Most people (well, probably older folks at this point) have had a DVD or CD player that plays discs that are scratched or covered in smudges, but another (generally cheaper) device cannot play it at all, or poorly. Why are some devices able to play flawed discs while others struggle?
In: 5
https://en.wikipedia.org/wiki/Cross-interleaved_Reed%E2%80%93Solomon_coding
Flawed discs have missing info. However, CDs have built-in algorithms and you can figure out what the missing data is based on the neighboring data (you can literally drill a small hole in a CD and it’ll work).
Now, as to how some players are able to read more flawed discs that other players can’t, I sadly can’t answer that. If I had to give an educated assumption, it’s based on their processors for using these algorithms to find the missing data.
Those devices work by shining a laser beam at the disc and reading the flashes and pauses that come back. Depending on how many sensors you have and where they are positioned, and how the player is programmed to respond to what they see, will effect the end result.
Cheap players with a single cheap sensor in the one ideal position might not get a good enough glimpse at the laser light that is reflected. Maybe the laser itself is cheap and doesn’t shine brightly enough or isn’t columnated enough to get through the scratched plastic.
Direct audio is so poorly stored on CDs compared to normal data that you can skip huge parts of the data and it still “sounds” pretty much okay.
Audio “mode” if you like, on a CD, gives you only 74 minutes of music on a 640Mb device. With 128Kbps MP3, that music would only occupy 56.25 MB.
And because the audio is “uncompressed”, as such, on a CD it can easily skip or ignore parts and just carry on without noticing (there might be silence or a brief blip but you probably wouldn’t hear it). Compressed files like MP3 tend to be very reliant on having to have a continuous stream of data and referencing earlier parts correctly in order to work.
And CDs do still have error correction in their design, so a brief blip might even be undetectable.
And depending on how well the device was designed, whether it buffers audio data or tries to play it direct, whether it tries to “interpolate” when it doesn’t have data available from the disk, whether it gets the laser to retry reading the sector or not before the listener gets to that bit of the song, or whether it just has to skip over all that and hope you didn’t hear it, means that some quality players will handle the situation better and others won’t.
For their time, CDs were incredible. Now, they are very inefficient compared to modern audio storage because of all that correction and redundancy and “slack” encoding of the data in order to just be able to play it without lots of integrated circuitry to decode the signal.
DVDs have the same issue – they were MPEG-2 for video but DVD-Audio uses the same kind of linear PCM as the CD used to.
By today’s standards, they are not very dense at all and not even really compressed (e.g. compared to H265 or modern MP3 etc.), and that means they could afford to lose some data and it not matter as much.
Latest Answers