Thứ Tư, 3 tháng 8, 2011

RightMark Audio Analyzer

 RightMark Audio Analyzer

ROSE COLORED GLASSES (updated 3/3/11): RMAA, in some ways, is impressive software for the price (free). A lot of people, websites and organizations are using RightMark Audio Analyzer ( aka RMAA from ) to publish audio measurements. It's a useful tool when you use it properly and know its limitations and I'm thankful for the work the developer put into it and making it available for free. But RMAA has many limitations and potential problems. And a lot of people use it incorrectly and publish misleading RMAA results.

THE SHORT VERSION FOR NON-RMAA USERS:  You can rarely compare one person's RMAA results to another person's RMAA results. This is especially true for evaluating anything other than a PC audio interface (aka “soundcard”). It's very easy to have a better piece of gear actually show worse results on RMAA because of different test conditions. So don't put too much trust in any RMAA results unless you made them properly yourself under carefully controlled conditions. Unless you know the absolute levels used, load applied, PC sound hardware used, and settings on the device being tested you just can’t trust the results. Changing any of these things can cause a greater change in the results than most realize.
SETUP IS EVERYTHING: With RMAA there are many things to get wrong. Examples include hidden mixer controls, sound processing settings in the PC driver, sample rate conversion by the operating system, level settings—both at the PC and for the device being tested, loading, ground loops, cabling, etc. And if you’re doing loopback testing, you have no way of knowing if poor performance is caused by the A/D, the D/A or both. Even having a cell phone nearby can corrupt RMAA measurements with no error indication at all—just erroneous measurements.

FOR RMAA USERS: If you use RMAA I'd suggest reading through the technical points below so you can be aware of how to get the most from it, what to watch out for, its limitations, and perhaps some better alternatives.

14 IMPORTANT THINGS RMAA DOES NOT MEASURE (revised 4/9): There are several things that can make an audible difference in sound quality that RMAA does not test for. And many of these are difficult or impossible to do at all with a soundcard-based test setup even using other software, test jigs, etc. They include:
  • MAXIMUM CLEAN OUTPUT LEVEL - RMAA has no concept of absolute levels. So it can't measure voltages, power outputs, etc. So you have no way of knowing, for example, how loud that portable player or headphone amp can play without obvious distortion. You also have no way of knowing what level you’re testing it at. Is it right on the edge of clipping or is the level so low it’s affected by the noise floor?
  • OUTPUT IMPEDANCE - Anything designed for driving headphones including PC's, portable players, headphone amps, USB DACs, audio interfaces, pro gear, etc. has an output impedance. It varies widely and can have a huge impact on the sound quality--especially with certain types of headphones (see Headphones and Amp Impedance). RMAA does not measure output impedance.
  • DAC LINEARITY - A key difference between a cheap D/A converter and a good one is how they behave at very low levels. Many exhibit considerable non-linearity at low levels. This is especially important for the huge number of devices that have digital volume controls. Because RMAA can’t measure absolute levels it can’t directly measure linearity.
  • SQUARE WAVE PERFORMANCE – 1 Khz and 10 Khz square waves reveal a lot of information about analog and digital audio components. This includes stability, speed, bandwidth, rise time, and for digital devices, the effects of the digital and reconstruction filters. Filters in digital audio gear differ widely in their design and can have audible side effects. A square wave can help reveal the type of filtering used (linear phase, etc.). RMAA doesn't have a square wave test. And even if you use oscilloscope software for a soundcard, it will only measure up to the cut off frequency of the PC’s sound interface. This is often too low to see many ultrasonic/RF, ringing, oscillations or instabilities.
  • JITTER - Jitter has been proven to be audible in some circumstances. RMAA provides no accepted way to evaluate jitter performance.
  • VARIOUS TWIN TONE TESTS – RMAA has a single IMD test that’s similar to the SMPTE test, but there’s no capability for other important twin tone tests such as the popular CCIF 19 Khz/20 Khz standard. These tests can reveal problems the SMPTE test does not. And, from what I’ve seen, RMAA doesn’t calculate SMPTE distortion properly.
  • THD20 & TIM – It’s very useful to measure THD at 20 Khz for a variety of reasons. It’s a good indicator of stability, feedback loop design, and even slew rate and TIM (Transient Intermodulation Distortion). To perform this measurement properly, you should be able to measure the first 3 harmonics at 40 Khz, 60 Khz and 80 Khz. Even if you get RMAA to work with at a 192 Khz sampling rate, it still can’t perform a proper THD20 measurement. And it has no option to adjust the measurement bandwidth of its THD sweeps.
  • POWER vs THD: RMAA cannot perform the classic measurement of output power versus THD. This is a standard benchmark amplifier test that’s widely performed as it’s very revealing of the amplifier’s behavior into various loads and various frequencies. RMAA has no way to do it nor am I aware of any other software that allows this sort of measurement with a sound card.
  • SELECTIVE SPECTRUM TESTS: It’s very useful to perform different spectrum testing with various input signals. RMAA doesn’t give you any control over its signal generator or its analysis. You cannot control the FFT points, averaging, weighting, filtering, etc.
  • RESIDUAL ANALYSIS – Analyzing the residual distortion products can be very revealing—for example it can determine if an amplifier suffers excessive crossover distortion. RMAA has no way to do this.
  • REAL TIME RESULTS - Many problems may only show up briefly or intermittently. And many adjustments (like finding the clipping point) are best performed in real time. For example, maximum output is commonly defined as 1% THD. So with a real time audio analyzer you just raise the level until it reads, in real time, 1%. With RMAA it takes a long time to run a test sequence which makes if very difficult to see what affect adjustments have on the results. And if you have a bad cable, or some other intermittent problem such as outside noise or interference, during the test, you will likely never know like you would with a real time analyzer. I’ve had a cell phone randomly mess up RMAA measurements for example in ways that just made the product being tested look bad.
  • SLEW RATE – RMAA cannot measure slew rate which, especially for audio power amplifiers, can be an important measurement.
  • DELAY & LATENCY – RMAA cannot measure delay and latency for digital hardware.
  • HARDWARE LIMITATIONS – Depending on the PC sound interface used, and external equipment, test aids, etc, RMAA can be unable to perform some tests. For example bridged and certain other amplifiers cannot have any of their output terminals grounded or connected together. But typical PC sound interfaces have common grounded inputs. To make matters worse, these grounds often go back to the PC’s 3 prong power plug and are also AC mains grounds which creates even more problems. Some of these issues can literally seriously harm the device being tested and/or your PC’s sound interface or even the PC itself.
ALL THE NASTY DETAILS (non geeks probably want to skip this section):

If you're not familiar with RMAA, here’s a typical screenshot. Note the 3rd column in the results labeled "RMAA Problems" and the highly questionable numbers. More on that later (click for larger):
PC HARDWARE ISSUES: RMAA results are only as good as your soundcard and how RMAA is used. Lots of people are running RMAA on whatever sound hardware their computer came with. And that's often a rather serious limitation. Built-in sound hardware is prone to all sorts of problems--especially the A/D section which is often only used for digitizing voice these days so manufactures don't put much effort into making the built-in A/D signal path work especially well.

Plus nearly all built-in sound hardware amounts to little more than a cheap CODEC chip stuffed onto a crowded and electrically very noisy motherboard. Modern CPUs create spikes of current that can exceed 100 amps. That generates a lot of electrical noise and it's hard to keep it all out of the nearby audio circuitry where it can confuse RMAA measurements.

And if you're using your PC's audio output, other applications or your operating system can play random sounds while you're running the test (like you get new email). Even CPU loading and hard drive activity can affect the noise floor of built-in audio hardware--especially the line inputs and A/D. And while RMAA should log an error if extra sounds are played during the analysis, it just corrupts the results in non-obvious ways instead (more on that later).

And some PC's (especially laptops) don't even have a line level input. They only have a microphone input. Trying to use a mic input for line level audio is a disaster and will yield very misleading results. Yet some still do it and publish the results with no indication of how misleading they are.

Lots of PC's and sound cards perform various audio processing in the digital domain. It's sometimes impossible to turn all this processing off hence many PC sound devices are not "bit accurate". Some perform internal sample rate conversion, for example, regardless of how they're configured (Creative's devices are famous for this). Others attempt to optimize or enhance the audio in various ways. This internal processing affects the accuracy of RMAA in some odd ways and works against you when trying to use PC sound hardware as a lab instrument for measurement.

Finally, even lots of outboard PC sound devices have problems--especially with their A/D side. Some have microphone preamps always in the circuit which seriously compromises their performance for line levels. Others have very inexpensive A/D chips and circuits in them. Some are not bit accurate, have noisy USB-derived power supplies, problematic level controls, etc.

GAIN MATCHING: With many PC sound interfaces, the mixer settings operate in the digital domain. And when you lower the level settings you may lose digital bit resolution. The effective number of bits might be 16 bits with the controls maxed, but if you have to reduce the gain, you may get less than 16 bit resolution. At some point this can become the limiting factor in the test setup (insufficient ENOB). This is especially critical for measuring very low levels like noise or crosstalk (channel separation).  And changing levels is made more difficult because PC sound hardware levels controls are rarely calibrated in known increments (like dB). And even the few I have seen that are calibrated in dB (in software) don’t “track” well at all with the actual settings. For example, some steps are close to 1 dB but at other settings I’ve seen as high as 3 dB. Again, the manufacture never intended for it to be used as a lab instrument. So they probably didn’t much care about such accuracy as it’s not a big deal for 99% of applications.
So, ultimately, you have to verify levels with an external meter and not just any meter (more on that later). And any changes in the controls may require making external measurements  over and over again to re-establish the actual levels and channel balance. This can get very tedious when dealing with audio signals that often vary over a wide range during testing. And if you don’t re-adjust the level controls to operate the A/D near full scale, you can lose lots of resolution and dynamic range making the device being tested seem to perform much worse than it really does.
There can be multiple issues here depending on the topology of your PC sound hardware and how it does gain control. Some of the external USB or Firewire devices have their own physical pots for levels controls. I’ve never seen these calibrated in any meaningful way. They typically cover an extremely wide range of gain (65 dB for the E-mu products). These knobs, because of their wide range, are extremely “touchy” and it’s just about impossible to make precise small adjustments or get the two channels precisely balanced. And the settings can even change on their own which can really throw off RMAA if you don’t realize it happened during a test.
Better devices use programmable gain amplifiers (PGAs) ahead of the A/D converters and control these PGAs from software. This can be a good solution, but only if you’re given calibrated control in sufficiently fine steps, and the PGAs themselves are accurate. I’ve yet to find any that meet these criteria but they may exist. Many of the less expensive ones are not terribly uniform in their steps over their full range. PC sound hardware is not designed with absolute precise values in mind. But that’s exactly what’s needed for a good measurement set up.
Some PC sound hardware uses a fixed gain amplifier and some sort of “digital pot”. These are often less accurate than the PGAs above. And often have fewer, and more coarse, non-linear steps. And the high fixed gain amps can have more noise issues.
Some PC sound hardware doesn’t have any sort of attenuator or PGA on the inputs. They simply design the circuit so the maximum expected input corresponds to roughly 0 dBFS in the A/D and then do everything in software from there. While this is fine for a person’s voice for Skype, it’s far from ideal for measurement purposes. As the signal level is reduced, the effective resolution of the A/D goes down with it.
So the gain increments are often too large, variable and/or unknown. For example, if you want to remove a 0.7 dB channel balance error, but only have 2 dB steps to work with, you can’t. And they often don’t correspond to any convenient number of dB because they’re derived from whatever the hardware supports. Most electronic volume control chips, for example, only have 64, 128 or 256 steps. So what you get is the full range divided by 64 uneven increments—whatever that happens to work out to.
Proper audio analyzers have calibrated analog stepped attenuation built into the input and output circuitry so regardless of the signal level they maintain the resolution of the D/A and/or A/D—to always be within a few dB of full scale. You can also set levels in their software to a resolution of at least 0.01 dB. I’m not aware of any PC audio devices that can do anything even close.

NO ABSOLUTE LEVELS: Even if you run RMAA on a decent PC audio interface with good specs, you still have no idea of the absolute levels. Are you testing too low? Too high? Near the device’s clipping point? This also prevents knowing the true output capability of the device being tested (i.e. dBu, dBv, dBm, volts, watts, milliwatts, etc.). It also means the channels may not be properly balanced which affects RMAA's measurements—the RMAA calibration routine compensates for channel imbalance in your device when you really want to be measuring that imbalance. The actual levels can make a BIG difference in the RMAA numbers you get.
AN EXAMPLE: Joe posts his RMAA results for Player A, and tested at a relatively low level. This will yield worse noise numbers but probably fairly good distortion numbers. Bill posts his results for Player B but, unknown to Bill, it was on the edge of clipping. So the noise numbers look really good but the distortion numbers look bad. It’s hard to draw many valid conclusions from the published results because the tests used very different levels. But that doesn’t show up anywhere in the RMAA report or output data. Player B might really be the better player, but Bill had no idea it was starting to clip so the results make it look like the inferior product.
REFERENCES ARE CRITICAL: The proper way to do audio measurements is to use standard reference values. For example 0 dBm is one milliwatt, 0 dBu is .775 volts RMS (consumer audio), etc. Without knowing the actual levels of the signals being measured by your PC audio hardware, you have no way to use proper references or even know what the levels are so you can test the next device, or even retest the current device, at the same level. And even if you use an external meter (most of which are designed for 60hz AC power and not even close to accurate across the audio band) you're still likely stuck with the un-calibrated mixer/level controls of your PC audio device. So the instant you change any of the controls, your meter measurements are rendered useless.
NO STANDARDS: RMAA isn’t very clear about what standards it conforms to, if any. Did the guy who wrote the software build in A Weighting to the noise measurement? What if you want A Weighting and it’s not there? If you use a higher sampling rate, are the THD values only calculated over the audio spectrum or do they also include any out-of-band noise shaping, or in the case of a Class D amp, ultrasonic carrier artifacts, etc? There are lots of unanswered questions about RMAA works internally and it makes comparing RMAA data to real measurements that conform to accepted standards difficult and dubious. I even suspect RMAA was designed “backwards” to match the specs of a reference soundcard which is a seriously flawed approach.
TEST LOADS ARE CRITICAL: Loading is often overlooked. For example, portable MP3 players often measure far better unloaded driving just the line input of your PC audio device than when properly loaded with real headphones or a suitable test load. Nearly all the RMAA measurements I see published never mention anything about what the device was loaded with so I'm guessing it's usually nothing. This results in essentially worthless test results! Some measurements, like crosstalk, are hugely affected by the load. And if you're trying to measure a line level output (rather than a headphone output) most PC sound devices don't have a well defined input impedance. Here again, this makes repeatability, comparing, and verifying results difficult—especially at higher levels of performance.

USING HEADPHONES AS THE LOAD: This can be good and bad. The good news: It presents a realistic non-linear load to the device being tested--especially if they're the very headphones the user intends to pair with the device. The bad news is someone else needs the exact same headphones (which they likely don't have) to conduct a fair test that's valid for comparison. And for many devices the headphones will alter the measured frequency response in ways more related to the headphones than the device (and example is below). And, unknown to many, headphones also act as microphones. So, for example, they pick up background noise in your room and this can raise the noise floor measured by RMAA making for misleading noise and distortion measurements. A proper resistive test load is better if you want to compare the results, see the true frequency response of the device, and avoid the "microphone effect".

Here are the RMAA results using a 15 ohm resistive load, and 2 different kinds of headphones--the UE SuperFi 5 Pro's have a really wide impedance swing from about 10 ohms to 85 ohms and a pair of Sony MDR-EX76's that only vary by a few ohms at higher frequencies. The levels were within 2dB of clipping at the worst case frequency using a Benchmark ADC1:

The best numbers are with the 15 ohm resistive load. When using the Sony's the THD more than triples and the noise floor rises up a few dB. And what's with those SuperFi 5 numbers? Does the otherwise low distortion source suddenly produce 4% THD? No, but RMAA makes you believe it does! What likely happened is the signal was clipped because of the frequency response swings. But you have no way of directly knowing that, and from the average person’s point of view, the levels were properly calibrated. Here's a "zoomed in" graph showing what effect the Sony headphones have on the frequency response:

So if Joe tested with a "lab" 15 ohm load you'd see the white graph in his results. If Bill used his Sony headphones you'd see the green graph published and probably think less of the product being tested. And if Bill used his SuperFi 5's here's what you'd see (in green)—a whopping 15 dB of response deviation:

Here's a Sansa e260 MP3 player with no load and a 15 ohm load:

Notice it has more than 8 times as much THD, more than 4 times as much IMD, the crosstalk is far worse, and the frequency response much worse when loaded. Here's the frequency response difference:

There's likely a coupling capacitor in the output amplifier of the Sansa. And, unloaded it has little effect, but with a typical load it's down -3dB at 40 hz. There's also a weird rise at high frequencies likely caused by marginal feedback design in the amplifier. Here's the swept IMD vs frequency:

Unloaded the distortion is down about -73dB worst case, but loaded it's more like -57dB which some might argue is audible. With no load, as many use RMAA, the Sansa rivals some of the better players out there. Loaded, however, it's closer to the bottom of the pack. It makes a huge difference! These are typical example and not some weird worst case test case.
GROUNDING: Even an external PC audio interface is grounded to your electrically noisy PC via the USB/Firewire connection. And the inputs and outputs share a common ground. These both can create potential ground loops, noise sources, or worse, with whatever device you're trying to test. If the device has bridged (analog or class D) outputs it may not take well to having the outputs grounded. Some amplifiers can oscillate or even self destruct if you connect their input and output grounds together externally. You can use transformers to isolate the device, but they introduce lots non-linear distortion on their own. Proper audio analyzers have differential and/or floating inputs and outputs that are not only isolated from ground but from each other.

OVERLOAD: If you're trying to measure something intended to drive speakers, it's rather likely to overload and possibly damage your PC sound input which can only handle, at best, a few volts of signal while speakers usually require tens of volts. You have to make an external divider network which is another potential source of problems and an accuracy issue. There's also still the grounding problem mentioned above which can literally damage the gear you're trying to test as well as your PC hardware.

SAMPLING RATE: RMAA is limited by the sampling rate of your audio hardware and what the drivers support. Often it doesn't work (or work right) above a 44 or 48 Khz sampling rate. This restricts the bandwidth of what it can measure to around 20 Khz. Many devices may have instabilities that show up at ultrasonic or even RF frequencies. RMAA is usually completely “blind” to these problems. See Testing Methods for an interesting real world example. And Class-D amplifiers are increasingly being used used in everything from MP3 players to A/V receivers. These amps switch at high frequencies and often put out noise that would be entirely missed by typical 44 Khz PC audio inputs. Even if you don't think it's audible, the amount, and kind, of out-of-band noise can still be a useful indication of the quality of the device being tested.

BIT DEPTH AND NOISE FLOOR: RMAA is typically limited to 16 bits resolution by either the PC audio hardware and/or driver limitations. But you often can't use the full 16 bit range because of gain limitations (see GAIN MATCHING above). So, in reality, you might end up with less (or similar) useable dynamic range than the device you’re trying to test.  Ideally the test set up should have far better dynamic range than whatever you’re testing. Even if you're lucky enough to get RMAA to work correctly with 24 bit drivers, you won't get anywhere near 24 bit performance from typical 24/96 or 24/192 PC sound hardware--you're lucky to get 17 or 18 bits of effective resolution (ENOB) due to the noise floor, power supply noise, grounding issues, cheap A/D and D/A converters, etc.
FALSE SECURITY: People often do a "loop back" test to first test their PC sound hardware. And, more often than not, RMAA reports fairly impressive results. So they think they're good to go. But there are often problems being masked, or caused, by one or more of the above issues. For example, level, loading, and grounding issues won't show up in a loop back test. And then there are the things RMAA doesn't test for at all.

RMAA DEFECTS: On top of all the above issues, the RMAA software itself is buggy and prone to problems. The last release was roughly 2 years ago and development has apparently been abandoned. Some of the FFT/math used to calculate the results is apparently wrong and/or has serious limitations that are neither properly documented nor obvious. There are lots of quirks and some obvious bugs. Try right clicking on any of the icons for the combined results graphs--instant total crash and you lose all your results.
Perhaps most important, RMAA often just outputs bad data instead of indicating an error condition with the measurement. To use an extreme example, you can stop playback of the test file half way through the tests, and instead of issuing an error, RMAA goes right on calculating and randomly spews out half truths and half garbage. It's obvious the developer didn't bother to put in many checks for validity of the input and output data. This can take much more subtle forms such as bad cabling, random noise (RFI from a cell phone, sound events from your PC, etc.), etc. not being detected during the test.

Here's an example of RMAA inventing impossible results out of nothing. It was run with no test file at all and, instead of timing out, it ran without a single error or warning and here's the result:

Wow, 245% IMD, that's pretty bad! I didn't know you could have more than 100%. But the THD is only 1.7%. The frequency response doesn't look so good though. I joke, but you get the idea. This doesn't inspire confidence in the software. And then there are messages like this one:

So clipping occurred but 0.000% of the samples were actually clipped? Hmmm. The funny thing is, in this case, there was zero clipping by the ADC (my Benchmark ADC1 has clip lights that stay on if it even clips a single sample until you reset the LEDs). And what's even more interesting is lowering the level by several dB and running the test again (as the error suggests doing) yields the exact same error! It appears to just be a case of RMAA getting confused trying to test this particular device (that tests just fine on my Prism dScope). Again, this does nothing to boost confidence in RMAA results.
RMAA DESIGNED BACKWARDS? I’ve seen enough weird results from RMAA I have to wonder if the developer designed it backwards to match the specs of a reference soundcard? Whatever test signals and analysis being used might have been “tweaked” until he got numbers that roughly matched the specs of some soundcard he respected. If true, this would explain a lot. Ideally RMAA would just do all the math correctly and the results would be verified against a professional audio analyzer to make sure the math was accurate. But it often doesn’t agree and I’m at a loss to otherwise explain why unless there are either bugs in the calculations or it was designed to produce the expected numbers when testing soundcards.

RMAA TIPS: So how is a person supposed to make RMAA-style measurements more accurately? First, use the best PC audio hardware you can. A good device would be something like the RME Fireface UC but, in my experience, their proprietary low latency drivers designed for multi-track recording don’t work well (or at all) with RMAA. Typical devices that do work are more like the E-Mu products:
- Creative/E-Mu 0202 - This device has pretty good A/D and D/A performance but it has un-calibrated level controls that cover a wide range of gain and are very "touchy". This makes setting the level the same for both channels just about impossible. So you can’t know the actual channel balance of the device you’re trying to test. The 0202 also has a mic preamp in the signal path on just one channel. Not surprisingly, that channel has higher noise than the line-in only channel. So any device you test with the 0202 is prone to rather lopsided results, weird crosstalk numbers, etc. It's also at the mercy of the noisy USB power supply. And it has serious issues on some PCs with its proprietary drivers. So, overall, I can't recommend it or many similar products.

- Creative/E-Mu 0404 - This device is better than the 0202 in that it has its own dedicated power, and both channels have the same signal path (unfortunately both have mic preamps). It also has slightly better metering but still suffers from the "touchy" gain control problem and the inability to exactly match the channels without some external help. See the section below on Levels. Another downside of the 0404 is it requires the same special drivers as the 0202. The drivers haven't been updated in years and especially can have issues with Windows 7 and 64 bit systems.
- Setting Levels - This is critical for many reasons. First, it's useful to know if the device your testing has a channel imbalance. But if you use the meters in RMAA to say adjust your E-Mu 0404, you will remove any channel imbalance anywhere in the signal chain. You can either feed known matched levels (i.e. verified with a meter) into both inputs and do your calibration, or use a "Y" cable to temporarily split one channel into two outputs to set the levels. That way you know any channel imbalance that shows up during testing is the device, not your set up.

- Measuring Actual Levels - As discussed earlier, it's important to know what levels you're really testing at. So it's best to use an external true-RMS meter that can accurately read audio frequencies. Most cheap DMM's, even many that claim true-RMS, are only designed to be accurate around 50 – 400 hz. They often "roll off" dramatically or behave unpredictably at higher audio frequencies. True RMS calculation is non-trivial and it’s expensive to have it be accurate at higher frequencies. So most reasonably priced meters don’t bother as they’re not intended for audio use. So if you're testing at say 1 Khz, they might read only half the real value. Looks at the specs for the meter, or if you already have one, test it on the output of your sound interface with test tones across the audio spectrum to see how flat it is (or isn't). It's a hassle to do the math at different frequencies to apply a correction factor but it’s cheaper than buying a more expensive meter.

- Choosing a Test Level – If you're testing a portable player or headphone amp, how far do you turn up the volume? I would suggest picking a value that's a bit below the maximum output to help assure the headphone amp won't be close to clipping. But if you test at too low of a value, the noise and distortion numbers will be much worse.
- A Good Reference Level - A good guideline for headphone outputs is 1mW into 32 ohms as that's used in the manufacture's specs of many devices and a value nearly any device can manage without clipping. P = (V*V)/R which means V = SquareRoot(P*R). So the square root of (.001v * 32 ohms) is 0.179 volts or about 180 mV RMS. This also happens to be about the typical power level most listen to their music at with typical headphones. So use your (accurate or calibrated) meter to set the player/amp to 180 mV RMS while playing a 0dBFS 1 Khz reference file. Then set your levels on your PC sound interface/mixer controls to be just under clipping (0 dB) while monitoring the 180 mV signal. But, beware, such low reference levels may challenge your PC hardware when making noise and crosstalk measurements.

- Test Your PC Audio Interface At The Same Level - Regardless of what sound hardware you're using, test it with a "loop back" test by connecting the inputs to outputs as instructed on the RMAA website to know what the "baseline" performance is. Try to do this test at the same absolute level you're going to use for testing your piece of gear (i.e. the 180 mV mentioned above). The signal to noise ratio, distortion, crosstalk, etc. of your PC interface will change at different test levels.

- Use the Proper Load - See the section above on loading. Ideally use a resistive test load. For devices intended to drive headphones, use resistors in the range of 15 - 32 ohms or test at several different impedances if you want. Even small 1/4 watt resistors are fine. They should be carbon film, and not wire wound. If you're more curious how the device will perform with your headphones, use them as the load. If you use headphones put them in a quiet place (not on the same surface your PC is on) and keep the room as quiet as possible during the test.

- Use Good Cables and Test Them - I'm not talking about $150 audiophile cables, but $1 ones from eBay are not a good idea either. Because of the way RMAA works, if you have a cable problem you may never know and just get marginal numbers. So test all the cables in the signal path by listening to a test tone on them while moving them around. If you hear static, or worse, find the culprit and replace it with something better. And, when testing crosstalk of headphone outputs, cables become a huge problem if there’s any extra wiring in the shared ground leg to the load.

- Run the Tests Multiple Times - A good way to help rule out intermittent problems is to run the tests multiple times. If you get consistent results, the results are more likely to be valid. If the results vary by more than 5%, something is likely wrong.
- Use a Real Oscilloscope  - If possible, it’s worth checking the output, especially driving a real world load, with an oscilloscope that has a bandwidth out to at least 5 Mhz. This may show problems not seen in RMAA using a sound card limited to 20 Khz – 96 Khz. You can also measure the slew rate on a real scope and better evaluate square wave and impulse performance.
HOW I DO IT: For RMAA testing I use a Benchmark ADC1 for A/D, Benchmark DAC1 Pre for D/A, a 6 1/2 digit Agilent bench DMM that's extremely accurate and flat from 10 hz - 100 Khz for exact levels and other measurements, and one of several oscilloscopes with 60+ Mhz bandwidth. See my blog post Testing Methods for more informatio