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How Chesky surmounted digital audio's flaws

technofile  by al fasoldt

Columns and commentaries in a life-long dance with technology
 

Simple gray rule



How Chesky surmounted digital audio's flaws 


By Al Fasoldt

Copyright © 1989, The Syracuse Newspapers

I am astonished. I am amazed.

I've been listening to a revolutionary compact disc. The sound is incredible.

The disc looks like any other 5-inch CD, but it was recorded by a method that is unique in all of audio. If the new technique catches on, it could change the expectations of millions of music lovers around the world.

The disc—with the improbably long title "Johnny Frigo with Bucky & John Pizzarelli Live from Studio A"—comes from Chesky Records of New York. Chesky issued the disc and two companion CDs to launch a new series of recordings made with a device that bypasses the biggest problem with current digital recordings.

That problem, stated as simply as possible, has to do with the way the audio signal is measured. If the circuit takes too long to measure the signal, the resulting digital code isn't likely to be an accurate reflection of the sound.

The standard way that compact discs are recorded relies on a complex circuit, called an analog-to-digital converter, that measures the signal about 44,000 times a second. This provides adequate fidelity for sounds that have low or medium pitches, but cannot work well for high-pitched sounds.

Since sounds are made up of vibrations, they can be reproduced accurately only if the vibrations are measured—or "sampled," in the technician's term—many times for each vibration.

Audio engineers have known for a long time that the sampling rate of digital recorders will have to be raised beyond the 44,000 figure before this problem can be solved. Some recorders now use a rate of 48,000 times a second, but this is not much of an improvement.

The goal, according to some recording engineers, is a sampling speed of at least 1 million times a second. But such a vast improvement is virtually impossible with current technology, using the standard system of digital recording.

The approach that Chesky uses in its new CDs abandons the present system, called pulse-code modulation (PCM). Chesky's method measures the audio signal at a much higher rate than normal without using pulses. The digital signal is then converted to PCM so it can be played back on a normal CD player.

This sort of technique, known as oversampling, has been used for the last year or two at the opposite end of the recording chain—in CD players—to try to improve the sound of the playback. But it makes little or no audible difference, since the fault lies in the recording itself.

But Chesky uses oversampling right at the start of the chain. And rather than employing the two-times or four-times oversampling that is sometimes used in CD players, Chesky uses 128 times the normal rate.

In terms of samples per second, Chesky's CDs outstrip even the 1,000,000-cycle recorder sampling rate dreamed of just a few months ago. Amazingly, the master recorders used to make the company's latest CDs operate at 6,000,000 Hertz. That's 6 million signal measurements each second.

What makes this all possible is a set of integrated circuit chips engineered by Robert Adams, director of research for dbx Inc. of suburban Boston. Adams, who was born in Syracuse and educated at Tufts University, developed a variation of "noise-shaped oversampling" for his chips.

Adams says his circuits provide "lower distortion and linear phase response" compared with PCM digital methods, and at the same time are much cheaper than PCM circuits would be at the same sampling rate.

In theory, as documented in a technical paper by Adams, the dbx-Chesky system should have a smoother overall sound and less background noise.

In actual listening, I found this to be true. I also heard something I had never before sensed in a digital recording, even in the ones that I had carefully made myself using a previous non-PCM digital circuit also designed by Adams.

What I heard was the utter smoothness and effortlessness that I have always associated with the best non-digital recordings. These analog recordings sound good in part because they are "natural"—in other words, they haven't been chopped up by the relatively low-speed sampling circuit of regular PCM digital recorders.

The dbx-Chesky system still must use the normal sampling rate when the music is placed on the CD, but it avoids any sense of unnaturalness in its master recording because the sampling rate is far too high to give any evidence of choppiness.

I was one of the loudest critics of CDs when they were introduced about seven years ago. I was worried that inferior digital recording standards were locked into the design of the CD.

But I am a non-believer no more. I salute Robert Adams and Chesky Records.

Chesky Records can be reached at 212-586-7799.


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