Special Coverage: Blu-ray

By the Numbers: Comparing DVD and Blu-ray Specifications

	DVD	Blu-ray
Disc Diameter	120mm	120mm
Disc Thickness	0.6mm x2	0.1mm (data) + 1.1mm (substrate)
Track Pitch	0.74µm	0.32µm
Min. Pit Length	0.40µm	0.15µm
Beam-Spot Size	1.32µm	0.58µm
Lens Numerical Aperture (NA)	650nm	405nm
Working Distance	1.7mm	0.5mm
Capacity	Single-layer: 4.7GB Dual-layer: 8.5GB	Single-layer: 25GB Dual-layer: 50GB
Maximum Data Transfer Rate	10.08Mbps	Video: 40Mbps Audio: 27.7Mbps depends on compression type, sample rate, bit resolution, no. of channels)
Video Codecs	MPEG-2	MPEG-2, H.264, VC-1
Audio Codecs	Dolby Digital, DTS, PCM, MLP	Required: Dolby Digital, DTS, PCM Optional: Dolby Digital Plus, Dolby TrueHD, DTS-HD, DTS-HD Master Audio
Maximum Duration of SD Material**	Single-layer: 2.25 hours* Dual-layer: 4.5 hours*	Single-layer (MPEG-2 @ 4–7 Mbps): 9 hours Dual-layer (MPEG-2 @ 4–7 Mbps): 18 hours
Maximum Duration of HD Material**	N/A (HD on DVD using VC-1 is possible, but not part of DVD spec)	Single-layer (MPEG-2 @ 25Mbps): 150 minutes Dual-layer (MPEG-2 @ 25Mbps): 300 minutes Single-layer (H.264 @ 12Mbps): 300 minutes Dual-layer (H.264 @ 12Mbps): 600 minutes

Notes
Track Pitch: Distance between adjacent turns in the spiral track.
Min. Pit Length: Minimum length of data region (pit).
Beam-spot size: Diameter of laser beam.
Lens Numerical Aperture: Measure of how tightly the laser beam can be focused.
Laser Wavelength: Wavelength of laser light.
Working Distance: Clearance between pickup head and disc surface.

*Maximum duration of SD material on DVD is highly dependent on the bit rate and specific MPEG-2 encoder used, so these numbers are very rough estimates based on typical bit rates.

**The figures given for maximum duration of SD and HD material on Blu-ray include audio information, but the bandwidth required for the audio portion is not included in the specified bit rates.

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