DVD Capacities
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DVD Capacities

One of the main advantages of the DVD format is the large storage capacity. There are many variations on how a DVD is put together and therefore, many variations on how much a DVD will hold. There are several factors that determine the actual viewing time for any given format. There are several reason why the DVD can hold so much more than a CD even though they are both the same size. For your convenience I have tabulated the information below.

Reasons for Added Capacities of DVD

Factors:

Channel bit code 8/14 + (3 merge bits) -> 8/16 EFM
Larger surface area utilization 86.0 to 87.6 square centimeters
Decrease in RS code 25% to 13% of channel rate
track pitch 1.6 to 0.74 microns
min pit length 0.972 to 0.4 microns
Reduction in packet overhead (2048/2352 to 2048/2060 bytes)


Gain:

Areal increase in channel bits 5.254 ((1.6*0.972)/(0.74*0.40))
Packet overhead reduction 1.142 (2352/2060)
Tigher FEC 1.16 (0.87/0.75)
Increase in usable area of disc 1.019 (87.6/86)
Tighter channel code 1.0625 (17/16)
Overall gain 7.5 4.7 / 0.65 GBytes

DVD Storage Capacities

Format Sides/Layers Capacity TIme
DVD-5     (12cm) SS/SL 4.38 gig (4.7 G) of data over 2 hours of video
DVD-9     (12cm) SS/DL 7.95 gig (8.5 G) about 4 hours
DVD-10   (12cm) DS/SL 8.75 gig (9.4 G) about 4.5 hours
DVD-14   (12cm) DS/ML 12.33 gig (13.24 G) about 6.5 hours
DVD-18   (12cm) DS/DL 15.90 gig (17 G) over 8 hours
DVD-1     (8cm) SS/SL 1.36 gig (1.4 G) about half an hour
DVD-2     (8cm) SS/DL 2.48 gig (2.7 G) about 1.3 hours
DVD-3     (8cm) DS/SL 2.72 gig (2.9 G) about 1.4 hours
DVD-4     (8cm) DS/DL 4.95 gig (5.3 G) about 2.5 hours
DVD-R     (12cm) SS/SL 3.68 gig (3.95 G)
DVD-R     (12cm) DS/SL 7.38 gig (7.9 G)
DVD-R     (8cm) SS/SL 1.15 gig (1.23 G)
DVD-R     (8cm) DS/SL 2.29 gig (2.46 G)
DVD-RW  (12cm) SS/SL 3.68 gig (3.95 G)
DVD+R    (12cm) SS/SL 4.38 gig (4.7 G) of data
DVD+RW (12cm) SS/SL 4.38 gig (4.7 G) of data
DVD-RAM (12cm) SS/SL 2.40 gig (2.58 G)
DVD-RAM (12cm) DS/SL 4.80 gig (5.16 G)

Factors Affecting Capacity

bulletNumber of sound tracks. Each sound track takes up space. This reduces the overall length of movie that can be stored. This includes commentary tracks and foreign language support.
bulletNumber of channels in each soundtrack. It naturally takes more space or bandwidth to store the six channels required for Dolby 5.1 than it does for the two track Dolby 2.0.
bulletType of Audio Tracks.  DTS is almost 4 times as large as a comparable soundtrack stored in Dolby 5.1. This is because the DTS soundtrack is not compressed to the same degree as the Dolby 5.1. If the soundtrack is stored in PCM (pulse code modulation) the quaility will be better than a CD but is limited to two channels. The space required for storage is also much less for PCM than other audio formats. The processing of the six channel formats, DTS and Dolby 5.1, are similar to those used in most theaters and naturally take a lot more space on the disc.
bulletThe new SuperBit format reduces compression to achieve greater throughput but the trade off is there is no room for the much coveted extras. Many distributors go the opposite way and provide a two disc set with the film on one disc and the extras on the second.
bulletCompression of the MPEG-2 files. As anyone that has worked with ZIP files knows, there are various degrees of compression a given algorithm can achieve. Very complex scenes that change often will not compress to the same degree as a simple scene.  
bulletThe quaility of the original material. Material that is in a lesser quality will compress to a smaller size. If the producers feel that quality equivalent to a VHS tape is sufficient then less space will be used on the DVD. Fortunately, most movies released on DVD are processed to provide the greatest possible quality.
bulletSeamless branching. One ability the DVD has is to provide different versions of a film on one disc. This is done by storing the alternate scenes and using the DVD internal software to decide which pathway the presentation takes. This is often used to provide a director's cut with the theatrical, rated and unrated etc. Examples of this are movies like Independence Day.

Dual Layer Notes

You do not actually achieve twice the capacity when a second layer is added to the DVD. Since the laser has to "read through" the outer layer to obtain information from the inner layer there is some loss. The outer layer is about 20 to 70 microns. To compensate for this distance and signal loss of the laser passing through the first layer, the minimum pit length of both layers is increased form .40 um by 10% to .44 um, This assists in the reduction of crosstalk between the layers. The reference scanning velocity is also increased from 3.49 m/s on single layer DVDs to 3.84 m/s on dual layer DVDs. This is because longer pits that are spaced further apart read more easily, more accurately and are less prone to data drop out due to vibration. The net affect of this is fewer pits are read per revolution which translates to a lower overall capacity.

Miscellaneous

Tip, It takes about two gigabytes to store one hour of average video.

DVD-18 is also called two sided RSDL (reverse-spiral dual layer). Due to the difficulties in manufacture this format is falling to the side of the road. Using two discs instead of a DVD-18 provides less failures and is therefore preferred by most distributors. Artisan had been the leader in DVD-18 but have been quietly reissuing those titles in multiple disc sets.

On some (if not most) dual layer discs there may be a noticeable pause or disruption of play during the switch from one layer to the next. This is not exhibited in DVD-18 discs so far. The reason for this is DVD-18 titles tend to plan the layer change a bit better.

Dual layer discs can be identified by the greenish-gold color of the playing surface. Single layer discs appear silver in color.

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