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Not sure if this is the right place to ask this, but here it goes: Different optical discs have different rotational speeds, based on the size of the pits and the bitrate. Newer disks have smaller pits, so the disks can rotate slower at the same bitrate, and even a single disk can have different speeds on different sections, usually going faster near the center. Now, how does that work? A regular disk drive can usually support multiple types of disks, it doesn't really matter wether you are dealing with a DVD, a CD, or whatever. Do these disks maybe have markers that dictate a set speed/acceleration at the very beginning? Does it simply calculate the size of the pits and adapt to that speed? or is it something completely different? Thanks in advance.

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  • \$\begingroup\$ You could also ask how it knows it's capacity, whether it is R or RW and so on... While I don't know the answer, I would assume that there is some record containing the media information that is being read by the drive at the minimum safe speed, then it is adjusting accordingly. \$\endgroup\$
    – Eugene Sh.
    Feb 9, 2022 at 16:53
  • \$\begingroup\$ so as soon as the laser can focus and read data, it should know wether its a CD, DVD,.. then it will search for an initial mark at an known position I guess... at the higher levels then a header of the format is present. And estimating the rotation speed while reading is easy. if you have a format demanding a constant bitrate, a pll controling the motor speed can measure the drift of the bitrate and compensate for it. If its a variable bitrate format, you simply control speed according to the max-speed of the disc and an error-rate threshold. Should not be to hard to implement... \$\endgroup\$
    – schnedan
    Feb 9, 2022 at 17:23
  • \$\begingroup\$ Disks don't have one specific rotation speed, rather the device decides how fast it wants to read the disk. If it wants to read faster (for example to buffer video for decode) it'll spin faster. If it wants to read slower (for example, an audio only disk) it can choose to spin slower. In the old days you used to pay more more for CD/DVD drives with higher maximum read rates with the fastest devices being quite loud because of how fast they would spin. \$\endgroup\$ Feb 9, 2022 at 17:23

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How is the disc playback rate determined?

CDs and DVDs use a recording technique called Constant Linear Velocity, or CLV. The pit-and-land data on the disc have a channel coding rate for the raw signal. The read back system locks onto that signal, recovers data and clock from it, and uses that in a control loop to maintain servo lock, including focus, tracking and the disc spin rate.

So ultimately it’s the controller that sets the pace, and the disc follows along in servo lock. Audio CDs and movie DVDs are played back at their ‘1x’ rate; while a CD- or DVD-ROM data drive is free to read them much faster (and spin them faster too, sounding much like a noisy fan in the process.)

How does the linear track carry a clock?

CD and DVD track pit-and-land coding is designed to have enough transitions in it to recover clock. This is assured by a number of methods, including data scrambling, and by balanced run-length controlled 1/0 encoding into larger data symbols. On CD, 14 channel bits are used per byte; on DVD it’s 16 bits per byte.

The disc media pit-and-land dimensions and track pitch are carefully controlled to ensure compatibility with any player.

How does a drive identify the media?

DVD optics and electronics are a bit more complex than CD. Mainly, this is because DVD drives are required to be backwards compatible with CD, which uses different laser wavelength and numerical aperture (NA) than DVD:

  • CD: laser 780nm (near-infrared), NA = 0.45
  • DVD: laser 650nm (red), NA = 0.63

And for the sake of completeness:

  • Blu-Ray: Laser 150nm (blue), NA = 0.85

DVD optics thus have to support two optical paths: one for CD and one for DVD. This makes DVD pickups more complex, although economies of scale and mature technology ensure that the cost difference is tolerable.

Blu-Ray pickups are even more complex, with three optical paths.

And just because, a BD-drive pickup teardown: http://repairfaq.cis.upenn.edu/Misc/Blu-ray/site1/optics.html

When the disc is mounted in a DVD drive, the controller first starts at the middle to identify the basic media type: it tries each optical pickup path and attempts to achieve a valid read channel signal to choose the appropriate optics for disc.

With the optics locked on, the controller then gathers more information from the Table of Contents, or TOC, to determine the data format, and reports that back to the host.

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You simply spin the disk at whatever speed is required to maintain your data stream. A CD output for example produces 1.4 Mb/s. You stream data from the disk into a buffer at a somewhat variable rate and from the buffer at a constant rate to the output. If your buffer is running low you speed up the disk, if its getting full you slow it down.

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    \$\begingroup\$ There's probably a phase-locked-loop in there keeping the data rate constant. \$\endgroup\$
    – Transistor
    Feb 9, 2022 at 17:18
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    \$\begingroup\$ No need. The output is simply driven by the audio master clock that clocks both the DAC and the SPDIF output. The input buffer takes up some amount of slack, so the disk speed can vary and you can re-read sections that had bit errors during the first attempt. \$\endgroup\$
    – Hilmar
    Feb 10, 2022 at 16:38

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