A Chip Off the Old Clock

This article was published in the October 1993 issue of EQ magazine. The sidebar of the article talks about the problem of digital jitter.

Every few years a product comes along that changes the way you work. It has just happened again. If your studio contains any equipment that produces digital audio signals, you need to know about a new IC chip, the AD1890 from Analog Devices

Most engineers sooner or later experience trouble copying signals of different formats and incompatible sample rates. Larger studios face the problem of synchronizing all their digital equipment. Fully digital solutions to these problems are very expensive. This will change soon, since manufacturers are working now to bring you products based on this new circuit.

The AD1890 is a stereo sample rate converter that costs less than $50. It comes in 20-bit professional and 16-bit consumer grade versions, but here we will be looking only at the pro version.

WHAT IT DOES

Hook this device up to a few external components, and you have a box that does a lot of very cool things. Here's a short list:
  1. It will convert any input sample rate to any output sample rate within the range of 8kHz and 60 kHz. This means an average DAT recorder with standard SPDIF (RCA jack) connectors could make a digital copy of any signal - from 11.025 kHz used by computer sound cards right up past 48 kHz. Just select the output frequency you need, and the input side locks up to whatever you throw at it. Converting 48 kHz consumer standard to 44.1 kHz CD standard is effortless. Until now, the price tag for this feature has been up to $10,000. You'll soon pay less than a tenth of that.
  2. The AD1890 will track continuously changing sample rates. Look for digital varispeed to appear on more new tape recorders. Until now, varispeed has been done mostlly with analog tape decks.
  3. This chip will synchronize digital signals of varying sample rates. Watch for digital mixers that use one AD1890 for each input. They will mix together any signal you have, whether it be DAT, CD, DSP processor, digital editor, sound card - well, you get the idea.
  4. If that's not enough, the AD1890 also dejitters any signal you put through it. In plain English, this could improve the final sound quality of your mix. It acts somewhat like a high-quality line-level isolation transformer in the analog world. If you have digital cable runs that are longer than you wish, or a piece of equipment that seems to sound different depending on what digital device is feeding it, a simple box containing this chip could eliminate this problem. See the adjacent sidebar for details.
  5. The AD1890 "speaks" the correct data format of both consumer SPDIF and professional AES/EBU (Cannon XLR jacks), so you can expect many manufacturers to provide both kinds of connectors. This will help you freely mix consumer and pro gear. As a bonus, pro audio boxes may give you the option to strip off SCMS copy code, since the AD1890 ignores this data. Many studios will find this feature alone to be a major convenience. Consumer boxes will undoubtedly preserve the copy code in order to stay in compliance with current law.

IS IT AS GOOD AS IT SOUNDS?

All of these features make sense only if this device has excellent audio performance. Does it? Definitely, and for a number of reasons. First, it processes signals in 20-bit digital words, a great improvement over the 16 bits stored on a CD. This yields a dynamic range of over 120 dB, with outstanding noise and distortion specs. Second as you drop the level of the input signal, the noise and distortion scales down by the same amount. This is the way such things should work, but not all digital circuits do this. Third, the dejittering action may actually produce a signal that is better sounding than the original.

Okay - but how does it sound? Many people believe that great specs mean great sound. This is not necessarily true, though specs can often give you some good hints. There were two separate prototypes available for test, both built around the AD1890. It is reasonable to assume that production boxes will use a very similar circuit, since there are only a few ways to hook up this device.

Most people will use the AD1890 as a sample rate converter, so the test converted audio from 48 kHz to 44.1 kHz, as will as the reverse. The reference system was a high-quality D/A-A/D converter setup that I prefer for analog mastering. For a listening environment, I used our mastering facility, which includes two time-aligned monitor systems, custom amplifiers, tube electrostatic headphones, and similar tools of the trade. The source material was mostly original master tapes along with some CDs. Music ranged from rock to classical organ music.

The decision? Very close, except at the extreme top, where the the AD1890 was more detailed. The analog system lacked a bit of clarity by comparison. When I turned the playback system way up (on a quiet passage), I found the noise floor of the AD1890 to be noticeably lower. This might explain where some of the extra detail came from: less interference. Regardless, my frame of reference shifted, and the analog system now feels like a very subtle special effect to be used for adding top-end smoothness. The AD1890 is a trustworthy reference, and that is what studio engineers need.

It won't be long before you can get your hands on commercial processors based on the AD1890. Check one out on your own source material. In the meantime, rumor has it that Analog Devices can't get people to return their evaluation systems. This sort of backhanded compliment signals a winning product. And me? Of course I'll return the prototypes. Just as soon as I remember where I mislaid them. Check back with me in 1999.

WHY YOU SHOULD CARE ABOUT JITTER

No, jitter is not about too much coffee at 3 A.M. It's a problem that can cause distortion in digital audio equipment. How does it happen? Good question.

Analog-to-digital converters and their opposites, digital-to-analog converters (DACs), require a perfectly stable reference signal, called a clock, to work correctly. Both consumer SPDIF and professional AES/EBU data formats require that the clock be encoded into the same signal as the audio information. This allows one wire to carry everything needed to produce a stereo image on your speakers. When the data reaches the receiving end of the wire, the clock is separated from the audio. The DAC then uses the clock to reconstruct the audio signal. In a perfect world, we're done.

But what if the wire is too long, or of the wrong type? Or the method of extracting the clock - or even the DAC itself - is sensitive to minor problems with the incoming signal? This is not theory, it happens frequently. Many studios run audio through multiple conversions from analog to digital and back again, often through cabling that is less than perfect. Suddenly the clock is not a stable reference, it is "jittered." The DAC can no longer do it's job properly. Welcome to the Twilight Zone, where output does not equal input. Clarity is lost as your mix passes through what you assumed was a perfect audio transmission system.

Here is where the AD1890 shines. First, it is extremely forgiving of variations in the incoming data. Second, the signal it sends out is controlled by a clock that is completely isolated from the incoming clock signal. It "re-clocks" the data stream, and any jitter that was embedded in the original just plain disappears. The output is an improvement over the input.

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