This is Only a Test

Were you one of those people who hated tests in school? Does staring at a bunch of little bubbles to be filled in make your stomach upset? Do you still break into a sweat whenever you smell a freshly sharpened #2 pencil? If so, throw your mouse toward that back button because this month I've got two test CDs to talk about. While one is a straightforward piece of CD-player gymnastics, the second tests YOU. There's even some homework, and you'll need to plot a graph in the process. While it's not quite the SATs, the test discs from Digital Recordings are still pretty serious.

CD-CHECK

I've always thought that error-correction abilities were underemphasized in high-end CD-player reviews. As I see it, if the digital data isn't read correctly in the first place, no amount of D/A magic will make it sound right. One of the first widely available discs for testing the performance of CD players was the French Pierre Verany set. Quickly adopted as a standard reference for equipment reviews and evaluation, the Verany set consists of a number of tracks with successively increasing error rates via larger sections of "drop-outs." Testing is done by listening to tracks until you hear audible distortion, which is usually a nasty skipping sound. The end result is the track number of the last size error that was corrected by the player. A typical CD player will track defects up to around 1mm in size, and error correction up to 2.5mm is not unheard of.

While very useful, there's one substantial flaw in the Pierre Verany drop-out tests. They use a simple sine wave as the signal you're hearing. It's not very difficult to construct a player that will interpolate for data it can't read correctly, and a simple tone is very easy to recreate that way even if parts are missing.

Until recently, there wasn't really anything better available. In 1996, Digital Recordings, a Canadian company involved in audio research, introduced CD-CHECK. This five-track CD uses test signals specifically designed to make CD errors as audible as possible. When using the Pierre Verany set with error-detection hardware they developed, the company found many problems with reproduction that were obvious to the equipment but transparent to your ears on a sine-wave sample. The stated goal of CD-CHECK is to make every error audible, so that anyone can easily listen to the disc and determine the correction capabilities of his source player with the highest degree of accuracy possible.

CD-CHECK starts with a spoken intro describing what you should listen to, along with some warnings about potentially dangerous listening volumes. After that, there's a 20-second-long signal with an odd tone that's similar to a warble, but more complicated. It's not something you can describe easily. Following the test signal, there are 15 minutes of silence, then the next track of signal/silence. The first track is stated to test "standard manufacturing errors", and any CD player that fails this one is ready for the shop. Bench testing is one of the stated applications of CD-CHECK, along with general maintenance. Digital Records suggests using the CD right before your warranty runs out, to detect the subtle failures that are the precursor of obvious problems. I've noticed myself that dying players often start to have problems tracking some discs months before they totally stop working, so a regular CHECK-up isn't a bad idea at all.

The second track is where the real work starts, with errors of 0.375mm. I usually start immediately on this one, because the first track is rarely ever a problem and I hate having to skip over its spoken intro. After several comparisons, it became obvious that the error-size measurements listed were not comparable to the Pierre Verany drop-out sizes. I found players that easily tracked 1.25mm errors on the older test set, yet they couldn't accurately handle even the 0.750mm error of CD-CHECK's track 3. The Digital Recordings test suite is often considerably more difficult to track at the equivalent size. But there were exceptions to that, where the two test reports were very close. So I consider it impossible to "translate" between results on the different discs. You can't predict the score on either set of tests based solely on the data from the other.

The fourth and fifth tracks test at 1.125mm and 1.5mm, respectively. Some of the better-correcting players (typically inexpensive mass-market ones from big names) I've tried handle track 4, but I have yet to find something that will play all of track 5. So even though 1.5mm doesn't sound all that big if you're used to the Pierre Verany scale, with the test stimulus of CD-CHECK it seems quite sufficient to humble all but the best correcting units.

I'd really like to believe that this superior error-detection scheme will catch on, but unfortunately the Pierre Verany tests are too entrenched in the industry. Everybody from Stereophile to Stereo Review is using that industry-standard disc set, so it's awfully tough to unseat. As John Atkinson wisely pointed out to me one day when we were discussing audio test procedures, it's outright dangerous for someone to render an official measurement in print using an unproven system. If you make disparaging comments about a CD player's error-correction capabilities as part of a review, and legal proceedings from the angry manufacturer follow, you are on very dangerous footing if you used a non-standard test regime.

One interesting comment in the literature I received from Digital Recordings involves using CD-CHECK to test the effectiveness of the usual CD-player tweaks: mats, rings, green pens, and the like. Their logic seemed to suggest that in order for any of these modifications to be useful, they must lower the error rate of the player on some discs. Apparently, their testing revealed that some tweaks actually decrease the error correction of the player, inserting artifacts that some people happen to like the sound of. While I don't doubt that, I don't think you should generalize from that and dismiss all such modifications simply because some actually introduce problems. There's more to what makes the sound quality of CD playback vary from system to system than just the error rate. In any case, it's an interesting test to add to any look at CD tweaks.

I've become quite fond of CD-CHECK. The test is so simple and quick that I can get good results in a matter of seconds; pop it in, press skip track a couple of times, and you're done. The difficulty scale seems broad yet fairly sensitive, although another track or two in the middle wouldn't hurt. At the retail price of $39.95 USD, picking up CD-CHECK can easily be justified just for helping to select one CD player. For the independent listener who doesn't want to rely on test equipment, CD-CHECK offers an unparalleled CD-player evaluation tool. Nothing else I know of works quite as well for really ferreting out which players correct errors well and which don't.

AUDIO-CD

The second of Digital Recording's test CDs with useful capabilities but blasé names is AUDIO-CD. Designed as a way to run an audiogram test of hearing ability that can be used in the home with inexpensive headphones, this CD lets harried audiophiles too busy to schedule an appointment with a professional audiologist find out useful information about their ears.

The idea behind AUDIO-CD is straightforward. If you play a track from a frequency where you ear is most sensitive, that let's you determine how your hearing is at other frequencies by turning the volume up until you can hear them. After initial setup with this disc, you use 24 tracks spaced appropriately through the frequency range. Each one starts off very quiet, then gradually increases by 1dB each second, with an 80dB range. By looking at the time when you first could hear the test signal, you end up with how many dB down your hearing is at that point. While the idea is simple, actually constructing appropriate test tones and calibrating everything is the hard part. Digital Recordings has taken care of that. All you need is a CD player and a set of headphones. It can work with speakers, too, but the accuracy may not be as good, and you'll have to cover each ear in order to check the other.

I started out with the best system I could assemble for headphone listening. Parasound C/DC-1500 as transport, Lexicon DC-1 as DAC, Cosmic HeadRoom amplifier, and Etymotic ER-4S headphones. After a few minutes of testing, it became obvious that the Etymotics were not the right headphones for this. While the insulation from the environment is very helpful (quiet listening conditions are essential for this kind of test), the microphonics of the headphone cable were problematic. Every time I breathed or squirmed in my chair listening for the tone, that injected a signal that interfered with the test. Even worse, they are so sensitive that I can hear some sort of high-frequency modulation every time the test signal pulses. This is presumably something slightly noisy in the DC-1 which I've never heard under regular listening conditions. The noise was audible even if the test tone wasn't, which made it hard to distinguish when the tone really started.

Ultimately, I found the Beyerdynamic DT-250 best-suited for using AUDIO-CD. The better your headphones, the more accurate the test, but Digital Recordings claims that even inexpensive portable system with accompanying headphones can be used successfully.

During the approximately 15 minutes I was testing, I discovered just how loud the invasions into my listening room are. During that time, I heard two planes go by, the refrigerator compressor kick on, and some church bells ringing. Last time I try a test like this on a Sunday. After I was done, I had a list of numbers already calibrated in dB. So when I note that it was 70 seconds before I heard the 20Hz signal in my left ear, that means my threshold there is 70dB louder than my hearing in the midrange. By noting my left ear didn't pick the signal up until 75dB, I can tell it is less sensitive at that frequency. This reverses at 40dB, where my right is 7dB more sensitive. Looking over the data and plotting it against the average lets you tell if you've got any quirks in your hearing to worry about. There is a program you can run at their website that graphs a full audiogram for you. I used a simple spreadsheet myself. The CD booklet includes a form you can photocopy and mark up, which also works just fine.

While this initial plot is helpful, there are a number of long-term uses to AUDIO-CD once you've established a baseline under typical conditions. This time of year my high-frequency hearing is worse than normal, due to congestion from seasonal allergies. I can quantify that now, once I get decongested and repeat the test. Similarly, you can check to see if medicine, environment, or other factors you suspect are bothering your hearing.

One unpleasant test you can try is measuring your Temporary Threshold Shift (TTS). Unless you're exceptionally lucky, I'm sure at some point you've been to a concert or similarly loud event where at the end of the night you had a ringing in your ears. That ringing means you just lost some of your hearing sensitivity, permanently. But even before you can see if there's a long term effect, you can measure the temporary loss. As soon as you get home, run through AUDIO-CD and see what your thresholds have changed to. Dr. Marek Roland-Mieszkowski of Digital Recordings assures me that measuring TTS accurately is scary enough to inspire a person to take earplugs with them everywhere. I know I'm leaning toward that myself on the weekends, because so many of the places I go out to anymore play their music at very high volumes, easily in excess of 100dB. The tell-tale ringing is there when I get home, and the damage stays long after I've forgotten the drinks I was enticed into leaving my apartment for.

Even though it's a bit disturbing to think about and measure hearing loss, AUDIO-CD lets you check your own ears at your convenience. The only way the results could be more private is if they shipped the CD to you in a plain brown wrapper. Anyone who would like to find out more about how they hear under various conditions should find the $39.95 AUDIO-CD costs to be an investment well worth making.

.....GS (gsmith@westnet.com)

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