Whitedragem
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Sep 29, 2019
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Part 2 - Physical Media How are they made?
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A very high quality CD, in ‘great’ condition. (dust included) CDs, being digital, contain binary information; either a ‘zero’ or a ‘one’, and this is etched in the production process, by a ‘burner’ as a pit (non modified substrate layer, ‘the valleys’), or a groove (creating a raise in the metal dye, ‘the mountains’). These 0s and 1s are why we refer to them as ‘digital’. The mediums that came before digital, such as tape and vinyl, that contained music in less 'mathy fashion' we refer to as analogue. It takes a complex computer, that we often refer to as a DAC (Digital to Analogue Converter) to turn these 0s and 1s back to an analogue wavelength that our ears are developed to process. The quality of DAC circuit design and then the amplifier to speakers/headphone pathway will all have effects on the end sound. Adding a dedicated DAC is an easy upgrade that can make massive changes to the perceived sound as a DAC circuit has so much to do right, and takes expertise to make it sound 'natural'; if done well then digital mediums have as much a right in HighFidelity sound as good analogue components do. Many people seem to think that a CD, by its nature, is perfect,.. but really it is just a great storage format. The actual sound that CDs make has gotten so significantly better, generally about every seven years or so DACs get significantly better that upgrading a DAC is probably the first thing we should all be considering if we are unhappy with our 'modern world sound'. If analogue equipment sounds great in your home stereo system, but digital sources sound cold and analytical, then an upgrade in the quality of DAC being used should level the playing field, to making digital audio 'sound nice again'.
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The TOC (Table of Contents) runs around the centre of the disc, being the first bytes of data on the 'inner ring' of the disc surface - generally up to twenty megabytes of data is used here as reference to where the files are located on the CD. The TOC is quite simple for audio discs as it is just telling our CD player (DVD/Blu-ray players etc)how many songs the disc contains and where the starting location for each track can be found. [The TOC gets a lot larger and more complex when 'burning' multi-session discs (not relevant to audio discs); - where if a user wants to update data elsewhere on the disc, writes can then modify the TOC values to ‘point’ to the newly added or 'changed' data.] On disc spin up, an 'initialisation' process scans the TOC to learn what the disc contains, and where the data can be found. For audio discs this is simply to learn how many tracks are on the CD and where each one begins. So the TOC is where the laser starts to read the pits and grooves’ (0s and 1s) on the disc.. After first initialisation (where the CD player reads the TOC to know how many tracks are on the disc and where they are found),.. the player can go on to play the tracks (if the user hit the 'Play' command to close the disc tray), or it can 'report to the display' how many tracks the disc contains and the total running time of the disc. It seems that in order to keep things predictable for playback equipment, song limits of 'the first twenty tracks' had the track lengths given, allowing 'count down' timers to display, counting down in seconds until the end of the song. For track twenty-one and onwards this neat ability to countdown song times is lost, not really an issue for most music listeners, as filling a CD with twenty tracks is pretty easy to do. If ANY DAMAGE is located on the inner part of the CD, the likelihood of the CD initialising in a player (telling the player what it contains) drops immeasurably. (Don’t buy discs with inner circumference damage).
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These two photos show examples of CDs that have had A) a label, and B) a full coating placed on the top surface of the disc. (Picture disc, even if the ‘picture’ is “just a coating”) Ahem, back on topic,.. When lasers focus on a disc, the pits and grooves are read by a process of reflecting the laser light through the substrate. A coating on top of the disc can help reflectivity. (some even swear by taking a permanent marker and ‘darkening’ the 1mm outer and inner ‘sides’(circumference/edges) of the CD with ink, to ‘aid’ disc reading). Anything that can be done to aid reflectivity, be it a gold disc, or a well looked after disc collection (sans ‘scuffing’) will make for an easier to read disc, with the flow on being better audio to be read from the disc surface.
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Layers; The dye/metal substrate actually sits 5% of the way from the top of the disc, or, 95% of the way from the underside of the disc. For this reason, a few scratches in the underside of a disc may not have affected the ACTUAL DATA contained therein, but just about ANY DAMAGE to the top surface pretty much means the DATA in that AREA is gone! <data area; 5% from top/95% from bottom of disc> Polishing CDs to remove ‘scratches’ is a thing. I might consider it if the disc is unreadable, but I do not like ‘thinned discs’ in my collection. The CD spec is pretty specific about a lot a little things, one being the ‘distance from the bottom layer that the data is contained’; lasers literally focus to a fine point to read the pits and grooves. Polishing the disc and making it thinner can have the laser be ‘unfocused’ when attempting to read the disc. <“do not polish!” (laser focus considertion)> The blank CD can hold roughtly 74 mins of music data when stored as REDBOOK Audio (44khz 16bit) <previous post should highlight why this is so!(not yet)> Some media allowed filling ‘right to the edge’; example being -around the 1990’s quite a few CDs started coming to market that were 80 minutes long. (eg ‘best of’ Pet Shop Boys- Discography) As most CDs were repressings of music from the Vinyl era that preceded them, it is common to find a lot of recordings that offer around 45mins worth of audio (Suzanne Vega albums come to mind). These discs are often only ‘half filled’ or have a lot of ‘disc edge’ that contains no data, nor has the laser read over the area. Scratches may not matter if they do not run into the data area. < damage to top of the disc is VERY BAD but only if ‘in the data area’ to be read > Scratched discs may sound ‘trebly’/missing bass (long frequency that is easy to lose if the disc is ‘hard to read’) as the sheer number of perfect samples to interpret for a musical not whose wave stretches for 20 metres in length, will not work out if the signal stops supporting the frequency mid way through..(discussed further in part 4 (although not yet!)) continue to: Part III - Buying Considerations https://drop.com/talk/26089/dragems-how-to-buy-second-hand-compact-discs/2513460

(Edited)
Sep 29, 2019
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