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?"

I think..

[Kris_J]

• 2 head VHS quality is approximately equal to 320x256 with heavy fringing and poor colour reproduction. Bit depth doesn't work (it's analog), think 30% JPEG quality.
• 4 head VHS quality is about 640x256 with less fringing, say 50% JPEG quality
• 6 head VHS is much closer to 640x512, but more via good interpolation than actually recording all the lines. A high quality deck and good tapes will all but eliminate the fringing, say JPEG at 80%.

They're PAL line counts. I thing NTSC has fewer lines - 200/400? As far as the fact that they're moving images, think motion JPEG rather than MPEG, the frames are all independant.

I hope this helps.

Re:NTSC and VHS quality.

[GodSpiral]

I'd say the colour accuracy is closer to 10-11 bits than 8. about 1500 bues.

VHS quality though has no standard, and is a marketing term. It is meant to be a description of sufficient quality to replace the VHS experience. It is extremely vague, and since the technology proponents wish to glorify their products, you should assume that their definition of VHS quality is a cheap vcr with dirty heads playing a 20th generation tape for the 100th time.

Analog artifacts are very different than digital ones. To me, and probably most viwers though, they are less irritating than digital artifacts.

I'd rather have less colour and sound depth with a smooth 30fps, than high res that skips and pauses, or occasional blocks where motion is occurring.

NTSC info off the top of my head.

[meldroc]

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.

Re:NTSC info off the top of my head.

[benenglish]

>>>VHS/NTSC quality is truly awful

Well, OK. I'll grant that in theory. But y'know something? Most people that speak ill of NTSC standards have never seen the full potential of the standard. Most TV sets are so horribly adjusted, right from the factory, that the pictures are flat awful. (In fact, most TVs don't even have user-accessible controls adequate to properly display an NTSC-compliant picture. The really good set-up guys use expensive testing equipment and usually have to screw around with things inside the chassis.) Much of this is done so that they are viewable in over-lit rooms. But if you've ever had the experience of seeing a PROPERLY adjusted, high-quality TV in a dark room, you would be VERY pleasantly surprised by the quality of the picture. It ain't real life, but it's astoundingly better than what most people are accustomed to.

Poke around at www.theperfectvision.com if you're actually interested in getting the best possible picture on your TV.

VHS Quality is...

[Tower]

an oxymoron.

--

Re:NTSC info off the top of my head.

[rizzo242]

The signal is interlaced, so the odd scanlines are displayed on one frame, the even scanlines on the next frame

IANABE (I Am Not A Broadcast Engineer), but truthfully, each 'frame' is the composite of both 'fields', one for the even scanlines, one for the odd scanlines. Each field is drawn in 1/60th of a second to compose a full frame in 1/30th of a second (30 frames per second).

If you ever look into really serious (read "expensive") video digitizing boards such as a TrueVision Targa or Bravo card, you'll see they promote "full 60 fields/sec" digitizing, which is incredibly important for professional video editing intended for output back to video.

VHS, SVHS, and Digital

[AgentRavyn]

While I may be a recent HS graduate, I have worked with quite a bit of video equipment over the last four years, most of it semi-professional or professional grade. One things that I noticed about VHS is how quickly the quality deteriorates. The quailty is piss-poor to begin with, but after the 3rd generation it becomes unbearable. If you don't know what I mean, imagine looking at the picture through cloudy saran wrap. SVHS loses quality much slower, but is still bad. Our digital was great. Too bad we didn't order a decent editing program. But, the big problem was getting the digital to the school broadcast system--we taped it to SVHS for a while, but the the whole point of digital was lost. Eventually we jury-rigged it, but... *shrugs*

It seems that no one but the people in my video classes noticed the difference between VHS and SVHS. A few noticed the digital, but not many. The whole point is, no one's gonna notice except whose with an eye for it. So, if you're doing something like cheap pr0n, no one will care.
___
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VHS on NTSC

[yerricde]

I thing NTSC has fewer lines - 200/400?

NTSC televisions generally have 224/448 viewable scanlines (320x224, 640x224, 640x448) and worse color fidelity (Never The Same Color).

As far as the fact that they're moving images, think motion JPEG

For an idea of what intraframe transform coded video (e.g. motion JPEG) looks like, look at full-motion video from a PlayStation game.

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adopt a normal bird
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XGNOME vs. KDE: the game! [8m.com]

Re:NTSC info off the top of my head.

[Just6979]

Even though the "resolution" seems horrible, it's actually not too bad since TVs have round pixels. When watching movies with my VCR plugged into my TV-capture card, the picture is noticably blocky because of the square pixels of a computer display, even with 3/4 of a million of those square pixels (1024x768). The square pixels really show up when the image has stripes in it. Very annoying moire patterns. Now, anti-aliasing would fix this, but I have yet to see a consumer graphics card with FSAA for _2D_.

Round pixels are one of the reasons HDTV looks so good

How game engines get away with high res

[yerricde]

Because only odd scanlines are drawn during odd frames, and even scanlines are drawn during even frames, games such as Tobal No. 1 and Ehrgeiz on PlayStation, as well as most of the Dreamcast games, can get away with drawing polygons only to half the screen every frame. Add textured polygons and the general blurriness of the NTSC standard (cheap texture filtering), and Ehrgeiz looks almost TV quality.
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XGNOME vs. KDE: the game! [8m.com]

Re:I think..

[Anonymous Coward]

VHS playback always uses only two heads. Four head VCR's generally use their extra heads for still frame/slow motion effects, six+ head VCR's have specialized heads that are used for different purposes (e.g. different heads for recording than for playback, or for EP(SLP)/SP modes.) Perhaps you are thinking of SVHS.

IIRC, VHS has approx. 240 lines horiz. but it's been awhile I could be wrong. VHS color fidelity is horrible, nothing compared to Laserdisc or DVD. VHS quality is much inferior to broadcast NTSC. I think most people don't realize how good broadcast NTSC can look, they never see it, most people have cable with is usually worse than VHS. Oh well...

See:

Society of Motion Picture and Television Engineers [smpte.org]
Imaging Science Foundation [imagingscience.com]
Widescreen Review [widescreenreview.com]

Resolution

[LMacG]

I always thought this was discussed in terms of resolution; e.g. a typical NTSC broadcast provides 330 lines of horizontal resolution, a standard VHS tape 240, and a DVD 420. This is discussed a lot more clearly (npi) at the Secrets of Home Theater and High Fidelity [sdinfo.com] site. Don't be alarmed by the date, the information hasn't changed.

Video Bandwidth

[Detritus]

I believe "VHS quality" is directly related to the video bandwidth. A video tape recorder (VTR) has two important qualities, bandwidth and signal-to-noise ratio. A broadcast quality NTSC signal uses about 4.2 MHz of bandwidth. A video signal has three components, luminance, chroma and audio. Luminance is what you see on an old black & white television set, brightness, ranging from black to white. Chroma contains the color information, hue and saturation. Television signals devote more bandwidth to luminance than chroma, the idea being that chroma doesn't need as much resolution as luminance. A consumer grade VTR isn't good enough for broadcast quality NTSC. How do they put NTSC video on a VHS VTR? They cut corners by reducing the bandwidth used for luminance and chroma, and accept a poorer signal-to-noise ratio. This mean that compared to the original signal, the output of a VHS VTR has less resolution or detail in the luminance and chroma plus more noise. The chroma is the most obvious casualty of this process. VHS throws away about 50% of the luminance and even more of the chroma.

Re:I think.. not.

[adolf]

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.

There are two levels of reviews

[anticypher]

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

Re:I think.. not.

[Kris_J]

Okay, that's two people now that have told me that there's no direct technical link between the number of heads and the picture quality of video recorded at normal speed on a VHS tape. Fair enough, but it's still a good "rule of thumb" to simply gauge the quality of a deck.

This is the issue with analog stuff. There is no one quality, it depends on the equipment -- just like a brand new LP on a several thousand dollar turntable can sound better than a CD.

Instead of the direct link to 2, 4 & 6 head VCRs on my original post, just think low, medium & high quality decks. And don't think the tape doesn't make a difference either.

I only have a 50/60Hz 68cm TV. I see a medium-sized interlaced picture regardless of the quality of the signal put into it, and as such my top of the line Sony VHS (not S-VHS) with good BASF tapes reproduces my recordings from cable at such a quality that I don't notice the difference between a recording and stuff that's live. (Not that that's saying much, I've spotted MPEG compression in some of the cartoons).

If we're going to start talking about DVDs, I have to say this - for me the screen dictates the apparent resolution of the image, making DVD seem no better resolution than a good VHS records, however the colour is much more vivid on a DVD than a tape, better even than broadcast and cable TV. It's just a shame about the region lockout crap...