Originally Posted by AdamCalifornia
Here is some more info regarding HD (High Density) layer on a SACD disc and a comparison with a DVD-Video disc. As we all know music CDs (so called Red Book), which you buy in stores, hold only 650MB of audio data. This allows for about 74 mins and 33 secs of music. (They can also achieve over 80 mins of music by slightly varying the CD standards.)
Now, as concerns SACD, due to technological improvements since 1982 (when the first CDs were launched), a standard one-layer SACD disc holds the same amount of data as a typical DVD-Video disc. They BOTH can hold 4.7 GB of data. (By the way, mechanical and optical properties of SACD disc are similar to DVD-5 disc properties.) Since the capacity of one-layer SACD is about 7 times the storage capacity of a regular CD they call it High Density (HD) layer. In case of the "2-channel stereo" this corresponds to about 110 minutes of music. When a SACD contains 2 (yes, two) HD layers then the capacity is doubled to 8.5 GB. Finally, the hybrid disc contains 2 layers: one is the HD layer and the other one is the regular CD (Red Book) layer. In this case the total is: 4.7 GB + 650 MB.
That the one-layer SACD disc can contain BOTH the "2-channel stereo" and the "5.1-channel" areas on the same layer is due to the following facts:
1. The layer itself is the HD (High Density) layer
2. Proper compression of both the "2-ch" and the "5.1" audio data is applied