Review-- Last-gasp 5" B&W analog TV featured at Big Lots...

[YamahaFreak]

Originally Posted by old_tv_nut

Dc restoration means the set keeps a constant black level independent of scene content. In sets without DC restoration, the average video level is constant because the varying DC level is blocked by a coupling capacitor. So if the original scene has lots of DC due to having large bright areas, the set lacking DC restoration will reduce the average level, and all areas will get too dark, and the shadows will be cutoff and just be black blobs. On the other hand, in a scene with lots of dark areas, the DC level will be too high and the scene will show as washed out and foggy.

Oh, I get it now!

[ChrisW6ATV]

Yes, old_tv_nut described it perfectly. This is one reason an old B&W TV can look so good.

[Whirled One]

Frankly, I've never understood why DC restoration has been so conspicuously absent on B&W sets in general since the early 1950's. I can see why manufacturers started leaving it out during the "dollar engineering" days of the mid/late 50's, but it never really made a comeback even after the TV manufacturers generally stopped cutting quite as many corners in the 60's.


There did seem to be a bit of "revival" in DC restoration around the early 1970's (probably thanks in part to publications like _Consumer Reports_ that kept pointing this out), but it was pretty spotty, especially among USA-made sets; even my high-end Zenith 19" B&W from 1973 doesn't have DC restoration.

I've never really understood this, even on these 'modern' 5-inch cheapie B&W sets-- I mean, all you need for 'basic' DC restoration is a diode and perhaps a resistor and bypass cap, which today would cost at most a few cents-- or even nothing if the componentes were integrated into an existing IC used in modern sets.

But, yeah, I find lack of DC restoration rather *annoying*-- I never understood why it pretty much disappeared on B&W sets (and even *color* sets often didn't have DC restoration either), since it's a pretty obvious picture fault (at least to me)-- I would think that a manufacturer that included DC restoration could have turned that into a useful selling point. ...Perhaps give it a fancy "sales ready" name, like "Automatic Black Level Regulator" or "Lumi-Matic" or something like that.

The only possible explanation for leaving it out that I've ever heard is that TV stations at the time often didn't do a very good job at keeping black levels consistent anyway, so lack of DC restoration in the receiver wasn't considered to be a big deal. I dunno how true that was though.

[Whirled One]

Originally Posted by ChrisW6ATV

Yes, old_tv_nut described it perfectly. This is one reason an old B&W TV can look so good.

Yep. For those who might still not be sure what effect DC restoration has on a picture, here are a couple of common examples where the effect becomes really obvious:

- Credit roll with white text on a black background-- on a TV with good DC restoration, the background will stay black no matter how much (or how little) white text is on the screen, and the white text will stay the same white. With no DC restoration, the "black" background may not go totally black, and will shift up and down in brightness depending on how much white text is on the screen, and horizontal "stripes" of darker grey may form around the white text as it rolls up the screen.

- Fade to black-- a TV with good DC restoration will actually fade to black. On a TV with no DC restoration, the scene will instead wash out into a mid grey.

Also, while only applicable to modern times, try watching any sort of "letterboxed" content on a TV with no DC restoration, and you'll see the "letterbox" areas of the screen pulse up and down in brightness as the overall brightness of the scene changes.

[ChrisW6ATV]

My first color TV in 1978 was a thrift-shop 19" Sears set that also lacked DC restoration. Yes, I learned what it was from Consumer Reports magazine, and once I knew (and looked at my brother's Sony set to see it), I had to have it.

I have never tried to watch any widescreen content on a B&W display (without DC restoration), but I can imagine it would look even more distracting than the set would on other content in general.

[MRX37]

I have a 5 inch B/W TV made in 2001. There's nothing remarkable about it except for the fact that it can tune up to channel 83. Seeing as Channels 70-83 were converted to mobile usage in the 80's, it's weird for this TV to be able to tune those channels.

Comes in handy for listening to dispatchers, almost like a police scanner.

[old_tv_nut]

Color sets always had *some* DC restoration. It was often deliberately made to be 85% or so. The argument at the time was that it reduced the visibility of the variations in the transmissions. About the time the networks got things under control, cable came along and generally screwed up everything possible in terms of differences between channels.

Another unspoken reason for lack of DC restoration is that it reduces beam currrent on scenes with lots of bright areas - so, you didn't have to be so precise in making sure the customer couldn't overload the HV section if he cranked the contrast to max.

Also, before the days of video processing ICs, where the "contrast" (actually now a "picture" control) could be controlled by DC, the beam current limiter had to work by reducing the black level- so, if the cutomer pushed the "contrast" too high, when an extra briight scene came along, the black level would be deliberately pulled down to limt the beam current. This could be for two reasons: protect the HV/horizontal sweep and/or limit Xrays.

Once the picture drive level could be controlled by a DC voltage, it could be used ot limit beam current, and there was no excuse for not having 100% DC restoration. This was the last gasp for B&W chassis, however, and the philosophy of less than full restoration hadn't died yet.

Studio monitors, of course, always had full DC restoration, but generally they made lousy TV's for other reasons, like maybe they expected clean signals and wouldn't sync worth a damn on noisy, ghosty over-the-air signals.

[ChrisW6ATV]

Another big reason professional monitors did/do not work well in a home environment is the general public's desire to have their TV as bright as the sun if possible. Studios (other than in the on-camera area) are pretty low light.

[compucat]

Originally Posted by Whirled One

- Fade to black-- a TV with good DC restoration will actually fade to black. On a TV with no DC restoration, the scene will instead wash out into a mid grey.

I had a 12" Zenith B&W from 1987 that must have had DC restoration because when the scene would fade to black, it looked like the set was off. I never knew what it was at the time but I always noticed that the better sets could do that. I don't think any of the black and white sets I have now feature DC restoration.

[Carmine]

Originally Posted by MRX37

I have a 5 inch B/W TV made in 2001. There's nothing remarkable about it except for the fact that it can tune up to channel 83. Seeing as Channels 70-83 were converted to mobile usage in the 80's, it's weird for this TV to be able to tune those channels.

Comes in handy for listening to dispatchers, almost like a police scanner.

MRX, Don't you live near me in Detroit? I've never been able to pick anything up on UHF 70-83, and I've tried DX'ing that before.

I have 88.7 FM set as a preset on my upstairs '78 Zenith set!

[YamahaFreak]

Originally Posted by Carmine

MRX, Don't you live near me in Detroit? I've never been able to pick anything up on UHF 70-83, and I've tried DX'ing that before.

I have 88.7 FM set as a preset on my upstairs '78 Zenith set!

I didn't know you could do that!

[MRX37]

Oh yes! Channel 6's audio spectrum is VERY close to the start of the FM radio band! Just about ANY dial TV can bring in 89X (88.7 FM) just by fine tuning the dial. Some TV's I used to have could get up to 95.5.

And as for using a TV to pick up dispatchers. It's possible, but inreasingly rare these days due to encryption and frequency switching. I used to pick em up all the time.

[YamahaFreak]

Originally Posted by MRX37

Oh yes! Channel 6's audio spectrum is VERY close to the start of the FM radio band! Just about ANY dial TV can bring in 89X (88.7 FM) just by fine tuning the dial.

And as for using a TV to pick up dispatchers. It's possible, but inreasingly rare these days due to encryption and frequency switching. I used to pick em up all the time.

I gotta go try that with my '63 GE.

[Sandy G]

You used to be able to pick up at least "half" of cell phone conversations on the upper UHF band...Not really very well, & not often, but I've heard 'em there...Really more of a technical oddity than any serious eavesdropping...

[MRX37]

Here's another thing I used to play with:

Those toy walkie talkies that operate in the 49 MHZ range, usually they have an adjustable coil inside them. I could break into Channel 2 and 3's audio carrier and hear my voice on TV.