This post is not intended as a technical document but more a help page for those that deal with audio and video evidence that is now more frequently found on a disc rather than a tape. Many police forces now habitually convert analogue and digital video material to DVD-Video but may also serve the material to defence solicitors as copies of the original data disc, this post should help you to understand the differences between the two types of disc format and allow you to view the material.
Lets start with what a DVD is so we all know where we’re starting from.
First:What does DVD stand for?
If I was on the television program QI I’d hold up the “Nobody Knows” placard, DVD is one of those acronyms that has passed into our vocabulary but for which nobody seems to be able to agree on the meaning. The people who first developed the format used Digital Video Disc but the DVD Forum decided it should be Digital Versatile Disc but later decided that as the meaning couldn’t be agreed on and ‘DVD’ was an international standard then the three letters were exactly that, just letters. So although it does seem a bit strange it is correct to refer to a disc as a ‘DVD disc’.
What is it?
DVD is a format for storing data on a disc with the same physical size as a Compact Disc but has the capacity to hold a minimum of 4.7GB of data. Commercially created discs are referred to as DVR-Rom (Read Only Memory) and can not be written to or altered. Writable discs, DVD-R (DVD dash R) and DVD+R (DVD plus R) store data in a similar way as that used to create a Compact Disc (CD) and can be read by an optical laser and converted by hardware and software into a format that can be viewed and/or heard. A writable disc can be termed as Write Once/ Read Many (WORM) or Re Writable (RW). The +R and +RW format are not recognised by the DVD Forum but are supported by the DVD+RW Alliance.
Discs containing audio and/or video evidence should be of the WORM type to prevent data being deleted or added.
Formats and duplicating
So we’ve established that DVD is an optical storage medium, but the next issue is that there are many variations of DVD formats, the most popular WORM formats are DVD-R and DVD+R with DVD-RW, DVD+RW and DVD-RAM being Re -Writable formats. You are most likely to come across DVD-R and DVD+R discs, the difference between the formats is more relevant to the Re-Writable versions of these formats and are not relevant to this discussion. If you want to duplicate a disc you will need to use the same format of disc. Discs can contain up to 4.7 GB of data there are also Dual Layer (DL) single sided discs available which have a capacity of 8.5GB which also need a specific type of disc for copying. There are also double sided single layer and double sided dual layer discs but these are very rarely used.
Playing the discs
The main difference that will effect end users is whether the disc contains data or DVD-video.
If the disc contains data exported from a Digital Video Recorder (DVR) it is often exported to a disc, depending on the amount of data then this could be a CD or a DVD. This disc will not play in a set top DVD player, a PS3 or xBox 360 but can be accessed using a computer which has a DVD drive. To view the video files you may need to install the proprietary software player which should be included on the disc.
The disc may alternatively contain other types of data and/or audio and video files that can be viewed on a computer using software such as Windows Media Player or Quicktime.
The content of these discs can be viewed using most modern set top DVD players or a games machine such as a PS3 as well as your computer providing it has a DVD drive and software that is capable of playing DVDs. The content may be structured with a menu or may simply just play the video contained on the disc.
One reason that the DVD wont work in your DVD player is that it contains data and your player is expecting it to have DVD-Video, so before sending the disc back as defective you may want to try it in your computer first.
If you have any questions about audio and video evidence contained on CD or DVD discs or other aspects of audio and video evidence please call or use the contact form (Contact Us) to request a call back.
In my next post I will talk more about how the fabled non destructible disc is in fact very fragile, despite what Raymond Baxtor said about it in 1981 on Tomorrows World (yes I am that old)..