Q&A # 33 - October 14, 1997
Staff
Q I just heard something that was really disturbing, namely that Paramount will no longer be pressing their movies on DVD. They will
press any movies that they have made a commitment to, then stop. Have you
heard this? If this is true, I think DVD will be headed for some deep
trouble. Don't throw away that LD player yet!
A Paramount has not pressed any DVD movies, as they are one of the hold-out
studios. There is a rumor that they are going to skip DVD and only do DIVX. Paramount made the same mistake with CD-I which was a flop before launch, and the studio did
not listen to consumers saying that they would not buy it. Now the same thing is
now happening with DIVX.
Q I have a question about whether or not there is a difference between how
amplifiers are rated for Home & Car use. I believe that I read in a
magazine once that power ratings for home audio were regulated by the
federal government, but that car audio was not. What I read was suppose to
be an answer to someone's question about how car audio amps can be rated so
wildly. A discussion with a friend brought this up. He said that car audio
is regulated by the government, of course me believing what I thought I
read, I disagreed. The Oct.(maybe:Nov.) issue of "Car Audio" answered a
letter that seems to agree with him. We both know RMS and the other
electronic terms (it's our job), but I believe the problem is with the
marketing departments. If I am indeed wrong, I would like to know what it is
that I misinterpreted. So, are car audio power ratings federally regulated
like home audio ratings are?
A It really does not matter whether the ratings are regulated for either type (home and car) because marketing departments always find a way to make the numbers sound great, with or without regulations. Frankly, I don't know what the regulations are, but I do know that the one thing they cannot legally do is publish an outright lie. Even rms ratings can be misleading though, because how they are measured can hide weaknesses. If the tester applies a very short pulse (a few milliseconds) with a specific frequency, say 1 kHz, an amplifier can look pretty good. But if applied on a continuous basis, in all channels, this is more stressful. Regulations or not, the specs will have to say the power output in rms per channel, into 8 Ohms or 4 Ohms, all channels on simultaneously, from 20 Hz - 20 kHz, if that is the way the amp was tested. If it was not tested that way, they can't say it was, legally. Otherwise, it is fraud. Car amplifiers are usually rated into 4 Ohms, because they are dealing with a 12 volt battery that has to be increased to higher voltage, and the lower impedance allows more current to pass at any given voltage (I=ER). Harmonic distortion should be stated at full bandwidth (between 20 Hz - 20 kHz), rather than just at 1 kHz, and at 0.5% or lower. The power supply is very important with car amps, just as it is with home amps. Big power supply capacitors are better than small ones. When I do some audio/video shopping, I look to see if the product I am considering sounds good, looks good, is built well, and priced right. THEN, I look at the spec sheet. If I decide to buy it, I make sure I can return it within a reasonable time (30 days) for full refund, and that I have a good warranty (at least 1 year parts and labor). This goes for in-store purchases or mail order.
Q There seems to be a lot of debate on the effectiveness of bi-wiring. Is
there any theoretical basis for bi-wiring, as opposed to simply using a
larger gauge wire to begin with?
A The theory is that with bi-wiring, the high frequencies will travel in one cable and low frequencies in the other. I performed a single blind test with regular and bi-wiring. The test showed no difference. Negative results don't prove anything however. In any case, I have not been able to hear any difference with bi-wiring vs. a single set of the same cables.
Q What is the best way to position rear speakers?
I have Phase Tech 800HO towers in front with Technics center and rears and
an Infinity BU-2 subwoofer, all powered by a Yamaha RX-V690, with a DV
Mistubishi 35" TV.
I have my rear speakers pointing at each other, but I am tempted to angle them towards the
listening area for more background influence. Should I crank the volume of the rears, lower the center volume, and crank the overall volume?
A Rear speaker placement is not all that difficult, even though Pro Logic is meant to have a diffuse rear sound field, and digital surround is meant to have more a specific rear sound field. My own personal preference for the rear, whether it is Pro Logic, DD, or DTS, is to have the volume of the rear a little less at the sitting area than the front. Otherwise it is too distracting. However, you probably will enjoy the surround sound more if the speakers are pointing at the listening area rather than at each other. Put on a good surround sound movie and have someone turn the speakers while you listen. Depending on your room setup, you may even prefer one rear speaker pointing in a slightly different direction than the other. There are not really any rules for placement as such. Only guidelines as a starting point. If you want more background, then crank up the rear. That's what the adjustments are for. You will probably find that the overall balance changes depending on the movie or music that is playing, and you will have to turn up the rear, turn down the rear, turn up the center, etc. Perhaps that is an idea for the manufacturers: some memory settings (five would be nice) for the sound field and volume balance of the various channels.
Q I have two observations/questions based on recent articles in home
theater publications. First, one magazine suggested that adding a good quality DAC to a DVD or laserdisc player can make quite an improvement. I'm not going to argue, but what is the point, especially if you have a good CD player already? The DAC could only be used for stereo listening of music/sountracks if a Dolby/DTS decoder/receiver has one input from the DVD/laserdisc digital. The DAC outputs analog RCA/BNC to the receiver etc.
Second, in a recent article concerning DVD, it was postulated that DVD has a substantial jitter, much
more than DAC/transports. Would it be possible to place a jitter
device (Monarchy D.I.P., Sonic Frontiers Ultrajitterbug etc.) between DVD and AC-3/DTS decoder/receiver? Would this mess up internal DVD circuits/clocks etc.?
A If you have a good CD player, then using a different DAC could very well make the sound worse rather than better. However, if the CD player is not so good, as is the case with many mass market players, adding a high quality DAC can bring out the detail of the sound. DACs and jitter reduction components are designed to handle specific sampling frequencies, not just any old frequency. AC-3 and DTS decoders have DACs in them for the AC-3 or DTS bitstream, while DACs made for regular CDs will convert the bitstream from the 44.1 kHz digital tracks on those CDs. Video players, including laserdisc players and DVD players, do appear to have less-than-ideal audio capabilities when regular CDs are being played on them. An outboard DAC and jitter reduction component are very likely to improve sound in these cases. For the AC-3 and DTS bitstreams, we will have to wait for improved decoders and jitter reducing components specifically designed for those bitstreams. I would hope that sooner or later, the manufacturers will just put specific jitter reduction circuits in the players along with better DACs. One promising idea is to use memory buffers to store the leading edge of the bitstream for reclocking before it goes to the DAC.
Q Why is the compressed 5.1 DD signal on LD's right analog track modulated in RF?
A The two analog tracks on LD have always been modulated in RF (radio frequency). This is done using Pulse Width Modulation (PWM) where the length of the pits varies on the LD, modulating different frequencies onto the RF carrier. When the carrier is demodulated, the analog sound remains. PCM, or Pulse Code Modulation, is used for the digital tracks (for both LD and CD), where the 1s and 0s of the digital bitstream are embedded in a carrier. Following demodulation, the bitstream remains. When an LD with the AC-3 bitstream in the right analog track is played, the DAC in your player tries to turn the bitstream there into analog sound. That is why you hear a buzzing sound in the right analog channel when playing the LD through a regular audio input of the receiver. So, for AC-3 on LD, you have to take the RF output from the right analog audio track, send it to a demodulator, where the carrier is removed, then the remaining bitstream is sent to the AC-3 decoder. By putting the AC-3 into the right analog channel, and leaving everything else pretty much the same, this makes the AC-3 backward compatible with existing laserdisc players (after having an RF output installed), and only needing the addition of an RF output jack on new players, at minimum expense to manufacturers and consumers (about $50 to the consumer price).
Q Will there be a way to convert the signal from HDTV to the old format (NTSC) or
will everyone have to buy a new TV once everyone switches to HDTV? When is
the switch to take place?
A HDTVs are supposed to be commercially available in the United States in late 1998, but I am not holding my breath. For a number of years, HDTV will co-exist with the standard NTSC and PAL. Converter boxes will be available so that you can watch HDTV programs on older TVs. HDTVs will be able to show multiple formats. However, HDTV programming will start out slowly and build. Completely switching over to HDTV with no NTSC/PAL will take at least a decade or so, in large part because of the TV studio expense involved.
Q I have read the basic primer of your Vol 1, No 1, 94 on televison. But I am
getting more confused after reading some manufacturers' specs, especially
for LCD projectors. For example, a typical model said it has a pixel
resolution of 644 dots x 480 lines and a horizontal resolution of 500 TV
lines. How can the 500 TV lines fit into 480 lines of pixels?
Also I would like to know about how effective an S-Video cable running at
say 20 feet long will be vs. a composite video cable at the same length. Will the performance of such a long S-Video cable get so degraded that I should just
stick to using composite? (I refer to a DVD source and not an LD source.)
A There are two resolution specifications you need to know about: Vertical
and Horizontal. Vertical resolution is fixed at 525 horizontal lines for our NTSC system, 625 for PAL, and 1080 for one of the HDTV options. This is different than the horizontal
resolution (number of vertical lines) that you would see from various sources like VHS (240), Broadcast (330), LD
(425), and DVD (480).
How does the 640x480 fit in? Well, you have 525 lines, of which
only 480 are actually viewable, while the others are used to sync the set with the
incoming picture. You will notice that your TV is not square, and that is how
you get the aspect ratio (width of the image divided by the height) 4:3, or 1.33:1. A square TV might be 480x480, but since the ratio is 1.33:1, we
must multiply that 480 by the 1.33, which results in approximately 640. This is where
640x480 comes from in the computer world, and because a computer can do
progressive line scanning ("non-interlaced"), their 640x480 looks a little nicer. This is also why WebTV seems to be fixed at 640x480. So a 525 line system (NTSC) is also 640x480.
A 20 foot run of S-Video, depending on the cable, will work (I
have a 6 meter Monster M1000-SV that is fantastic and has no problems).
S-Video is more sensitive to long runs because of the smaller cables and a
possible time delay between the two signals. Camelot is offering a powered
S-Video cable that they claim will go to 150 feet with no loss. I am
awaiting the arrival of their cable to test. *NEVER* use the composite
out of your DVD player, unless that is all your TV accepts. Use the S-Video instead, or even better, the component video if your player and TV have the connections.
� Copyright 1997
Secrets of Home Theater & High Fidelity
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