How come a CD can only hold 80 minutes of music, while a DVD can hold over 2 hours of video/sound etc,? Is it because all the stuff on a DVD is compressed and if they did that to a CD the sound quality would be compromised?
The "codefor both of them just consists of 1’s & 0’s right?
Or am I missing something?
Yes the data is all ones and zeros, but the actual recording method differs slightly. When info is encoded on either a DVD or CD the disk is actually imprinted with small channels of varying lengths, fixed width and depth (different length for ones and zeros). A laser is used to read the impressions and software is then used to translate that back to video and/or sound. The difference between a CD and DVD is in the width of the channels. Newer lasers used in DVDs can be focused to a narrower diameter, thus permitting the channels to be narrower - since width does not affect the content of the data. By making narrower channels, more “rings” of channels can fit on a disk thereby increasing storage capacity.
I think I remember getting this explanation from a Crutchfield manual not too long ago but I might be mistaken.
CD can hold 700 Megabytes of data.
DVD can hold 4.7 Gigabytes, or 9 if it is a dual layer disc.
Its just the fact that the DVD can hold many more 1’s & 0’s.
So just how much music could I record on a DVD from CD, assuming that I had the kinda dough to buy DVD recorder?
A straight calculation using effac3d’s numbers (single layer) would imply between 6 and 7 CDs worth of music.
DVDs also can do a neat trick called RSDL, or Reverse Spiral Data Layer. This superimposes two spiral data tracks on the disc - one clockwise, one couter-clockwise.
Exactly how the laser makes sense of it is black magic, as far as I can tell. It’s actually more to do with how deep into the disc the laser is focused.
Sorry for the hijack, but does anyone know if DVD players will become commonplace as replacements for auto CD players? It would be awesome to burn, say all of Led Zep’s albums onto a single disc and just put that thing in and go!
Information is storied on laser-optical discs in the form of “pits” that are stamped or molded into the plastic and coated with aluminum in order to make them reflective. The size of the pits and the distance between them determines how much information the disc can contain.
DVDs are read with a finer-wavelength laser than an audio CD, so the pits can be much smaller: one quarter to one half the length of the pits in an audio CD, resulting in two to four times as much storage on a disc of the same diameter. A DVD can also have two different layers of pits per side, doubling its information-carrying capacity on top of that.
While the pits themselves correspond to ones and zeros, the arrangement of the data is different. CD audio is stored in a format called LPCM (Linear Pulse-Code Modulation) which is a series of numbers describing the amplitude of the audio signal as it changes over time. This data can be converted back into music by feeding the numbers through a circuit called a DAC (Digital-Analog Converter) which synthesizes an audible waveform from the pattern described by a series of PCM values.
DVD can also record audio in LPCM format, but video requires so much of the disc’s overall capacity that it’s almost always recorded in a compressed format like MPEG-1 Layer 2, Dolby Digital or DTS. These formats rely on sophisticated algorithms to reduce a long series of raw LPCM values to a short series of encoded values which are stored on the disc in its place. When the DVD is played back, the compressed information is decoded to a series of LPCM values closely resembling the original signal, and converted into audible music using the DAC circuit previously described.
If a DVD-5 (single-sided, single-layer, 4.7 * 10^6 bytes) is used solely for audio, it can hold 6 hours in stereo LPCM format or more than 54 hours in Dolby Digital (AC3 2/0 at 192 kbps).
I have seen the future, and it is good. Here, I will reveal my prophesy for all to hear: No, they will not.
They just approved a standard for ‘Blue laser’ DVDs which will have an even higher capacity then todays current DVDs.
With CD-RW, the layer with the pits that is read for playback, is actually melted by the laser and new pits (the new material) are put into the dye while it’s soft. The color of the film is different because the material is different.
You can already do that with an MP3 CD player (available in handheld, automobile, and stereo component forms). You can fit about 10 albums on a single CD.
Really, CD’s vs. DVD’s is a very close analogy to past history – LP records vs. 78’s.
They both record data in similar ways, but in each case, the newer technology ‘minaturizes’ the method, so as to record more music in a smaller amount of physical space.
The more things change, the more they stay the same.
Welcome to the SDMB, t-bonham@scc.net!
To answer your question: no you DO NOT put apostrophes in the abbreviations. DVD’s, CD’s and PC’s are all incorrect. It’s DVDs, CDs and PCs.
In regards to the replacement of CD-Audio: There are two competing standards, DVD-Audio and Super Audio CD. Both allow many more channels of audio than CD (7.1 for DVD-Audio, 5.1 for SACD), and provide much higher audio quality. DVD-Audio has a peak datarate of about 9600kbps, SACD about 14000kbps, ten times that of CD. Both may have their data compressed using lossless compression, which allows them to roughly double playtime without quality loss. According to the SACD guys, SACD sounds better than even DVD-Audio. It uses a technology called Direct Stream Digital to store audio data, instead of PCM used by CD-Audio and DVD-Audio.
From the looking I’ve done, its still not possible to go out and by an album from your favorite new artist in DVD-Audio or SACD format. neither seem to be catching on big, but there are players available for both formats, and some discs have been released.
To answer whose question?
Forming the plural using an apostrophe is always incorrect in the case of common nouns and surnames (The Smiths, not The Smith’s), but it was once widely accepted in the case of letters of the alphabet, numerals, and abbreviations. That practice is waning but, while most modern style guides recommend against using an apostrophe in forming the plural in any case, that preference has not matured into an absolute rule, at least not a rule that is universally accepted. For example, from The Blue Book of Grammar and Punctuation: