Q & A

Q&A # 96 - December 29, 1998

Staff

Q You mentioned something about Sony's new format of 192 kHz sampling. I had heard that it was 2.8 MHz. Can you elaborate?

A The new format is called Direct Stream Digital, or DSD, and is the basis for Super Audio CDs, or SACD, which is being promoted by Sony and Philips. It appears to be a Delta-Sigma encoding technology, so each sample is one bit, and is similar to the one bit DACs that are popular right now. The stated rate of 2.8 MHz refers to 2.8 million single bits, which is near the range of 192,000 (192 kHz sampling, or 4 x 48 kHz) x the conventional 16 bits. The instantaneous slope of the signal is encoded, rather than the absolute amplitude. So, if the bit is high, the slope is positive, and if the bit is low, the slope is negative. The bandwidth is the rise time defined by the sampling rate, and is 100 kHz in the case of DSD. I don't have all the info on this technology yet, but this is as near to what we can infer so far. We will publish more details, including corrections on any errors to the above statements, as they come in.

Q I am having trouble understanding the significance of the horizontal resolution specs given by TV manufacturers. Most new televisions have very high resolution stats, 600, 800, even 1000 lines, yet the number of horizontal lines obtained with a DVD player is always significantly different. For example, the horizontal resolution for the Toshiba TW40H80 is 800 lines according to the manufacturer, but the maximum horizontal resolution from a DVD measures just under 500 lines. Am I missing something or is this a huge contradiction? Do company specs not mean anything? How is someone to get true specifications if you can't believe what the company prints?

A The horizontal resolution is the number of vertical lines that the TV is capable of displaying. Although some of them may be able to display 1,000 vertical lines, no current video source has them. DVD and DSS have 480, LD has 420, over the air broadcast TV has 330, and VHS has 240. However, it is logical to assume that TVs with 1,000 line resolution capability will likely be able to display the 480 lines more accurately than a TV that has 600 line resolution. But, don't worry about the spec, because all the TVs will handle the highest resolution sources of DVD and DSS. Select your TV based on features.

Q I've been going through several sites that offer calculations for capacitor values based on speaker resistance and required frequency roll-off, I was wondering if the frequency you input means the very start of the roll-off or the -3db point. Since all the sites give the same computations and one site mentions the first order filter as a Butterworth, I am assuming the requested frequency to be input is the starting point of the roll-off (Butterworth has a peak at crossover point so I assume Butterworth uses initial roll-off as cross-over point as opposed to Linkwitz-Riley which matches -3db points to obtain flat response). Also one site suggested using Mylar capacitors or even better, polypropylene and mentions that as a note, to use non-polarized capacitors. How do I determine the capacitor type short of asking an expert. Are there descriptions on the capacitor?. What else should I know about the capacitor? Voltage or current capacities? Another thing, in what other significant ways will adding the capacitor affect my speaker? Impedance? Internal crossover?

A If you used a single capacitor, calculated for 1 kHz, the response would be down 3 dB at 1 kHz, but the roll-off would start somewhere around 4 kHz. There would also be a 45⁰ phase shift at 1 kHz, with differing amounts of phase shift on either side of this frequency. Phase shift can cause a major change in the sound, so you need to be careful when adding one to your system. When you buy the capacitor (I have used polypropylene), the type is stated, and 250 volts is a popular value. Butterworth may be first order, second order, third, etc., and they refer to complete crossover configurations to control woofers and tweeters. The same goes for Bessel, Chebychev, Legendre, and others. The single capacitor is really just one part of the complete crossover network. If you are going to use the capacitor to roll-off the signal below, say 90 Hz, put it between the preamplifier and power amplifier, and use the input impedance of the power amplifier in the calculation.

Q I own a Denon AVR-3600 and it includes pre-outs for 5 channels. Does this mean I can simply use a DTS decoder and plug it into the pre-outs?

A Yes, you can do that, assuming the DTS decoder has a pass through feature, for when you are using the processor in the receiver rather than DTS decoding. However, this also means you need a five channel power amplifier for the outputs after the DTS decoder.

Q What is your opinion on using the S-Video connection vs. the composite video connection? I was discussing connections with one of the dealers I use and he asked why I was using the S-Video on my Sony DVD player. He suggested that I may actually get a better picture using the composite and the Sony KP-46V15's comb filter. I am under the assumption that S-Video will always give you a better picture than composite video. All reviews I read always reference using the S-Video where available unless the equipment has component connections.

A DVD has digital component video with the chroma already separate from the luminance, so comb filters are not necessary. Therefore, always use the S-Video connection from the player to the TV. Using the composite video out from the DVD player combines the chroma and luminance in the player and separates them again in the TV, which causes some picture degradation. For laserdisc players, the signal coming off the disc is composite, so you have to check out which comb filter is better, the one in the laserdisc player or the one in the TV. If the player comb filter is better, then use S-Video. If the comb filter in the TV is better, then use composite. But for DVD, always use the S-Video (or component video if you have them in the player and TV).

Q What exactly does sensitivity of a speaker mean and how does it affect the sound quality?

A Sensitivity is a measure of how much amplifier power it takes to achieve a certain loudness from the speaker. If a speaker is rated at 88 dB sensitivity, this means that with an input of 1 watt from the amplifier, using a 1 kHz sinewave, a microphone placed at 1 meter in front of the speaker would register 88 dB, thus, the rating of 88 dB/w/m. Lately, this has been refined to be an input of 2.83 volts rather than 1 watt. The sensitivity per se does not affect the sound, but usually, lower sensitivity speakers have a flatter frequency response. An increase in 3 dB sensitivity will imply that it takes only half the amplifier power to achieve the same loudness of speakers at the lower sensitivity. For receivers, it is best to get high sensitivity speakers, say 91 dB/w/m.

Q My basement home theater is located directly below the master bedroom. This makes it difficult to enjoy a late night movie because the sound, especially the bass, carries through the floor/ceiling and annoys anyone trying to sleep in the bedroom. Headphones are ok for one or two people but you lose the nice effects of DD. How does one reduce the amount of sound that carries through a floor/ceiling (or wall)?

A The ceiling and floor vibrate with the sound, and they then act as a big speaker into your bedroom. So, in order to reduce this effect, you must reduce the vibration transmitted to your home theater ceiling and bedroom floor. You can do this by putting vibration dampers on the walls in your home theater (the walls transfer to the ceiling). Decorative rugs, used as wall hangings, work well for this. If you don't have a rug in the bedroom, adding one will help. Also, just turning down the subwoofer at night will make a big difference.

Q I'm a little puzzled. In your Q&A 86 you mentioned using two (I assume equal) powered subs can result in 6 dB more output. What gives? I always thought doubling (electrical or actual) output meant a +3 dB increase. What's even more baffling is that M&K advertises their push-pull subs as having + 6 dB more output per watt of amp power. Won't the internal amp power be divided between the two drivers and result in a similar dB/w/m as a single driver? The only thing I can think of is that the drivers are more efficient.

A When we use two subwoofers rather than one, we are doubling the amount of air that is moved rather than simply doubling the amplifier power to a single driver. This is why we get the 6 dB increase. Doubling the amplifier power to a single driver does not necessarily double the excursion of the voice coil. The M&K subwoofer's push-pull technology complicates the discussion, because push-pull improves the ability of drivers to perform in an enclosure of a given size.