[ELI5] Why did digital audio/cds allow for increased dynamic range and wider stereo separation compared to vinyl?


[ELI5] Why did digital audio/cds allow for increased dynamic range and wider stereo separation compared to vinyl?

In: Technology

A record player needle can only vibrate so much. That accounts for the higher dynamic range of CDs, the difference between the lowest and highest dB sound it can produce is lower for vinyl. This also accounts for the higher level of stereo (channel) separation. There is a lower dynamic range to work with on vinyl, so the channels can’t be as far apart.

Vinyl only allows a needle to lift or drop so much, limiting the range that it can produce. It is also limited in how fast it can go up or down, creating some limitations for how quickly you can change from a low note to a high note (or vice versa).

Vinyl records audio directly in a single groove. This means there are technical issues — how thick is the needle, how fast the disc is spinning, how precise is the manufacturing, how small of a feature the plastic can have, what is the needle’s inertia, etc. All of those things directly affect how precisely it can represent the signal. That’s also why not just any record player will do. How good is the driving mechanism and the quality of the needle matters.

In digital similar issues exist, but the important difference is that they don’t change how precisely the data is represented, they just affect how many bits we can squeeze into a given amount of room. We can allocate the available bits in any way we want. So if you want to put 24 bit, 192 KHz audio on a CD, you can. You won’t fit a whole lot of it, but that’s the only issue.

For stereo, since there’s only one groove, stereo is encoded at an angle. The incorrect ELI5 explanation is that one channel is left/right and the other is up/down. The real explanation is that this is that both are at a 45 degree angle from the vertical.

Meanwhile, in digital we can just interleave channels. Have a track that goes “5ms of channel 1, 5ms of channel 2”. You can have as many as you like and the separation is perfect because they have nothing to do with each other.

On a vinyl record, the sound is recorded in grooves molded into the vinyl. On very quiet sections, the groove is almost a straight line. On louder sections, the groove zigzags back and forth noticeably.

Vinyl has limited dynamic range because if the sound too quiet, the movements of the groove are too subtle for the needle to pick up, and if the sound is too loud, the groove would touch the groove next to it and so the record would skip whenever the needle reaches that point.

On a vinyl record, stereo is achieved through a process called “mid-side encoding” or “joint stereo encoding”. Basically, to produce stereo records that could still be played by older mono turntables, they devised a scheme whereby vibrations of the needle in one direction would represent “left plus right”, vibrations in another direction would represent “left MINUS right”. An old mono turntable would only be able to play the L+R signal… while circuitry in a stereo turntable would perform some analog math to reconstitute separate L and R signals from the L+R/L-R. But since everything involved here is analog, it’s less than perfect and can only achieve so much.

In digital media, since we didn’t have the old mono compatibility issue to think about, we can store the left and right signals as-is. And since digital media is all ones and zeroes rather than a physical groove in a record, we don’t have to worry about tracks physically touching, so we could give a digital format any amount of dynamic range we want.