Q&A # 50 - February 10, 1998
Staff
Q
I have a Yamaha RX-V793 and Velodyne F1800RII. I'm
trying to figure out how the bass management works. The Yamaha has a HPF at 90 Hz,
18dB/octave to the sub, and the sub has a direct option, bypassing its internal xover.
When I run it this way, there's a +6db peak at 80 Hz, which is fun for HT, but less than
desirable for music. So, if I use the Velo's internal LP xover (at ~75 Hz for my Linaeum
LFXs), and the Yamaha sends a subwoofer signal HPF'd at 90 Hz, do I end up worse than the
direct mode? When set up this way, there's a 5 dB valley from 120 Hz -160 Hz, but no peak
at 80 Hz. I guess the question is this: which would you choose: a peak at 80 Hz, or a
valley through 120 Hz -160 Hz? BTW, all measurements done with RS SPL meter calibrated
against a good RTA at the Eastman School of Music, using Stereophile's Test CD 2, bass
decade track. So I know the measurements are spot-on.
A Personally, I would prefer the
valley rather than the peak. Peaks in the 80 Hz region make the sound boomy, and peaks in
the 120 Hz - 160 Hz region make the vocals chesty, so having a valley in the 120 Hz
- 160 Hz region can make the vocals a bit thin, but this is not objectionable (to me).
Boominess and chestiness are very irritating. In fact, the center channel usually gets
bass loading (emphasis) off the front of the TV in this exact region (120 Hz - 160 Hz). We
use EQ to reduce this phenomenon with many center channel speakers. If you like the 80 Hz
emphasis with movies, then it is simple to just flip the crossover switch one way for
movies and the other way for music. Getting the response completely flat is not
necessarily the most enjoyable.
Q
Hopefully you are able to shed some light on my
receiver problem. When in "Jazz Club" mode, there is a serious imbalance in
volume
between the surround channels. As soon as I shift to another mode, the problem disappears.
It would appear that the problem is with the DSP. Is this an inherent problem with this
receiver? What can be done about it?
A The "Sound Fields"
that are part of DSP programs in receivers are not standardized. Each manufacturer
assembles the sound fields based on visits to various environments, such as stadiums,
churches, night clubs, and so on. They set up microphones in the environment, make some
sound pulses, and record the patterns of sound reflections. Depending on the actual
location tested, and where the microphones are positioned, the resulting information can
be quite different. The manufacturer puts the reflection data into chips as mathematical
equations (algorithms), and the equations are used to add the reflections into the
surround channels along with some of the original two-channel stereo information. I don't
particularly care for DSP, because it sounds artificial. I use Pro Logic mode with some
stereo CDs, but I hardly ever use DSP Sound Fields. However, many people do like them.
Although switching between DSP modes can result in large differences between the amount of
sound in the front compared to the rear (this is normal), I have not heard any DSP mode
that causes the left rear to be much different than the right rear in terms of volume. If
that is what you are experiencing, then I think your receiver needs to be serviced.
Q
How exactly does switching phase 180 degrees
affect subwoofer output? I'm sure it's different for each sub as well as room placement,
but is there any GENERAL rule of thumb for this?
A All crossover networks cause
"phase shift". This means the position of the waveform is "shifted"
(moved) in time with respect to where it was in the original signal. The maximum amount of
phase shift occurs at the crossover frequency , and it decreases as you move away from
that frequency. Let's say you set the subwoofer "low pass" frequency at 70 Hz.
The maximum phase shift would then be at 70 Hz (it might be 1800,
which is 1/2 the wavelength), and there would be less phase shift at 20 Hz. This phase
shift makes the sound from the subwoofer less able to "blend" with the sound
from the other speakers, because its bass frequencies are out of time with the mids and
highs. So, being able to adjust the phase on the sub allows you to blend the bass with the
mids and highs (making the sound "seamless"). With some subs, there is a phase
switch that toggles between 00 (no shift) and 1800
shift. With others, the phase is continuously adjustable between 00
and 1800. Regardless of the shift adjustment, however, you can
only align the bass with the mids and highs at one frequency, since the amount of shift
changes as you move away from that frequency. I have always found the phase control on
subs to be very useful (especially with multiple subs), and I adjust them to have a good
response at about 30 Hz - 40 Hz. This requires fiddling with the volume, phase adjustment,
and low pass frequency, so it isn't a simple task, but when it is done, you will like the
result. To do it, you don't need an SPL meter. Just pick out some of your favorite CDs or
movie sound tracks that have deep bass. While the music or movie is playing, set the low
pass frequency to the approximate area you want, then adjust the volume to where it sounds
the "best". Then adjust the phase, and readjust the volume if necessary. You may
have to tweak the low pass frequency, then readjust the other controls again. Using all
the controls will allow you to get the volume and blend that is most pleasing. Write down
the final positions of the controls, since knobs and switches are like magnets to kids.
Also, you might want to change one or more of the controls with an occasional movie, then
reset them afterwards. Be sure to remember Dr. J's cardinal rule (Rule # 1) in audio and
home theater: "Have fun!".
Q
I have some questions regarding Dolby Digital and
bass management / LFE 5.1 channel. Does DD-encoded source material contain programmed
information in the LFE 5.1 channel, or does the bass need to be "routed" there
through bass management controls via the DD processor? E.G., if you run all L/C/F/LS/RS at
full frequency, will you not hear anything through the 5.1/sub? Or, conversely, if the 5.1
channel does contain encoded info, and you cut off the bass to the L/C/F channels @100 HZ,
do you get both "doses" of bass through the 5.1/sub? One more: if you have a THX
sub with its own fixed 80 Hz crossover (M&K V-125 THX), and you have a DD processor
with a fixed 100 Hz crossover for bass as per Dolby specs, what happens to the 80-100 Hz
frequencies? As you can see, I have a little confusion on this topic and would appreciate
your help.
A It's confusing to everyone.
The LFE channel in DD, and DTS for that matter, is separate from the front
left/center/right, rear left/right channels. It has its own separate track. If you run all
channels at full spectrum, the LFE channel still has its own signal. If you route the bass
from any of the other channels to the LFE, the LFE will indeed be getting more bass.
However, many of the DD movies just duplicate the bass from the other channels in the LFE
channel rather than having completely different sound effects go there. With THX
operation, the receiver or surround sound processor controls the bass low pass to the
subwoofer, so the subwoofer does not need to have, and should not have in operation, its
crossover at that time. THX subwoofer functioning also starts rolling off the bass in the
really deep frequencies so you can get more volume at around 40 Hz where the bass sound
effects are concentrated. There should be a switch on the back for THX operation, which
takes the crossover out of the signal path. You will need a THX receiver or processor mode
to use the subwoofer in true THX mode.
Q
I've got a Meridian 565 7.1 processor and I wonder
if you guys know how much the upgrade to 24 bit DACs would cost me? I read your report of
the CES where you stated that the 24 bit DACs would be the same as in the 508.24. Would
the upgrade of the 565 also include sound shaping algorithms, or is there simply not
enough processing power? Is the only option of benefiting from the upgrade, to use the 565
in combination with the 518? I figure however, that I would get better sound quality from
DTS material -- when it's got 20 bit resolution.
A I believe they will be
upgrading the four 18-bit DACs with four 24-bit DACs, and the one 16-bit AD with a 20-bit
AD. No, I do not believe they will be putting in any noise shaping algorithms. This will
remain in the 518. You should get higher sound quality from everything, not just DTS.
There is no word on when these upgrades will begin, but I (Stacey Spears) will do a
follow-up once I have them.
Q
I want to know the best way to set up and
integrate the following items:
TV
Cable
VCR
Video Game (Playstation)
Stereo
A Plug your Cable into your VCR
via coax. This will allow you to use either your VCR or TV as the cable tuner. It will
also allow you to run
your Cable through your stereo. Run the coax out of your VCR to your TV. Run the
audio outputs of your VCR and Playstation into your Stereo. Run the video outputs straight
into your TV. It is best to bypass the stereo with your video cables.
Q
I have been looking at several options for
upgrading to a DD-based separates system or receiver. Of the products I've auditioned, I
like
the Marantz SR780 (because it fits in my price range and has features that I like). I've
also considered going to a budget-minded separates
system that would include the new Marantz AV550 and a 5 channel amp like the Carver or
Rotel. My question is this: can one use the digital inputs on the Marantz for CD audio
from a CD player with a digital output, or are all the digital inputs on the Marantz units
strictly for DD? Also, have you heard the AV550? I haven't been able to find any
information from consumers like myself. Do you have any reviews planned for the new
generation of DD preamps like the Marantz, the new Adcom, or the Acurus ACT-3? Your input
is important to me. Thanks.
A Marantz is one of the
companies that likes to process the signal in the digital domain. So they have A/D and D/A
in the receiver. Yes, you should be able to use your CD player digital output into the
Marantz receiver as long as you use the proper input jack on the receiver. However, that
does not necessarily mean the sound will be better. It will depend on the quality of the
D/A (DAC) in the CD player vs. the D/A in the receiver. Just try the system both ways and
your ears will make the right decision. We do plan to review some of the new dedicated
processors.
Q
I just purchased a Hitachi Ultravision 50SBX70B
projection television and a Yamaha RX-V2092 AV Receiver. My living room size is 24' x 28',
with 12' ceilings, tecate floor tiles, mostly windows on two walls, and open space (to the
kitchen and dining area) on the other area where walls would be.
I am of the opinion that having a good television (visually
appealing picture) and a good receiver are most important to a home theater system.
That purchasing a subwoofer and surround speakers from Costco made by JBL would be more
than adequate for a family theater system. My reasoning for this is that in an audio store
where the assortment is state-of-the-art, one assumes that purchasing the best that one
can afford is satisfying to the ego once the system is set up at home. However, wouldn't
middle of the road JBLs, at home - with nothing else to compare to - be adequate?
My girls are 7 and 2, and we have a DSS satelite and are thinking about adding a DVD
system also.
Is my logic on speakers wrong? And yes, I have read all of your Q&As but still am
confused.
A Your question brings to mind
Dr. J's Rule # 2 in Audio and Home Theater: "You don't have to justify to anyone, or
apologize to anyone, for getting the equipment you like." We all have different
priorities. Some of us like amplifiers, speakers, TVs, or whatever. Since the TV and
receiver are most important to you, then, by all means, stay with that focus. You don't
have to buy a big set of fancy speakers or subwoofer just because your friends have them.
You have a great receiver with lots of flexibility, so you can always upgrade to other
options later if you decide to.
� Copyright 1998 Secrets of Home Theater & High
Fidelity
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