Rolloff, Lamps, HDTV

Going Down
In the measurement section of some speaker reviews, I've often read that a speaker's -3dB point is at a particular frequency and the -6dB point is at another frequency. Could you please explain from what level the speaker is down 3 and 6dB?

Ronald Benson

All speakers output a certain volume when fed an input signal at a certain level or amplitude. Hopefully, that output volume remains constant for a given input-signal level across a wide range of frequencies. (Of course, most speakers are not perfectly "flat" in this way, but the output of good ones varies only a little as the frequency changes, and that variation is normally no more than ±1dB or ±3dB from an average or nominal value.)

But below a certain frequency, the speaker can't output the same volume for the given input-signal level, so the output volume "rolls off," dropping more and more as the frequency falls. The same thing happens at the top end, but in most cases, the -3dB and -6dB points are specified for the low end of the frequency range.

As the frequency decreases, the output level falls, and at some particular frequency, it reaches -3dB below the nominal level. As the frequency decreases farther, the output level falls to -6dB below the nominal level. The frequencies at which the output level reaches -3 and -6dB below the nominal level are called the -3dB and -6dB points.

Brother Can You Spare a Lamp?
Is there any downside to buying a spare lamp for a projector? I have a Sony VPL-VW100 projector, and I was thinking of getting a spare lamp, but then I saw that the price of the lamp is around $800! If I buy a spare now, is there a chance it won’t be as good as if I wait until I really need it (maybe gas leakage or glass age)? Do lamps ever go out of production?

Ron Bolanowski

I don't think there's any downside to getting a spare lamp before you need it. I can't imagine they "go bad" or anything like that. I suppose that lamps do go out of production at some point, but I don't think it happens very often or quickly. If you can afford to spend $800 now as insurance against the moment when the current lamp dies—which will most likely be in the middle of a big movie or sports party—then go for it.

When is HD Not HD?
Some HD channels broadcast programs in a half-screen, showing a big HD logo on both sides. CNN, ESPN, CBS, and many other stations do this on many programs. In these cases, the quality of the image suffers lot. Why do they do that?

Also, TBS broadcasts Everybody Loves Raymond in HD but not Friends. Both shows were produced more than 10 years ago, so how can either one be in HD? Why one and not the other?

Chung Han

I agree that many HD channels don't do a good job with their HD broadcasts. When CBS, CNN, and other channels put a 4:3 window on the screen with big HD logos on the sides, that usually means the content they are showing is standard def, which is why it looks so bad. I often see this in the middle of a newscast—the in-studio image is HD, but the remote image is SD, which means they don't have an HD camera for remotes or perhaps the footage they obtained simply isn't HD.

As for Friends and Everybody Loves Raymond, both were probably shot on 35mm film and transferred to SD for their original broadcast, but they can easily be transferred to HD at any time. It seems that TBS (or maybe the owner of the show) did the HD transfer for Raymond but not Friends, which seems odd to me, since I think Friends was much more popular.

If you have a home-theater question, please send it to

EWL5's picture

Towards the end of its run, Everybody Loves Raymond was in fact broadcast in HD on TV. I have never seen Friends in HD and the show ended well before HD had secured a proper foothold.

Duncan's picture

One reason I have heard for not buying spare bulbs too early is that if the waranty runs from the time of purchase and you find it is a dud you may not be able to get a replacement.

willdao's picture

And, there's the (perhpas slight) chance that the bulbs may come down in price in the future.

Scott Wilkinson's picture

Duncan has a very good point that I didn't consider in my response. Willdao may also be correct, but there's no way to know for sure.