Digital First Aid

Quick and easy solutions to some of the problems encountered when connecting digital audio equipment.
First Aid
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Who's in charge?
The most popular e-mail question regards the interfacing of digital recorders with digital consoles. Sometimes audio intermittently toggles from clean to the loudest possible fuzz. Other times, the signal appears on the meters, but the machine refuses to go into record. The first question to ask is which device is in charge. The second question requires, unfortunately, more questions.

Think about making a video-to-video copy. Transparently to the user, the recorder slaves to the player and pictures come out the other end. Now, switch to the audio world using a digital console and a tape machine. When tracking, the machine is slave to the console. When mixing, the console is slave to the machine. When overdubbing, which device is in charge? The answer is the master clock. Not every user has one, nor are many users versed in concepts beyond the simple making of connections.

Now, consider a digital console with several digital sources: CD player, Mini-Disc player and DAT machines. In this case, which device is in charge? Few of the standard players can slave to an external (master) clock, and the console cannot lock to all of them without each stereo input having a sample rate converter.

DIY jitter test
By Ken Smalley

The inability of most digital converters to resolve a jittery signal is not always apparent with typical program material. The human ear can resolve signals into a given noise floor, provided psychoacoustic masking is not constant. The following test allows the listener to increase the likelihood of jitter and determine if it corrupts the conversion process:

First, generate a 1 kHz, -0.5 dBFS digital audio signal. Pass it through 200 feet (61 m) of average-quality audio cable and into the digital-to-analog converter to be tested. The jitter produced should be enough to cause clock problems with most D-to-A converters.

Next, reduce the cable length 10 feet (3 m) at a time until no clock problems are noticed. Most will now assume the audio signal will be unaffected.

To prove jitter effects are still degrading the D-to-A signal path, monitor its output through a very sharp 1 kHz notch filter. A distortion analyzer with an audio monitor is usually ideal. In average- and poor-quality D-to-A converters, you will hear harmonic components of the digital signal's sample rate, for example, 12 kHz for a 48 kHz clock and 11.025 kHz for a 44.1kHz clock.

Smalley is with Studio Consultants, New York.

Sample this
OK, last challenge. A personal digital multitrack recorder/mixer is feeding a DAT machine via digital I/O. In record ready, the machine registers an input signal but will not go into record. The tape machine is sent in twice for evaluation; both times, there seems to be no apparent problem. After much questioning, it turns out that vari-speed is being used on the personal multitrack, resulting in the sample rate being out of range for the DAT machine. It wisely refrains from going into record. The fix, as it turns out, is to put a sample rate converter on the output to make the multitrack compatible with the outside world.

These are just a few real-world examples of stuff that I have experienced either firsthand or via e-mail. In many cases, I have been able to help people without ever actually seeing the gear. Imagine what I could do with gear really in my hands. Seriously folks, analog gear does not interconnect as easily as it should, and digital has turned many pieces of equipment into a sealed black box. When my cup is half full, I see lots of work for third-party developers creating the various missing links to pull things all together.

Final note
Every day, I get as feel as though I am getting smaller and smaller. I service DATs, ADATs and Tascam's DTRS (Hi-8 eight-track) format. My most popular (and sometimes missing) hex key is less than a millimeter. Tensions are 5 centimeter-grams to 40 centimeter-grams, and when necessary, capstan motors are being shimmed by a few thousandths of an inch. My eyes are augmented by magnifiers and oscilloscope probes, trying to zoom in and extract the maximum amount of data from what looks like fuzz.

Eventually, the machine with which I am working gets connected to another of its kind, and that is where my life begins to intersect with yours. Because I have been writing about digital tape machines from the inside-out (in print and on the Internet), e-mail pours in when the interconnected fail to communicate. Not only are panties in a bundle, but getting water from a stone also seems an easier challenge than getting answers from those who should know.

There are three sides to this coin. One is simply to read the manual a few times—admittedly hard to do when you are in a hurry. The other is for manufacturers to provide more online interconnect tips rather than point fingers at one another. I am not pointing any fingers yet, but I would suggest that users and installers create detailed feedback. How it gets to the right person and whether anything gets done about it will be fertilizer for future articles.

Ciletti spent 19 years chasing hums and buzzes in New York City. He now chases Baby Luca in the Twin Cities area. Drop by www.tangible-technology.com for a virtual visit.

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