Q&A # 100 - January 26, 1999

Staff

Q I recently purchased a Sony 36" XBR TV with a 3D digital comb filter. After watching it for a few weeks, I've noticed dot crawl on the screen quite often. Especially when watching football, I see fuzziness/jaggedness in the lines/numbers on the football field. I thought 3D digital comb filters would eliminate dot crawl. Is something wrong with my new TV or am I expecting too much? I have normal cable for my TV.

A There are at least two causes of jaggies in the TV picture. One of them is dot crawl which can be seen along horizontal or diagonal lines even when there is no motion in the image (nothing moving). The 3D digital comb filter will reduce this type of jaggie. There is another type of jaggie that is a result of the fact that NTSC TV is interlaced, meaning that a full image on the TV is made of two interlaced fields taken 1/60th of a second apart. The two fields are shown one after the other, and together, they produce a full screen image, called a frame. Because they are taken with the camera 1/60th of a second apart, when something is moving in the field of view (such as the camera panning across a football field during the action), objects will appear jagged due to their edges being in slightly different positions in the two fields. A digital comb filter (for that matter, any comb filter) won't improve this artifact. Only progressive scanning will do that, which is something we will get in 480p, 720p, and other "p" formats with HDTV.

Q Can I use a two channel amp to drive one center channel speaker and if so how would I run the hookup? It is an 8 Ohm speaker. Would I then use the 4 Ohm amp ratings or 8 Ohm amp ratings?

A The best way to do this is to use a Y connector so that the channel you are sending to the amplifier is split and goes to both the left and right inputs on the amplifier. For this to work, you need a speaker that is bi-ampable, meaning that it must have a way to disconnect the woofer/midrange from the tweeter on the back of the speaker. Usually this consists of two metal strips that are held in place by the speaker binding post nuts. You MUST disconnect these before connecting the outputs from the two amplifier channels to the speaker. What you end up with is a bi-amped speaker, in your case, the center channel speaker. You can adjust the high frequency output and low/mid output by adjusting the volume on the two individual channels of the amplifier. The other way to do it is by bridging the outputs from the two amplifiers so that you have one output instead of two. This entails using the two + speaker binding posts from the two channels, usually the red posts. The black or - posts are not connected to anything. For this to work, you need a bridging switch on the amplifier which connects one input to both inputs and inverts the signal on one channel. This procedure reduces the low impedance driving capability, so that, if it was rated into 4 Ohm loads, it will now only drive an 8 Ohm load.

Q Much has been said in regards to insufficient amplifying power causing clipping. I have an A/V receiver that produces 85 watts per channel and floor speakers with an efficiency rating of 92 dB. While listening to some CD music with the volume control set at -25 dB, the sound emitting from the speakers was quite loud due to the high efficiency of the speakers. At that volume setting I assume I was feeding the speakers approximately 1 watt of power. However, during one particular song at a specific passage where the music hit a higher frequency, I noticed a sparkling/sizzling (like firework sparkles) sound from the right speaker. It was very brief and it always occurred at that particular spot of the song. I also noticed that even if I lowered the volume to -40 dB the sparkling noise was still noticeable. Is it possible that what I heard was not due to amplification clipping but the inability of the speakers to produce that frequency? Or perhaps there was a defect in the recording? I don't think the speakers are damaged because everything else I've listened to appears to be fine.

A In this case, I would say it is a defect in the recording. The way to test this is to play the CD on another system, say in your car, or at a friend's home. If it does not occur on other systems, then it might be a problem with the tweeter in the right speaker, but I doubt that is the culprit. You could test this by reversing the channels right to left by switching the cables from the CD player or by switching the speaker cables.

Q Is there any advantage to having a high power amp with 300-400 watts per channel when you use speakers that themselves have an internal amplifier? This seems on the surface that it is just a waste. That that kind of power should be used for speakers that do not include internal amplifiers.

A Speakers with built-in amplifiers use that amplifier power just for the woofer, except for a few special cases such as Paradigm and Meridian. Low frequencies demand the most power, but it is also nice to have lots of power for the mids and highs. You can never really have too much power, only not enough power. Having 400 watts per channel allows the transients which are at the leading edge of many sounds to come through. Those transients can be very demanding too. If you are listening at about 5 watts per channel, a transient might demand several hundred watts just for a fraction of a second. Having that power available makes the difference for a you-are-there experience.

Q I've read a lot about DTV and HDTV, and I'm now wondering: What will studios use to record resolutions above 480p? I would think that even Beta-SP is out of a job; and what can home consumers expect for recording High Definition signals? Some new tape format, or maybe re-writeable CDs? I havn't seen anything that talks about it, and was hoping you might have the scoop. I hope we won't be stuck with recording down-converted signals on plain old VHS!

A Re-writeable DVDs are already starting to become available, and it will really be just a matter of space on the disc, rather than finding a way to record the data. 1080i will use up the DVD space very quickly. Current DVD movies are already in 480p, but are downconverted to 480i for our NTSC TVs. The real problem will be getting past the copyright issues since digital copies will be essentially perfect replicas of the original. Once recordable DVDs are mass market, I suspect they will be more successful than home digital tape recorders.

Q I'm trying to decide between a Denon 3600 and a Yamaha RX-V2095. I was leaning toward the Denon until I realized it doesn't have a six-channel input for a processor for the next audio format to come along (the newer 3600s do have DTS built in). Specifically, I'm concerned about the DVD Audio format. I don't want to spend that kind of money on a receiver and then be locked out of that format. Do you know if the Denon can be upgraded -- with some sort of internal card or something -- when DVD Audio becomes a reality or is using the five pre-outs and an additional amplifier the only option? The Yamaha has a 6-channel input. I think I'll be happy with either amp -- as long as they don't become obsolete in two years.

A The terminology for DVD audio is confusing because it does not specify what format is contained, and there are several. DVD audio could be DD 5.1, DTS 5.1, 24/96 two-channel, 24/192 two-channel, and other formats. The Denon and Yamaha will decode DD and DTS audio discs, whether they are on CDs or DVDs. For 24/96, 24/192, and the new Sony format that is 2.8 MHz sampling, the decoding will be in the players, so you don't have to worry about having the decoding in the receiver. The only situation where you would have a problem is when the 24/96 and others are more than two channels, say they are 5.1, and that is when the six inputs on the receiver would be needed, but I think for awhile, these formats will just be two-channel, and you can use the analog outputs from the player to the two-channel analog inputs on the receiver.

Q Sony states that you should not play video games on a projection TV as it might damage the picture tubes. Is this true, and if so, why would a video game be more harmful than a DVD movie? I have a Sony 41" that I use with my computer and Super Nintendo so I am very concerned about this matter. Could you please elaborate?

A The reason video games could be harmful is that they have stationary backgrounds in many cases that could burn their image into the face of the picture tube when they are on for extended periods. This would be the case for any TV, not just projection TVs. I would suggest turning the brightness and contrast down as low as you can stand it.

Q I have learned a lot since I "discovered" your website about 18 months ago. Thank you very much. I have a two-part question regarding setting delays for surround speakers. A) In addition to distance from the seating area, do you need to take into consideration the difference in length of the speaker wire running to the front and rear speakers? and B) My DVD player has delay settings and my receiver has delay settings. Do I set the delays on both of them or one of them?, (if one, which one?) I was concerned that I would be doubling the delay if I did both.

A The length of the wire will not make a difference for the delay setting since the electrical signal travels at almost the speed of light. If your cables were thousands of miles long, maybe then it would make a difference, but not for room length cables. Use the delay settings on only one component, probably the receiver if that is the one easiest to adjust.

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