Signal and noise - Compact Disc microscopy
While refining my CNC microscope setup, I thought I'd have a look at some CD surfaces. Unfortunately CDs are not especially useful as calibration artifacts, since the track pitch and channel-bit length can be varied over ~15% ranges and still be in spec (manufacturers played with these parameters to pack more audio onto the disc). Still, it got me interested in the possibility of reading the data using these optical images.
The image below was taken with a Nikon 20x M Plan objective (NA=0.4, 210/0, LWD), a Logitech C270 webcam, and a 3D-printed epi illuminator with microscope-slide beamsplitter, all attached to the carriage of a Lulzbot Taz 4 3D printer:
Some spherical aberration is present. In fact it's a great deal worse if you image through the polycarbonate---a 1.2 mm slab of n=1.55 material is enough to seriously blur the image at this NA. I'd like to find some sort of SA compensator that I could hang on the end of the objective (there is plenty of working distance, ~5 mm). Perhaps just a (spherical) achromat would suffice---I don't want to pay $400 for an Edmund Optics asphere plate. This image is taken from the CD's label side, and there's still ~0.1 mm of polymer acting as a "cover slip". The objective is optimized for an air medium and no cover slip, so this is still suboptimal.
Anyway, here's a section through the image plotted as a waveform.
The eye is open, so slicing detects the following symbols:
0001111100001111100011110001111000001111111000000011000000111111<br>1100000111000001110000000011111111110000011111110000011110000011<br>1111110000011110001111000011111000001110000011110000111110000111<br>0001110001110001110000011111000001111000111000001111100011111100<br>0001111110000000001110000011100001111000000111111110001110001111<br>0000011100011100011111110000011110000001110000111111111000011111<br>0000000001111111000000111100001111000001110000011111111100001110<br>000011110011110000011100011100001111000000
Decoding the NRZI line code (transition is a "1", no transition is a "0"), the channel bits are:
0010000100010000100100010010001000010000001000000101000001000000<br>0100001001000010010000000100000000010000100000010000100010000100<br>0000010000100010010001000100001000010010000100010001000010001001<br>0010010010010010010000100001000010001001001000010000100100000100<br>0010000010000000010010000100100010001000001000000010010010010001<br>0000100100100100100000010000100010000010010001000000001000100001<br>0000000010000001000001000100010001000010010000100000000100010010<br>00010001010001000010010010010001000100000
No frame-sync pattern in this short segment (in either direction). I'll have to take several images and stitch them.
A specification for the CD physical format, channel coding, ECC, framing, and metadata is available here:
https://archive.org/details/RedBookAudioRecordingCompactDiscDigitalAudioSystemIEC60908SecondEdition199902ISBN2831846382
Archives
August 2025 (3)
July 2025 (1)
January 2023 (1)
December 2022 (1)
August 2022 (1)
February 2020 (1)
February 2019 (1)
December 2018 (1)
May 2017 (2)
May 2015 (1)
September 2014 (3)
July 2014 (3)
June 2014 (1)
December 2012 (1)
June 2012 (2)
February 2011 (3)
December 2009 (1)
January 2001 (1)