display of output vs. input.
ETF is a Windows-based acoustical analysis program which works in conjunction with your own sound card, microphone, and preamp (they recommend using a Radio Shack SPL meter!) providing measurements of frequency response using either a frequency sweep or MLS statistical test signal, RT60, SPL, and it offers a 3D “waterfall” display of frequency response over time (energy-time curve). A demo version to allow you to verify that ETF will run properly on your computer and with your audio hardware is available as a free download (www.etfacoustic.com).
The RightMark Audio Analyzer suite (RMAA) is a Windows-based program that turns your computer into a fairly complete test generator and measurement system. Originally designed to test the sound card in a computer by looping the output back to the input, it's useful (once you've established the limitations of your system by measuring your sound card) for measuring the characteristics of outboard equipment.
Operation is simple and results are fairly clear. Select a test and RMAA generates a test signal and applies frequency and level analysis algorithms to display frequency response, TMD, IMD+Noise, usable dynamic range, and provides a spectrum display to evaluate noise content. With a microphone and preamp, you can do some rudimentary speaker testing. There's a non-real-time mode for testing recorders. It also includes a utility which creates a test CD for checking the playback of a single-ended unit such as a CD player. Now, how much would you pay? That's the best part – RightMark Audio Analyzer is free for the downloading from audio.rightmark.org.
Over on the Mac side, SpectraFoo from Metric Halo is both a comprehensive metering system for recording, mixing, and mastering and an audio analysis program. SpectraFoo has probably the most comprehensive digital bitstream display and analysis of any of the tools discussed here. As with SmAArt, system transfer function can be measured using any source including music. The test generator offers white and pink noise, bursts, sweeps, and plain ol' sine waves.
That CD player that you replaced with a DVD player last Christmas can be recycled into a useful test generator. The Sound Check 2 Test and Demonstration CD from InnerStudio is an example of one of several commercially produced test CDs. Another is published by the National Association of Broadcasters (NAB). Both contain a wide range of test tones, pink noise, and band-limited pink noise for acoustic measurements, as well as some off-the-wall specialized test signals. For example, the Sound Check CD, co-produced by engineer Alan Parsons and acoustician Stephen Court, in addition to test signals, includes a wealth of musical instrument and vocal tracks, sound effects, and five minutes of SMPTE timecode. The NAB disk includes IEEE standard VU and PPM meter response tests.
You can of course roll your own test CD. There are many utilities that generate fixed frequency or swept WAV files and pink noise that you can burn on to your own disk. You won't get the comprehensive tests found on the commercial disks, but you can custom-make whatever you need, from an hour of 1 kHz tone available whenever you want it to a set of alignment tones for analog tape adjustment or reference levels, pink noise for sound system adjustment, or your favorite reference music for auditioning monitors.
The advantage of a test CD is the convenience. The downside is that most CD players have unbalanced outputs, and the maximum output at full scale digital level is usually in the vicinity of around +12 dBu. This may not be hot enough to fully drive a modern digital input, but it's fine for routine bench testing.