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CD-R Read Speed Experiments

May 21st, 2011 by Multimedia Mike

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.

Profiling
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

Empirical Data
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.

Implications
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.

Source Code
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.

  1. #ifdef _arch_dreamcast
  2.     #include <kos .h>
  3.  
  4.     /* map I/O functions to their KOS equivalents */
  5.     #define open  fs_open
  6.     #define lseek fs_seek
  7.     #define read  fs_read
  8.     #define close fs_close
  9.  
  10.     #define FILENAME "/cd/bigfile"
  11. #else
  12.     #include <stdio .h>
  13.     #include <sys /types.h>
  14.     #include </sys><sys /stat.h>
  15.     #include </sys><sys /time.h>
  16.     #include <fcntl .h>
  17.     #include <unistd .h>
  18.  
  19.     #define FILENAME "/media/Full disc/bigfile"
  20. #endif
  21.  
  22. /* Get a current absolute millisecond count; it doesn't have to be in
  23.  * reference to anything special. */
  24. unsigned int get_current_milliseconds()
  25. {
  26. #ifdef _arch_dreamcast
  27.     return timer_ms_gettime64();
  28. #else
  29.     struct timeval tv;
  30.     gettimeofday(&tv, NULL);
  31.     return tv.tv_sec * 1000 + tv.tv_usec / 1000;
  32. #endif
  33. }
  34.  
  35. #define READ_SIZE (20 * 1024 * 1024)
  36. #define READ_BUFFER_SIZE (512 * 1024)
  37.  
  38. int main()
  39. {
  40.     int i, j;
  41.     int fd;
  42.     char read_buffer[READ_BUFFER_SIZE];
  43.     off_t filesize;
  44.     unsigned int start_time, end_time;
  45.  
  46.     fd = open(FILENAME, O_RDONLY);
  47.     if (fd == -1)
  48.     {
  49.         printf("could not open %s\n", FILENAME);
  50.         return 1;
  51.     }
  52.     filesize = lseek(fd, 0, SEEK_END);
  53.  
  54.     for (i = 0; i < 3; i++)
  55.     {
  56.         if (i == 0)
  57.         {
  58.             printf("reading inner 20 MB...\n");
  59.             lseek(fd, 0, SEEK_SET);
  60.         }
  61.         else if (i == 1)
  62.         {
  63.             printf("reading middle 20 MB...\n");
  64.             lseek(fd, (filesize / 2) - (READ_SIZE / 2), SEEK_SET);
  65.         }
  66.         else
  67.         {
  68.             printf("reading outer 20 MB...\n");
  69.             lseek(fd, filesize - READ_SIZE, SEEK_SET);
  70.         }
  71.         /* read 20 MB; 40 chunks of 1/2 MB */
  72.         start_time = get_current_milliseconds();
  73.         for (j = 0; j < (READ_SIZE / READ_BUFFER_SIZE); j++)
  74.             if (read(fd, read_buffer, READ_BUFFER_SIZE) != READ_BUFFER_SIZE)
  75.             {
  76.                 printf("read error\n");
  77.                 break;
  78.             }
  79.         end_time = get_current_milliseconds();
  80.         printf("%d - %d = %d ms => %d kbytes/sec\n",
  81.             end_time, start_time, end_time - start_time,
  82.             READ_SIZE / (end_time - start_time));
  83.     }
  84.  
  85.     close(fd);
  86.  
  87.     return 0;
  88. }

Posted in Science Projects, Sega Dreamcast | 10 Comments »

10 Responses

  1. Coin Says:

    Since I can offer insight on one point here I will: All original CD players (audio and data) ran in CLV (constant linear velocity mode). The read speed of 1X is constant from ID to OD, the spindle speed decreased as the head moved outward.

  2. Multimedia Mike Says:

    Thanks, “Coin”. That satisfies my curiosity on that point.

    Onto the first double-speed CD-ROM drives– were they only guaranteed to run at 2X on the outer rim?

  3. Pengvado Says:

    > How do you engineer an ISO-9660 filesystem image so that the timing-sensitive multimedia files live on the outermost track?

    See the -sort option to mkisofs.

  4. Multimedia Mike Says:

    @Pengvado: Thanks for the tip.

  5. Colin Says:

    I think up until 12x or so drives were still running CLV ID to OD. After that came the era of “max” drives which only reached their rated speeds at the OD via CAV, zoned CAV, and zoned CLV.

  6. Reimar Says:

    In the manufacturers’ defense there were good reasons to go with CAV besides boasting speeds the drive would only rarely reach.
    A motor that runs stably at a wide range of high speeds is rather difficult to build, but more importantly with a lot programs running directly from CD seek speed became very important. Accelerating (and even decelerating) a disk to high speeds needs a lot of energy and thus started to take a loooong time, making seeking from the inner to the outer part very slow.
    Removing the need to change speed when seeking made the drives a lot faster for hard-disk like use-cases.

  7. Brendan Says:

    You could probably find a 1x SCSI CD drive which would work on a reasonably modern PC. Macs and UNIX machines both would have used SCSI drives. It would almost certainly be caddy-loading though…

  8. Multimedia Mike Says:

    @Brendan: Probably. I think I even know where I could find a handy supply of ancient drives. I wouldn’t relish the exercise of making a SCSI adapter work on a modern computer, though. Unless there’s a SATA-SCSI adapter out there?

  9. Colin Says:

    If you’re looking for a relatively unique ancient drive I could send you a Nakamichi 5 disc SCSI changer.

  10. Multimedia Mike Says:

    @Colin: Thanks, but I doubt I would get around to using it. I trust your expertise on the single-speed matter.