Talk:Constant linear velocity

CAA information appended to CLV
Coming from the laserdisc article, I have added information of CAA, Constant Angular Acceleration, to this article citing the similiarities with CLV. I sincerely hope that this appropriate and acceptable, as I do not believe that CAA is worthy of its own unique article.--Kenn Caesius 19:30, 4 March 2006 (UTC)

Drive velocity is independent of how the information is stored
Linear velocity is a property of how information is written and read. For information that inherently has a time component, such as audio and video, it is also a property of how that information is stored on the disc. It can be thought of as a very slow nondispersive wave propagation velocity of the stored information along the track of the disc that is frozen in time.

However, for a given frequency component, the quantity that describes how that waveform is stored along the disc track is spatial frequency (units of e.g. cycles per metre), denoted by ξ (Greek small letter xi). If the read velocity differs from the write velocity then the resultant frequency differs from the original frequency. This is related to the spatial frequency as follows:

$$\frac{f_{\text{write}}}{v_{\text{write}}} = \frac{f_{\text{read}}}{v_{\text{read}}} = \xi$$

For information in general, the quantity that describes how the information is stored on the track has a dimensionality of information over length (units of e.g. bits per metre), which I suppose would be ‘spatial information rate’ or ‘spatial bitrate’. Analogously, if the read velocity differs from the write velocity then the read bitrate differs from the write bitrate in accordance with the following relation (bitrate is denoted by R):

$$\frac{R_{\text{write}}}{v_{\text{write}}} = \frac{R_{\text{read}}}{v_{\text{read}}} = \text{spatial bitrate}$$

The importance of this is that the read or write bitrate may not be constant. If the drive is reading at a constant angular velocity and the information stored on the disc has a ‘constant linear spatial bitrate’ (as might be obtained from writing at constant linear velocity at a constant bitrate, but not necessarily) then, seeing as the bitrate is proportional to the linear velocity which is proportional to the radial distance of the track at that point, the bitrate is not constant. If the disc was written in the same manner (i.e. with constant angular velocity and a bitrate proportional to the radial distance of the write head) then neither read or write had a constant linear velocity, as the stored data might suggest.

Where the bitrate is proportional to the angular velocity, the spatial bitrate is not constant but inversely proportional to its radial distance. In this case the stored information has a ‘constant angular bitrate’ (units of e.g. bits per radian or bits per cycle). If the drive turns at constant angular velocity then the bitrate is constant. For angular velocity, denoted by ω (Greek small letter omega), the following relation holds:

$$\frac{R_{\text{write}}}{\omega_{\text{write}}} = \frac{R_{\text{read}}}{\omega_{\text{read}}} = \text{angular bitrate}$$

Note that although audio and video stored on a track have an inherent velocity, the spatial information rate or angular information rate is independent of this because the quality of the audio or video may differ for different recordings (e.g. different digital audio bitrates) or even on the same record (e.g. digital audio variable bitrate). Information rate doesn't just apply to digital information. For example, the audio on a normal vinyl has constant angular velocity (and intended to be played at constant angular velocity) so one might expect the angular information rate to be constant but if the track has a limiting spatial information rate then the angular information rate may be closer proportional to the radial distance which means that the fidelity (such as upper-range frequency response) typically decreases towards the centre of the disc. (See also channel capacity.)

As it currently stands, there is barely any distinction between drive linear velocity, stored audio/video linear velocity, and the stored linear spatial bitrate (or drive angular velocity, stored audio/video angular velocity, and the stored angular bitrate). The article once mentions ‘constant bit density’ which is a more appropriate term for a constant amount of information per volume (and would have to consider track and layer pitch), which isn't a direct analogue to constant linear velocity as is constant linear spatial bitrate.

— James Haigh (talk) 2015-09-14 T 01:07:41Z

Cassette tape
Although it's not a disc format, cassette tapes sorta apply to constant linear velocity. In fact, when a cassette is played, the reading head reads the same length of tape every second. 104.172.119.172 (talk) 01:12, 28 July 2021 (UTC)