What is that beep sound an optical drive makes when recognizing a disc?

I know this question sounds trivial, but it has made me so inquisitive over time that I just have to share it here.

When a disc is inserted in an optical drive, it first starts spinning (earlier optical drives tend to check reflection first; no reflection = “no disc”), then I presume the drive tries out all disc types with its laser until it finds the correct disc type and then fine-tunes some laser parameters or similar.

But something I have noticed in all optical drives (half-height and slim type) and even some CD players is the beep after recognizing the disc, probably from the optical pickup unit. For dual-layer discs, there are two of these beeps, which means one per layer. Imminently after the beep, the disc accelerates and the computer can access the data.

I know it is a part of the disc recognition process, but what exactly is it technically?

I’m not sure it’s a beep, it always sounded more like high pitched short squeal to me.
I believe its caused by the coils in the pickup head being energised. The coil changes a magnetic field which allows the laser head to move closer or further away from the media.

The head itself has a tiny metal plate on the bottom of it, and its suspended by carefully calibrated springs. Apply a certain amount of current to the coil will pull the head down and away from the media by a certain amount. Reducing the current will allow the head to move up towards the media using the springs.

That’s my understanding of it. But don’t quote on it. :slight_smile:

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I see. Interesting, thanks.

But how come the sound only occur once during recognition and not the rest of the operation?

I would assume because when a media is first inserted and has to be recognised requires a full travel of the pickup head to focus on the media. Thereafter the movement from the pickup will be minimal, only requiring minimal adjustment to keep focus.

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