Paul McGowan:
Larry Borden sent me a link to my old friend Peter Moncreif’s musings on CD quality and what affects it. Peter and I go way, way back to his early Berkley days and few writers I have met have Peter’s ability to really hear and identify causes of what is going on in the music. He’s truly gifted.
After reading his article I am thinking seriously about getting the Audio Desk Edge trimmer. I know, spring’s a long way off and there won’t be edges on the grass to trim for some time, but this trims the edges on a CD. Up until reading this article I dismissed this as BS. Now I am not so sure. Peter writes:
“What technically does treatment with the Audio Desk edge trimmer accomplish? First, it makes each CD truer and better dynamically balanced. As you know, each CD is a stamped, molded piece of plastic. So its edge has significant irregularities. This means that each CD is out of round and not dynamically balanced. If you drove your car with its wheels and tires out of round and out of balance, you’d experience horrendous thumping and shimmying. A CD rotates pretty fast, so it too experiences considerable wobbling and shimmying forces from its being out of round and out of balance. Furthermore, it is the outer edge of the CD where the irregularities are the worst from the molding process, and this is precisely the worst location for such irregularities to be. Irregularities way out at the edge of any rotating system have the maximum, worst adverse effect, because they have the maximum moment arm or angular momentum out at the edge.
A major irregularity at one point of the circumferential edge would cause the disc to wobble or shimmy once per revolution. This in turn would place sudden great demands on the rotational servo and tracking servo (perhaps also the focusing servo), once per revolution. And this in turn would mean that these servos would draw a large burst of current once per revolution. When servos draw a burst of current, the voltage or current available to sensitive circuitry elsewhere in your CD player might drop, and this drop might either directly cause distortion (say by changing operating points) or might indirectly cause distortion by modulating the signal decoding process at some point.
Thus, the music might suffer distortion modulated at a once per revolution frequency. The CD spins at a rate from 200 to 500 rpm, which is equivalent to 3 to 8 Hz. Now, 3 to 8 Hz is obviously a very low bass frequency, and it would strongly modulate music’s bass, from 20 Hz to 100 Hz, probably making it sound less defined, more boomy, less tuneful. We normally think of digital as having problems at only very high frequencies, and digital is often touted as having supposedly perfect bass down to DC. But here we can see that the bass quality from CD could be degraded significantly, by something so simple as a once per revolution irregularity at the rim of a pressed and molded CD. And if you could remove this irregularity by trimming the disc edge to be more precisely uniform, then music’s bass quality should improve, since you could then hear the true bass sound of the original music, without the contamination of modulation distortion. That’s just what the Audio Desk edge trimmer does, and an improvement in music’s bass is exactly what we heard as a result.”
Thanks Peter. I would recommend reading the entire article. I still struggle with demagnetizing discs. I have heard the improvement, I don’t buy it.
You can also check out this YouTube DIY project for building an edge trimmer. The music’s great while watching the video.
