DVD vs VHS in North America: A comparison of Video and Aural output
Created | Updated Dec 13, 2004
In the early 1980s, a consortium of Japanese electronics manufacturers developed an analog, magnetic tape format for the storage of video information. VHS, as the format was dubbed, became the standard for consumer video storage and playback for the next 15 years. The late 1990’s saw the introduction of DVD, a digital medium, which vied to replace VHS. While the standard VHS cassette stores 120 minutes of video data and 3 distinct audio tracks, DVD stores as much as 4 hours of video and up to 8 separate audio tracks, each capable of delivering discrete channel output. In comparison to VHS cassettes, DVD offers superior video and audio playback. This can be demonstrated by comparing the resolution, the aural output, the data transfer mediums, and the storage systems for both formats. Through such a comparison, the benefits each format has to offer will be made clear.
A meaningful comparison of resolution requires an understanding of how an image is produced on the standard NTSC television monitor. The black and white image, which is carried by the luminance signal, comprises the majority of video data, and therefore, resolution. Colour resolution is defined by the number of colour changes in a given distance horizontally, and affects how crisp the border between different colour fields can be. Changes in the shade of a colour are controlled by luminance and are not affected by the limitations of colour resolution. A standard VHS cassette is capable of producing an image with 32 lines of colour resolution, but typically only produces 25. DVD is capable of displaying 270 lines of colour resolution, although this can sometimes fall as low as 240. The result is that colours displayed from a DVD have a higher degree of luminance, appear crisper, sharper and more true to life than its VHS counterpart.
Additionally, when an image is displayed off of a VHS cassette, it is in an interlaced format. What “interlaced” means is that, at any given time, only half the resolution of the image is being seen on the screen. An interlaced picture is broken into two parts, even numbered scan lines, and odd numbered. These are displayed separately on the monitor, although too fast for the human eye to tell. The result is an image, which is only half the resolution that the monitor is capable of producing. DVDs can output a progressive scan signal. Progressive scan, unlike interlacing, displays one full frame of image data. This delivers a much higher resolution picture, and also has the added benefit of eliminating motion artifacts; a common problem with interlaced images.
The VHS format allows for 3 distinct audio channels: left, right, and mono. While it is possible to record a distinct audio track for each of these, in common practice only the left and right channels are used, creating a stereo effect. DVD has the ability to facilitate up to 8 separate audio channels. Each of these channels is capable of carrying 5.1 discrete audio layers. Discrete audio layers allow for a unique audio signal to be sent to specific speakers, usually via an amplifier featuring an audio decoder. The 5.1 channels are; right channel, left channel, center channel, rear left channel, rear right channel, and subwoofer. Since the subwoofer only generates low frequencies, and not actual sound data, it is not considered a full channel on its own and is therefore counted as the ".1" in 5.1 audio. The dynamic audio range of DVD, coupled with the capacity for carrying up to 8 separate audio tracks with better-than-CD quality sound, clearly makes DVD the superior format in this comparison.
Video data means nothing if it can not be communicated to the monitor. To compare VHS to DVD, it is necessary to compare both formats highest quality data transfer medium. In the case of VHS, this is the S-Video cable. The practical limit for resolution on an S-Video cable is 120 lines of colour resolution, although the theoretical limit is 140. DVD uses component video cable, which has no theoretical limit for the transfer of colour resolution. Component cable sends 3 distinct video signals: red, blue, and green. This division of video information allows for supremely high quality image transfer. S-Video, carrying all of its video information in one cable at a maximum of 400KHz, is unable to provide the same limitless potential for resolution delivery.
VHS is an analog storage system. The consequence of this, as far as video quality is concerned, is that the image has not been compressed in any way. The image is unaltered from its original form, save for the lack of resolution and any possible panning and scanning (a technique used to crop a widescreen picture to fit a square monitor). DVD is a digital storage system. While it is possible to store video digitally without compressing it, DVD video is always compressed to maximize the amount if video information which can be stored on the disc. What compression does to an image is mathematically break it down, assigning blanket values to areas of similar or identical information. To put it simply, a black background is not stored as thousands of pixels of black information, but rather as a single value defining a large area of the image. While this does have the effect of losing visual data from the original image source, careful mastering of a DVD prevents such information loss from being visible to the naked eye. So, while VHS delivers an unaltered representation of the original visual data, DVD counters with higher resolution, resulting in a more detailed picture, despite the loss of visual data.
In all aspects of video playback, DVD clearly out performs VHS at every level. Resolution, while adequate on VHS, makes an enourmous leap in quality on the DVD format. Audio presentation, limited to stereo and mono playback in VHS, is outmatched in every way by the vast audio capabilities of DVD. S-Video is certainly a high quality video-transfer medium, but it simply cannot compare to component video, with its unlimited potential to deliver exceptionally high resolution. Finally, while VHS delivers an unaltered analog presentation of a video image, the inherent resolution deficiencies of the format prevent it from competing with the digitally compressed image stored on DVD. While it should be made clear that VHS is a reasonable format for video storage, the benefits of DVD are undeniable, and clearly make the format superior.
A meaningful comparison of resolution requires an understanding of how an image is produced on the standard NTSC television monitor. The black and white image, which is carried by the luminance signal, comprises the majority of video data, and therefore, resolution. Colour resolution is defined by the number of colour changes in a given distance horizontally, and affects how crisp the border between different colour fields can be. Changes in the shade of a colour are controlled by luminance and are not affected by the limitations of colour resolution. A standard VHS cassette is capable of producing an image with 32 lines of colour resolution, but typically only produces 25. DVD is capable of displaying 270 lines of colour resolution, although this can sometimes fall as low as 240. The result is that colours displayed from a DVD have a higher degree of luminance, appear crisper, sharper and more true to life than its VHS counterpart.
Additionally, when an image is displayed off of a VHS cassette, it is in an interlaced format. What “interlaced” means is that, at any given time, only half the resolution of the image is being seen on the screen. An interlaced picture is broken into two parts, even numbered scan lines, and odd numbered. These are displayed separately on the monitor, although too fast for the human eye to tell. The result is an image, which is only half the resolution that the monitor is capable of producing. DVDs can output a progressive scan signal. Progressive scan, unlike interlacing, displays one full frame of image data. This delivers a much higher resolution picture, and also has the added benefit of eliminating motion artifacts; a common problem with interlaced images.
The VHS format allows for 3 distinct audio channels: left, right, and mono. While it is possible to record a distinct audio track for each of these, in common practice only the left and right channels are used, creating a stereo effect. DVD has the ability to facilitate up to 8 separate audio channels. Each of these channels is capable of carrying 5.1 discrete audio layers. Discrete audio layers allow for a unique audio signal to be sent to specific speakers, usually via an amplifier featuring an audio decoder. The 5.1 channels are; right channel, left channel, center channel, rear left channel, rear right channel, and subwoofer. Since the subwoofer only generates low frequencies, and not actual sound data, it is not considered a full channel on its own and is therefore counted as the ".1" in 5.1 audio. The dynamic audio range of DVD, coupled with the capacity for carrying up to 8 separate audio tracks with better-than-CD quality sound, clearly makes DVD the superior format in this comparison.
Video data means nothing if it can not be communicated to the monitor. To compare VHS to DVD, it is necessary to compare both formats highest quality data transfer medium. In the case of VHS, this is the S-Video cable. The practical limit for resolution on an S-Video cable is 120 lines of colour resolution, although the theoretical limit is 140. DVD uses component video cable, which has no theoretical limit for the transfer of colour resolution. Component cable sends 3 distinct video signals: red, blue, and green. This division of video information allows for supremely high quality image transfer. S-Video, carrying all of its video information in one cable at a maximum of 400KHz, is unable to provide the same limitless potential for resolution delivery.
VHS is an analog storage system. The consequence of this, as far as video quality is concerned, is that the image has not been compressed in any way. The image is unaltered from its original form, save for the lack of resolution and any possible panning and scanning (a technique used to crop a widescreen picture to fit a square monitor). DVD is a digital storage system. While it is possible to store video digitally without compressing it, DVD video is always compressed to maximize the amount if video information which can be stored on the disc. What compression does to an image is mathematically break it down, assigning blanket values to areas of similar or identical information. To put it simply, a black background is not stored as thousands of pixels of black information, but rather as a single value defining a large area of the image. While this does have the effect of losing visual data from the original image source, careful mastering of a DVD prevents such information loss from being visible to the naked eye. So, while VHS delivers an unaltered representation of the original visual data, DVD counters with higher resolution, resulting in a more detailed picture, despite the loss of visual data.
In all aspects of video playback, DVD clearly out performs VHS at every level. Resolution, while adequate on VHS, makes an enourmous leap in quality on the DVD format. Audio presentation, limited to stereo and mono playback in VHS, is outmatched in every way by the vast audio capabilities of DVD. S-Video is certainly a high quality video-transfer medium, but it simply cannot compare to component video, with its unlimited potential to deliver exceptionally high resolution. Finally, while VHS delivers an unaltered analog presentation of a video image, the inherent resolution deficiencies of the format prevent it from competing with the digitally compressed image stored on DVD. While it should be made clear that VHS is a reasonable format for video storage, the benefits of DVD are undeniable, and clearly make the format superior.
