first understand the difference between analog and digital
... Analog is a continious spectrum with theoretically an infinite number of levels. For example in the normal human hearing frequency range an analog signal might be able to have every frequency between 0 and 20K Htz of 20,000 different levels.
Now digital on the other hand has distinct steps of values. The smaller the steps the better the convernsion. For example if the sampling is every 2,500 Htz you would get a very bad sound reproduction of the original signal: set -= 0, 2500, 5000, 7500, 10000, 12500, 15000, 17500, 20000.
However even a sampling rate a large as 100 gives prettty good sound reproduction of the original: 0, 100, 200, 300, 400, 500, 600, 700 ...
Besides the frequency range you also have to take into account the time of the interval. If for example you sample for 1 seconds and then break down the signal even to a low 10 Hz interval you would obtain a poor reproduction.
To be valid and repordocue qulaity sound you have to get somthing below the sound persistnace of the human ear, it you statrt to approach less than 10 milli-seconds with a frequency sampling of 100 Hz you begin to obtain good SQ.
Analog can be converted to digiatal and digiatl can be converted to analog. The quality depends on the the frequency interval and the interval length.
Any reasonable cost convetor will sucessfully convert analog to digiatl or vice-versa. However, look at the specs and listen before you buy!