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Websites made with MediaSPIP
2 mai 2011, parThis page lists some websites based on MediaSPIP.
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MediaSPIP Core : La Configuration
9 novembre 2010, parMediaSPIP Core fournit par défaut trois pages différentes de configuration (ces pages utilisent le plugin de configuration CFG pour fonctionner) : une page spécifique à la configuration générale du squelettes ; une page spécifique à la configuration de la page d’accueil du site ; une page spécifique à la configuration des secteurs ;
Il fournit également une page supplémentaire qui n’apparait que lorsque certains plugins sont activés permettant de contrôler l’affichage et les fonctionnalités spécifiques (...) -
Creating farms of unique websites
13 avril 2011, parMediaSPIP platforms can be installed as a farm, with a single "core" hosted on a dedicated server and used by multiple websites.
This allows (among other things) : implementation costs to be shared between several different projects / individuals rapid deployment of multiple unique sites creation of groups of like-minded sites, making it possible to browse media in a more controlled and selective environment than the major "open" (...)
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Google Analytics 4 and GDPR : Everything You Need to Know
17 mai 2022, par Erin -
CD-R Read Speed Experiments
21 mai 2011, par Multimedia Mike — Science Projects, Sega DreamcastI 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.C :-
#ifdef _arch_dreamcast
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#include <kos .h>
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/* map I/O functions to their KOS equivalents */
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#define open fs_open
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#define lseek fs_seek
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#define read fs_read
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#define close fs_close
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#define FILENAME "/cd/bigfile"
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#else
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#include <stdio .h>
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#include <sys /types.h>
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#include </sys><sys /stat.h>
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#include </sys><sys /time.h>
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#include <fcntl .h>
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#include <unistd .h>
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#define FILENAME "/media/Full disc/bigfile"
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#endif
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/* Get a current absolute millisecond count ; it doesn’t have to be in
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* reference to anything special. */
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unsigned int get_current_milliseconds()
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{
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#ifdef _arch_dreamcast
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return timer_ms_gettime64() ;
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#else
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struct timeval tv ;
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gettimeofday(&tv, NULL) ;
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return tv.tv_sec * 1000 + tv.tv_usec / 1000 ;
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#endif
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}
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#define READ_SIZE (20 * 1024 * 1024)
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#define READ_BUFFER_SIZE (512 * 1024)
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int main()
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{
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int i, j ;
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int fd ;
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char read_buffer[READ_BUFFER_SIZE] ;
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off_t filesize ;
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unsigned int start_time, end_time ;
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fd = open(FILENAME, O_RDONLY) ;
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if (fd == -1)
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{
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return 1 ;
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}
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filesize = lseek(fd, 0, SEEK_END) ;
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for (i = 0 ; i <3 ; i++)
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{
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if (i == 0)
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{
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lseek(fd, 0, SEEK_SET) ;
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}
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else if (i == 1)
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{
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lseek(fd, (filesize / 2) - (READ_SIZE / 2), SEEK_SET) ;
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}
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else
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{
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lseek(fd, filesize - READ_SIZE, SEEK_SET) ;
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}
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/* read 20 MB ; 40 chunks of 1/2 MB */
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start_time = get_current_milliseconds() ;
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for (j = 0 ; j <(READ_SIZE / READ_BUFFER_SIZE) ; j++)
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if (read(fd, read_buffer, READ_BUFFER_SIZE) != READ_BUFFER_SIZE)
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{
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break ;
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}
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end_time = get_current_milliseconds() ;
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end_time, start_time, end_time - start_time,
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READ_SIZE / (end_time - start_time)) ;
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}
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close(fd) ;
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return 0 ;
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}
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How to calculate ffmpeg output file size ?
25 septembre 2011, par poundifdefI am using ffmpeg to convert home videos to DVD format and want to calculate the output file size before doing the conversion.
My input file has a bit rate of 7700 kbps and is 114 seconds long. The audio bitrate is 256 kbit (per second ?) The input file is 77MB. To get this information I ran :
mplayer -vo null -ao null -frames 0 -identify input.MOD
So in theory, the input file should have (roughly) a file size of :
((7700 / 8) * 114) / 1024
That is, (7700 / 8) is kilobytes/second, multiplied by 114 seconds, and then converted to megabytes. This gives me 107MB, which is way beyond my 77. Thus I am skeptical of his formula.
That said, after converting the video :
ffmpeg -i input.MOD -y -target ntsc-dvd -sameq -aspect 4:3 output.mpg
The numbers seem to make more sense. Bitrate is 9000 kbps, and applying the above formula, I get 125MB, and my actual output file size is 126MB.
So, two questions :
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How do I factor the audio bitrate into this calculation ? Is it additive (video file size + audio file size) ?
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Do DVDs always have a 9000 kilobit/second rate ? Is that the definition of a DVD ? Or might that change depending on video quality of my input video ? What does "-target ntsc-dvd" guarantee about my video ?
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Why does my input file not "match" the calculation, but the output file does ? Is there some other variable I'm not accounting for ?
What is the correct way to calculate filesize ?
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