Q&A # 235 - June 30, 2001

Staff

Q As to the question in the previous Q&A, about software DVD decoders and judder, here is the answer.

A My feeling is that this might explain part of the judder, but it doesn't seem like it would explain all of what we were seeing. The difference between 59.94 frames per second and, say, 60.1 is only 0.3%, so you'd expect to get an extra frame about every 5.5 seconds, if my math is right. We were seeing judder in the range of several extra (or dropped) frames per second, as I recall, which would require that the refresh rate of the screen be off by +/- 2 Hz, which seems pretty huge.

I suppose a really brain-dead synchronization algorithm could cause huge judder with relatively small frequency mismatches, and given the other dumb things some of these DVD software companies have done, it's not completely improbable. But overall, I tend to blame resource contention. A modern Windows PC has tons of processes running even with a really clean install, and if another process interrupts the DVD software at a critical moment, like when it's about to update the screen buffer, it could cause it to miss its refresh window and hold the frame until the next vertical retrace. But I don't really know - we'd need to do a bunch more analysis to figure out the real culprit.

Q I bought a receiver and a separate power amp. I hooked up the amp to the pre-out of the receiver and removed the jumper pins. Then, I stupidly  replaced one of the jumper pins horizontally into the slot below the pre-out. Anyway, all was well until there was a big crackle of noise and the powered subwoofer for one of the speakers blew (as well as the fuse). I know I have to replace the speaker. The receiver seems to be fine, but I was  wondering if there could've been any internal damage to the receiver or the
power amp.

A The jumper pins usually connect pre-outs to pre-ins for the main front left/right channels. I am not sure which jack you put the pin back into, but I suspect there was a short as a result. If the fuse blew, the speaker may not need to be replaced, and I doubt if any damage occurred to your receiver or power amplifier if they are working. I think if they had been sent a bad signal, the fuse would have blown there as well. In fact, if the fuse in the powered sub did not blow right away, it might have been due to something else entirely.

Q Is there a difference between subwoofers and bass modules? If so, are there between music and HT? I ask this only because my Pioneer receiver will not give a sub-out signal when in stereo mode, but only in the DSP or one of the surround modes. The explanation is that 1) Subwoofers are only for movies, or 2) The subwoofers are not supposed to act that way. My amp only has low level inputs and thus cannot take the signal from the speakers without changing the signal. I can go on and on about the differences between small speakers with a separate bass unit, subwoofer augmentation, with and with out bass management. I guess my ultimate question is was I wrong to assume that a sub-out would still allow me to use my full range speakers as full range speakers?

Thank you for your website. I appreciate that you assume that the readers have read the primers so not much wording is lost in details except to point out the differences. Have enjoyed your articles more then any other publications I have read.

A It is a matter of semantics. Subwoofers and bass modules are similar in that each is an enclosure with a large (relatively) driver, and sometimes an amplifier, specifically designed for the low frequencies (bass). However, strictly speaking, a subwoofer is for the frequencies that are too low for the woofer. The woofer driver in an enclosure with a midrange driver and tweeter, or separate enclosure (bass module) that is part of a speaker having midrange drivers and tweeters in a different enclosure, is not a subwoofer. It is a woofer. But, even if you have full range floor-standing speakers with large woofers, a subwoofer can be very useful, because they are designed to go really low, e.g., down to 15 Hz. If you want to use your speakers as full range, with a subwoofer picking up the really deep frequencies, set them to "Large" in the bass management menu of your receiver. Usually, there will still be bass coming out of the subwoofer-out jack regardless of how you set your other speakers, except possibly if you set the subwoofer management to "None", in which case LFE information gets sent to the main front left/right speakers. But, if for some reason it does not work that way with your receiver, use the rec-out jacks on your receiver, and set the crossover on the subwoofer to about 50 Hz.

Q Some component-video cables with RCA connectors have effective impedances of about 25 Ohms. Of course, the ideal would be as close as possible to 75 Ohms from output to cable to input for DVD players and DTV decoders. In some areas of home electronics, impedance mismatches are probably not significant. However, I'd like your opinion on the picture-quality differences, if any, with a mismatch as described above. I don't want to spend $250 or more for 1 meter component-video cables with RCA connectors! The engineering claims for so-called "high-end" cables are quite annoying.

A For MHz frequencies that digital signals use, the 75 Ohm impedance is important. We have been shocked to find how many companies are selling digital cables with 50 Ohm plugs. Even BNCs can come as 50 Ohms. If you look into the plug you will see either a white plastic disc inside, indicating it is 50 Ohms, or the lack of that disc, which indicates it is 75 Ohms. The lower impedance connectors can result in reflections, which would smear the digital audio or video. When you buy digital cables, for use with connecting a transport to a DAC, or for connecting the component video outputs from a DVD player to your TV, insist on having the 75 Ohm connectors. I just installed some new BetterCables Silver Serpent component cables, which have silver on copper, and 75 Ohm RCA plugs. Even for 25 foot cables, the price is only $215. One meter is $100, so you can get them well below your $250 limit that you mentioned. There are other good cables out there too, but just specifically ask the question about the connectors being 75 Ohm. (For regular analog audio interconnects, this is not an issue.)

Q This question refers back to one that was posted in your 01/97 Q/A. I too, have a Sony STR-DE915 and have some concerns about the 48 vs. 96 kHz compatibility. Recently I had a digital cable box installed, with an AC-3 digital out jack. After purchasing a digital coaxial cable and connecting it to the receiver, I get nothing! The cable box manufacturer's manual does not list any specs. Is there something I'm missing, and is there a compatibility issue? Also, I've been shopping around for a DVD player.  Am I correct in assuming from your response in the previous Q&A that ANY DVD player will work with my receiver, or do I need to look for one that will switch between 48 and 96 kHz, like the Pioneer models?

A The digital output jack on your cable box is for 48 kHz PCM Stereo and Dolby Digital (DD). However, although DD started out being called AC-3, the digital output of your cable box should be connected to one of the standard digital input jacks on your receiver. The jack on your receiver that might be labeled AC-3 is for the AC-3 RF output from laserdisc players. If you plug the digital output from your cable box into that input jack, you will not get any DD decoding. Every DVD player has DD output and should work with any receiver that has a digital input and DD decoding.

Q I want to replace my aging Sony DVD player with the new Denon DVD-2800 (anxiously awaiting your review), but I was told by Hitachi that my 53SDX89B TV will only turn off its doubler if it receives a 480P signal. Does the Denon produce the 480P or does it try to produce higher resolutions?

A DVD players produce only 480p (when in progressive mode). The line doubling or other resolution scaling is done by TVs and outboard scalers. So, you should be all set to rock and roll.

Q Any general thoughts on the newer, smaller, high-powered subs, such as the B&W ASW2500? Any specific comments about this particular sub? Would this sub be a logical/good choice used with a system including the Denon AVR-5800, in a room sized 16' X 25'?

A The ASW2500 has a 10" driver and a 700 watt (continuous) power amplifier. It is rated down to 22 Hz. Your room is pretty big, so I would suggest that you get a larger subwoofer with 12" driver or, even better, a 15" driver. It won't have to work so hard to fill that large room of yours.

Q Regarding the audio connection between a DVD player and a receiver, if a long cable run cannot be avoided, what is the better cable choice: Toslink optical cable or RCA audio cable? What are the realistic maximum cable lengths of each before measurable signal loss and/or interference occurs? Are there other facts to be considered when deciding between these two cable types?

A This will depend on the quality of the cable, but I have used 12 foot Toslink with no problems. Finding a Toslink longer than that is not easy. It also depends on what the cable will be lying next to. If it has to cross over some power cables or be bundled with other audio and video cables, I would suggest Toslink.

� Copyright 2001 Secrets of Home Theater & High Fidelity
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