I want to know how fast I can really read data from a CD-R. Pursuant to my previous musings on this subject, I was informed that it is inadequate to profile reading just any file from a CD-R since data might be read faster or slower depending on whether the data is closer to the inside or the outside of the disc.
Conclusion / Executive Summary
It is 100% true that reading data from the outside of a CD-R is faster than reading data from the inside. Read on if you care to know the details of how I arrived at this conclusion, and to find out just how much speed advantage there is to reading from the outside rather than the inside.
Science Project Outline
- Create some sample CD-Rs with various properties
- Get a variety of optical drives
- Write a custom program that profiles the read speed
Creating The Test Media
It’s my understanding that not all CD-Rs are created equal. Fortunately, I have 3 spindles of media handy: Some plain-looking Memorex discs, some rather flamboyant Maxell discs, and those 80mm TDK discs:
My approach for burning is to create a single file to be burned into a standard ISO-9660 filesystem. The size of the file will be the advertised length of the CD-R minus 1 megabyte for overhead– so, 699 MB for the 120mm discs, 209 MB for the 80mm disc. The file will contain a repeating sequence of 0..0xFF bytes.
I don’t want to leave this to the vagaries of any filesystem handling layer so I will conduct this experiment at the sector level. Profiling program outline:
- Read the CD-ROM TOC and get the number of sectors that comprise the data track
- Profile reading the first 20 MB of sectors
- Profile reading 20 MB of sectors in the middle of the track
- Profile reading the last 20 MB of sectors
Unfortunately, I couldn’t figure out the raw sector reading on modern Linux incarnations (which is annoying since I remember it being pretty straightforward years ago). So I left it to the filesystem after all. New algorithm:
- Open the single, large file on the CD-R and query the file length
- Profile reading the first 20 MB of data, 512 kbytes at a time
- Profile reading 20 MB of sectors in the middle of the track (starting from filesize / 2 – 10 MB), 512 kbytes at a time
- Profile reading the last 20 MB of sectors (starting from filesize – 20MB), 512 kbytes at a time
I tested the program in Linux using an LG Slim external multi-drive (seen at the top of the pile in this post) and one of my Sega Dreamcast units. I gathered the median value of 3 runs for each area (inner, middle, and outer). I also conducted a buffer flush in between Linux runs (as root:
'sync; echo 3 > /proc/sys/vm/drop_caches').
LG Slim external multi-drive (reading from inner, middle, and outer areas in kbytes/sec):
- TDK-80mm: 721, 897, 1048
- Memorex-120mm: 1601, 2805, 3623
- Maxell-120mm: 1660, 2806, 3624
So the 120mm discs can range from about 10.5X all the way up to a full 24X on this drive. For whatever reason, the 80mm disc fares a bit worse — even at the inner track — with a range of 4.8X – 7X.
Sega Dreamcast (reading from inner, middle, and outer areas in kbytes/sec):
- TDK-80mm: 502, 632, 749
- Memorex-120mm: 499, 889, 1143
- Maxell-120mm: 500, 890, 1156
It’s interesting that the 80mm disc performed comparably to the 120mm discs in the Dreamcast, in contrast to the LG Slim drive. Also, the results are consistent with my previous profiling experiments, which largely only touched the inner area. The read speeds range from 3.3X – 7.7X. The middle of a 120mm disc reads at about 6X.
A few thoughts regarding these results:
- Since the very definition of 1X is the minimum speed necessary to stream data from an audio CD, then presumably, original 1X CD-ROM drives would have needed to be capable of reading 1X from the inner area. I wonder what the max read speed at the outer edges was? It’s unlikely I would be able to get a 1X drive working easily in this day and age since the earliest CD-ROM drives required custom controllers.
- I think 24X is the max rated read speed for CD-Rs, at least for this drive. This implies that the marketing literature only cites the best possible numbers. I guess this is no surprise, similar to how monitors and TVs have always been measured by their diagonal dimension.
- Given this data, how do you engineer an ISO-9660 filesystem image so that the timing-sensitive multimedia files live on the outermost track? In the Dreamcast case, if you can guarantee your FMV files will live somewhere between the middle and the end of the disc, you should be able to count on a bitrate of at least 900 kbytes/sec.
Here is the program I wrote for profiling. Note that the filename is hardcoded (
#define FILENAME). Compiling for Linux is a simple
'gcc -Wall profile-cdr.c -o profile-cdr'. Compiling for Dreamcast is performed in the standard KallistiOS manner (people skilled in the art already know what they need to know); the only variation is to compile with the
'-D_arch_dreamcast' flag, which the default KOS environment adds anyway.