Help and FAQs


If you have a broadband connection to the internet, you can watch the videos online using your browser. As they are stored as flash videos (flv), this requires the standard flash player browser plugin, which is available for all major operating systems (including Windows, Linux, and MacOS) and likely to be already installed on your computer (about 98% coverage according to some surveys). In addition, Javascript needs to be enabled in your browser.

If the bandwidth of your internet connection is too low or if you don't want to install the flash player plugin, you can download the flash video files for offline viewing. Note, however, that most media players do not yet support the relatively recent flv-format, so you may have to install additional software for viewing such as the free VLC media player.

Therefore, as an alternative, many videos can also be downloaded in the well established mpeg2 format which should be supported by all players. We encode the mpeg2 videos with a considerably higher resolution (e.g. 720 * 480 rather than 320 * 240 pixels), which means that the quality is much better with the text of presentation slides usually being readable. The drawback of the high resolution are very large file sizes (up to 1000 MB per hour of video) with download times typically being several hours. If you seriously want to work with a video and have an internet  flatrate, we think that the better quality is well worth the waiting time. Note, however, that such huge downloads cause a tremendous load (and cost) on the side of the server, so we may not be able to keep up the download service for high resolution videos in the long run. Also, in peak hours and after announcements the webserver may be overloaded.



Why does the video archive use the Adobe flash video format?

The advantage of the Adobe flash player (previously: Macromedia flash player; video file name extension: flv) is that it is the most widely installed internet video client, which - according to Adobe - runs on over 96% of all computers that are connected to the internet. Also, this player runs on a wide variety of platforms and operating systems. To our knowledge, no competitive open source format exists. For users who can not play flv-files, standard mpeg2 files are provided for download. These should be playable by any video software.

Watching videos online using the browser does not work

Try alternative websites such as and which use similar technology. If they also don't work, the problem should be on your side, e.g. a missing flash player browser plugin. If the other sites work, please contact us by e-mail, thereby specifying your operating system and your browser. Note also that, in contrast to most other sites, we allow the downloading and offline viewing of videos, so this may be an ad hoc solution to such problems.

Download of video files is very slow

Compared to most other types of data, video files are extremely large. So even at a reasonable download speed of e.g. 50 KB per second downloads will typically take several hours. Also, the webserver hosting the video archive may be temporarily overloaded, especially during peak hours or after announcements have been made to a wide audience. Therefore, please wait some days after announcements, and try downloading during the night (European time).

Is it better to watch the videos online or offline?

As a sustainable datastream of more than 50 KBytes per second is difficult to guarantee on the internet, the online videos can only be offered at a low resolution of e.g. 320 * 240 pixels (similar to other video websites). This may be ok for getting an idea if a video is of relevance, but makes serious viewing unsatisfactory. For this reason, if you want to seriously work with a video, we recommend that you download the high resolution version and view it offline. The download may take several hours, but the picture quality is usually much better. An additional advantage is that in case of repeated viewing (e.g. by several persons) the file has to be downloaded only once.

Why is the technical quality of some videos poor?

As the video archive is based on a decentralized community driven approach, it is not feasible to buy one set of expensive professional video equipment and carry it to the relevant places all over the world. That is, only equipment as available to the local volunteers can be used. Although nowadays even inexpensive camcorders can produce reasonable results, this is only true for good lighting conditions. Unfortunately, in typical conference environments these may not be given depending on the quality of the beamers available. With poor beamers, the room has to be darkened, thereby making recording conditions very difficult. Furthermore, the fans in some beamers, PCs, or air conditioners can be fairly noisy, thereby severely disturbing the sound recording. Should you be able to help us filtering out this kind of noise, please let us know.

The slides in the videos are not readable

In many cases it helps if you download the higher resolution files for offline viewing. Alternatively, in many cases separate PDF-files with the slides are available, which can be looked at in parallel. In principle it is possible to automatically synchronize the PDF-file with the video, but this requires considerable manual effort. If you feel that this should be given a high priority, please let us know. But please also consider helping us in this time consuming task, or in finding volunteers for it.

Why are the discussions sometimes not included in the videos?

Although discussions are an important part of a scientific presentation, the problem is that permission for internet publication should be sought from all participants of the discussion. This is sometimes difficult as names and addresses are often unknown. Also, there are often acoustic problems as usually only the presenter has a microfone. For such reasons, although in practice most scientists would probably give their permission for video publication, the published versions of the videos often do not include the discussion part.

What can presenters do to improve the technical quality of a recording?

Slides should use large fonts only and colors must be selected such that a high contrast is guaranteed. If only the presenter has a microfone during discussion, it is desirable that he repeats the questions.

What can camera persons do to improve the technical quality of a recording?

First of all, a tripod should be used as random camera movement has a very negative effect on the viewing experience. Secondly, lighting conditions are also important. Although rooms typically have to be darkened for beamer presentation, this should be done only to the extend absolutely necessary as otherwise the camcprder does not have enough light, with the typical effect of this being that a lot of image noise is produced. With regard to sound, the microphone should be as close to the speaker as possible to avoid echoes (typical for large rooms) and acoustic noise (e.g. produced by fans as used in air conditions, PCs, and beamers). Therefore, in practice, external microfones that can be placed close to the speaker (e.g. using a neck strap) often lead to much better results than the internal camcorder microfones. If the camcorder does not have a microfone input (as usually the case with low or medium cost camcorders), we recommend to use a good quality MP3 recorder such as the iriver T20. Note, however, that standard low cost MP3 players with recording function usually deliver very poor sound recordings, so the quality needs to be tested. As the recordings from the internal camcorder microfones are available in addition, when cutting the video there is a choice of two different sound sources, and the better one can be selected. If the camcorder has a microphone input, we recommend to use a wireless lapel microphone. For example, a $50 microphone from Radio Shack has been reported to work ok. Note that professional solutions offering redundancy in wireless transmission (two different frequency bands are used in parallel)  start at prices that are at least 10 times higher.

How can camera persons ensure the best possible compression rate for a video?

Most importantly, a tripod should be used to avoid camera movement. Without it, much of the picture will change from frame to frame, and the compression algorithms (which all utilize similarities between frames) can not work well. Another important factor is to have good lighting conditions. If there is not enough light, the camera chip produces random noise which is again different from frame to frame, thereby adversely affecting the compression rate. Similar considerations apply to camera quality: The better the quality, and the larger the lense, the better will be the picture and the higher the compression rate. With regard to the conversion of video formats (as required for web presentation), it is best to convert from the original uncompressed file as delivered by the camera. The reason is that pre-compressed files contain artefacts from the compression algorithm which have the same effect as the arbitrary noise mentioned above. That is, the artefacts change arbitrarily from frame to frame and make further compression less effective.

What level of recording quality can be achieved?

Even without professional equipment as used for TV productions, it is in principle possible to arrive at a technical quality that makes the overall experience of a viewer as enjoyable as what the live audience may experience. The sound gains from the flexibiltiy in the placement of wireless microfones, presentation slides are well readable if they are synchronized PowerPoint presentations, and the speaker can be seen well due to the zoom function of the camera. An additional feature is that the viewer has the possibility to repeat sequences at will.

A well known scientist will give a presentation at our institute's research colloquium. Should I ask him if he wants to be recorded for the video archive?

From our side, this would be more than welcome, and our experience is that this often leads to very pleasant encounters. However, whenever possible please try to ask well in advance, so that the speaker has enough time to think about the pros and cons of a recording. In addition, even if the presenter agrees, he should always have complete control of what happens with his recording. This includes that he can withdraw the permission for publication any time, and without having to give reasons.

I have seen links to booksellers on the webpages of the video archive. Are these advertisements?

When putting a presentation on our server, we try to come up with related links that might be of interest to users, and which we then include on the webpage. In particular,  if we are aware of a related book written by the presenter, we include a link to the book. In selecting a link, we try to point to the most informative website, thereby disregarding whether it is a commercial or non-commercial site. These links are meant as an additional service to the users, and we feel not showing such important information would be a counterproductive way of understatement. Note, however, that the video archive is not participating in any bookseller's promotional programme, so these links do not serve the purpose of generating revenue for the archive. Should this be the case for the authors of scientific literature, then our view is that this is well deserved.

How can I cut videos using free software?

Video files in the avi format, for example as they come from Camcorder MiniDV-tapes, can be cut using the free software VirtualDub. VirtualDub can also merge avi-files. For mpeg2 files (e.g. as offered for download on the video archive) the standard version of VirtualDub does not work, but there is a modified version of it available on the internet. Another possibility  is to convert the mpeg2-file to wmv using the free Windows Media Encoder (to be downloaded from Microsoft), and then to perform the cut using Movie Maker. Movie Maker is a software for video editing and capturing that is included in Windows. The highly compressed flv-files as offered on the video archive are not intended for editing and can only be played (conversion is possible using commercial software, e.g. Adobe Premiere Pro, but we are not aware of free software for this purpose) .

How can I transfer my video files to others in high quality?

Of course a video tape or a copy of it on DVD can be sent by mail. Note, however, that standard 60 minutes MiniDV tapes have a capacity of about 13 GB (20 GB in long play mode), so for copying to DVD it may be necessary to split files and to use several DVDs. Due to their enormous size the internet transfer of such files can be extremely time consuming. For internet transfer we therefore recommend converting the files to the mpeg2 format, which without noticeable loss in quality can easily lead to compression factors of 1:5 or 1:10. If the original recording is in PAL format, the frame size should be chosen to be 720 * 576 pixels, whereas for NTFS the correct setting is 720 * 480. A suitable data rate is 5000 kbit per second in both cases. Lower data rates give higher compression, but may lead to artefacts in passages of the video with a lot of movement. When dealing with videos, older file systems such as FAT 16 and FAT 32 should be avoided  as they can not handle files larger than 2 GB.

How is support of the video archive acknowledged?

We always aim at giving full credit to all persons supporting the video archive. As the ACL video archive is meant to be a joint effort of the entire scientific community,   we invite all persons interested to join us in it. The idea is that all contributors and supporters have equal status (with only the ACL executive supervising), but that the amount of each person's involvement becomes visible by the number of mentionings on the pages describing the videos. Contributions that are not related to individual videos are mentioned on the acknowledgment page.

I have some CL-related videos on another website. Should I also make them available via the ACL video archive?

Yes, we would greatly appreciate this. There would be two options for you to choose: One is that we add a pointer to your site under the heading "Links to videos on the web" (bottom left of the page). The other is that we physically include copies of the videos in the archive. The latter requires more effort but would be our preference because this way users have the same interface and the same file formats for videos from different sources. This makes it very easy to quikcly browse through the videos. In contrast, having to deal with the hundreds of different video formats that are currently in use is likely to be very discouraging for users.

If a presentation is online at  the video archive, can it also be made available on other websites?

Yes, but a permission must be obtained. As the copyright is not transferred to the video archive, there is no problem with this from our side as long as the copyright holder (usually the presenter)  agrees.