What makes Blu-Ray (TM) Blue?

In the last blog, I talked about the difference between continuous (analog) data and discrete (digital) data. One of the most popular, "hands-on", types of data that people use each day are that for audio and video.

In the case of audio, analog data are often considered to be the most "faithful" to the sound. Digital adherents say that the sound stays crisp and clear. They are actually both correct. When analog recordings are made they are able to reproduce all of the "between" sounds that are dropped during analog recordings. While one can debate as to whether it can be heard by most people, it does exist and, therefore, there may be a substantial difference even if only noticed by the subconscious.

Analog recordings primarily fall into two categories -- an engraved reproduction of the sound waves or a magnetic version. Each has the capability of continuous data recording. However, the use of such recordings requires destructive mechanical mechanisms to be "read" after being recorded. For the "engraved" (vinyl, records, wax cylinders -- yes all have been used) version this means a sharp object following the path of the engraving which will eventually start cause eroding the engraving and a deterioration of the sound. For magnetic versions, the media (tape usually) wears while being pulled and the magnetic material on the tape also gets worn by friction with the reading "head".

Thus, as time goes on and the recording gets used, the recording will get worse -- while, in general, a digital recording will stay the same. So the audiophiles and the digital adherents are both "right".

CHALLENGE: It should be possible to create a commercially viable analog recording medium that can be read non-destructively. With all the bright people and companies employing bright people this should be possible. Make it so!

As discussed in the previous blog, digital media (for audio and video especially) requires decisions as to how much data will be omitted. This is precision and sampling rate. For human speech, it is considered acceptable to take a sample 8000 times per second and the data can be recorded with the use of 8 binary units ("bits"). This means that a digital recording of human speed will require 480,000 bytes (8 bits), or 480 KB per minute of recording. In the case of "high fidelity" digital recordings of music, the sampling rate can be increased and the precision may also be increased. This ends up with a greater amount of data.

Currently, a popular way to record this data is on optical disks. The digital data are marked on the optical media with very, very small pits. A pit can be considered to be a "1" and a land (non-pit) can be considered to be a "0". Note that this is actually very similar to analog engravings except for the nature of the data. Please also note that the exact encoding is actually more complicated than I am saying -- check other sources for more precise descriptions.

A larger difference form analog data, however, is how the data are read once recorded. An optical disk makes use of a laser which can tell whether there is a pit or a land by the timing of the reflection from the medium. This reading is non-destructive and, as long as the optical medium is not otherwise damaged, should retain data unchanged for a long time.

We now enter into the third area of data recordings which is storage space. An audio CD makes use of a near-infrared laser (wavelength of 780 nm). This wavelength determines how dense the data can be placed on the optical medium. For an audio CD, using 780 nm wavelength lasers, about 737 MB (megabytes) of data can be stored in a single layer (a disk CAN have multiple layers with the laser reading separately from the different layers of the disk). Since this amount of data is considered to be around 80 minutes of music, we can see that, for an audio CD, each minute takes about 10 MB of data so the precision and sampling rate are much higher than considered acceptable for human speech -- greater "fidelity".

The wavelength of the laser determines density -- how "packed" the data can be. This limits the total amount of data in a predetermined physical size.  One method of increasing the density is by decreasing the wavelength of the laser.

A Blu-Ray disc uses a "blue" (blue is officially considered to be 475 nm) laser with a wavelength of 405 nm. A single layer Blu-Ray disk can store about 25 gigabyte (GB) of data. For audio, this would be about 100 hours using the same encoding as CDs. DVDs use a wavelength of about 650 nm ("true" red)..