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What is 'VHS Quality'?
Jon Ahrens asks: "Whenever you scan reviews on video technology, the articles consistently use a phrase "...VHS quality transmission". How do you measure VHS quality, qualitatively or quantitatively? And if quantitatively, what are the measures used, for example, number of pixels, depth of color, etc.? Are there specific IEEE or NBA standards that explain VHS quality?"
NTSC (the standard signal for American TVs) has 525 lines of resolution, but only 480 lines of visible picture. The remainder is taken up in the vertical blank interval. The signal is interlaced, so the odd scanlines are displayed on one frame, the even scanlines on the next frame. As far as horizontal resolution, there isn't a real pixel value since the signal is analog.
However, I remember some facts from when I used my Atari 400 computer. It had a maximum resolution of 320x192 pixels in graphics mode 8, in a single color. When I placed single pixels or vertical lines on the screen, they would be blue colored if they were on an odd column, and red if they were on an even column. This was commonly used as a hack to make graphics mode 8 display red and blue as well as black and white.
This shows the limits of a typical TV set. The dot pitch is so large that these color artifacts show up in 320x240 resolution. Your typical VHS VCR would be capable of the equivalent of 320 pixels across (the signal may have finer horizontal detail, but the TV won't be able to make use of it,) and 480 lines down, interlaced. Color is analog, so it can't translate to color depth, but NTSC got the nickname "Never Twice the Same Color" for a reason.
In short, VHS/NTSC quality is truly awful, the only reason it's viewable at all is because the picture is constantly moving, so your brain can interpolate more detail into the image from the multiple frames.
Meldroc, Waster of Electrons
an oxymoron.
--
"It's tough to be bilingual when you get hit in the head."
I like this 2- 4- 6-head paradigm espoused above. It's simple, concise, and understandable by mere mortals.
'Tis a shame that it's fundamentally incorrect -- at least, as far as NTSC goes. (PAL should be much the same on the VHS front.)
2-head VCRs are optimized for high-speed playback and recording - that is, two hours of video on a two hour ("T-120") tape. I don't recall why two heads are used - perhaps one for playback, and another for record, or to put luminance and chroma on seperate parts of the tape. Whatever the case, 2-head VHS looks as good with SP mode as anything else.
A fairly recent (say, less than 15 years old) 2-head VHS deck *can* record/play in other modes, but shoddily. The reason is that the heads in all VHS decks are spinning at high speed, while the tape moves at a relatively slow speed. Unlike analog audio cassettes (which use fixed heads), VHS uses a (helical scan?) spinning head, operating diagonally to the medium, in order to achieve the bandwidth required for video signals.
Obviously, you want to use as much of the magnetic media as possible, so the width of these diagonal scans is rather important. If you slow the tape down (say, because you want to get all of Titanic on one tape), they'll get closer together, and perhaps interact with eachother, causing obvious visual distortion that we all immediately identify with as a "shitty VCR." This is why nobody seems to like 2-head VCRs - not because their purchased video library (which is all produced in SP/2-hour mode) looks bad (indeed, it's just as good as a four-head deck, all else being the same), but because the stuff they tape themselves/"borrow" from friends is not quite up-to-par. (Side note: Ever notice that your 2-head VHS[C] camcorder only works at high speed? Ever wonder why?)
4-head VHS VCRs were introduced to allow people to record six hours of video on a T-120 tape, and do so with markedely higher quality. That's all, no mystery and no magic. Only pair of heads are ever used at once - those which are appropriate for the selected speed.
Playback/record quality of 6-hour tapes will be better on a 4-head deck, as opposed to one with 2 heads. Playback/record of 2-hour tapes will be identical between 2- and 4-head units (assuming the associated electronics are up to snuff).
6-head VCRs? Feh. Sounds more like a marketing ploy by JVC than anything else, unless it optimizes things for half (as well as 1/3 and full) speed tape travel. Or, I might be a bit behind the times, but this is all *really* old technology.
Of course, all of this negativity begs the question: What is "VHS Quality"? The answer is clear. If the end result looks like something that would make even a 1970s videophile grimmace and vomit repeatedly when compared to Beta, it's probably bad enough to be considered VHS quality.
Is that retort rather too subjective for the question at hand? Probably. But, the phrase "VHS quality" in reference to anything digital is highly subjective as well.
From the objective standpoint, consumer-grade digital video stuff tends to have nasty encoding artifacts of all kinds, including the holy grail of DVD. These artifacts are of such dramatically different nature from those introduced by VHS as to render *any* comparison completely invalid.
Kid-proof tablet..
If you are looking at cheap consumer grade products, then you will see comparisons with VHS. If you work on professional gear, you will see the term "broadcast quality".
:-)
VHS is a rather limited method of storing video signals. It roughly equates to under 480 scan lines with about 2.6 MHz of horizontal bandwidth and a fair amount of jitter in the timing signals. Compare that to a broadcast signal, which has 525 lines and 4.8 MHz of bandwidth, and very tight jitter for timing.
By cutting down on the bandwidth, you lose a lot of the clarity in the horizontal, and a little in the vertical. Since some of the scan lines are lost, the picture is a bit smaller and may be stretched or distorted to hide the fact. Text in credits is much harder to read and is usually blurry.
VHS quality means it is good enough for casual playback for the average couch potato viewer. The average viewer doesn't really care about signal losses, noise, jitter and jump in their picture, because their sets don't show the difference.
Do yourself a favor and take a look at broadcast quality gear sometime, and compare it side by side with VHS quality. There is a big difference, both is viewing pleasure and cost, you get what you pay for
But the term VHS quality just means its good enough for average consumers not to complain too much.
the AC
Hemos is like...sci-fi fans;he thinks technology is cool, but he hasn't bothered to understand the science it's based on