Q&A # 53 - March 3, 1998
Staff
Q
What is the real difference between a 1 bit and an
18 bit processor? Is it a tech term that one can really hear the differences or is it a
marketing scheme? Also I noticed that some DVD players have 9 bit and others have 10 bit.
What do they really mean?
A 1 bit processors usually refer
to the digital to analog conversion (DAC) method used, and 18 bit, 16 bit, 20 bit, or 24
bit processing can apply either to the same thing (DAC), or signal processing (DSP)
resolution. For a quick run down on digital, look at John Busenitz's recent article in
Secrets. Converting back to analog, both the single bit processor, and any variations of
the multi-bit, accomplish essentially the same thing, but use different techniques. The
single-bit processor uses digital signal processing to evaluate the equivalent values for
the multi-bit. They then provide the recorded quantized level by turning on full blast for
a ratio of that sample duration equivalent to the ratio of the recorded quantized level to
full output. After going through the filter, the signal loses the drastic switching part,
which is a very high frequency component of the entire spectrum, and smooths out the
length of samples to a relatively slower changing, and hence smoother, averaged output. In
multi-bit converters, each bit is assigned a voltage level in the converter, and for each
sample, the output is the sum of the voltage levels of the "on" bits.
Think of it this way. Most of our audio material is 16/44.1, meaning
that as it was sampled 44,100 times in a second (44.1 kHz), each sample recorded at
a signal voltage to the nearest quantized level, with 216 total
levels. Each bit can have a value of either 1 for "on" or 0 for "off",
i.e., two values, which is the basis of binary mathematics. With CDs, the 16-bit
"word" which represents the code for a particular voltage can be any combination
of sixteen 1s and 0s. There are a total of 216 combinations of words
that are 16 bits in length, and each bit is either a 1 or a 0. If the word were 10
bits long, there would be a total of 210 combinations. Say
we've got 3 bits, and sampling rate of 1 sample per second. Three bits provides 23,
or 8 quanization levels, 0 - 7. Now say we want to reproduce the level 5 during the
sample. In the single-bit processor, we'd turn the thing on all the way for 5/7 of a
second, and off for 2/7 of a second, so that the average level during the entire sample
period would be 5. With a 3 bit processor, however, we'd have a separate amplifier for
each bit, all of which would be on or off for the entire sample duration of 1 second.
The first bit would have a level 4 output, the second a level 2, and the third a
level 1. To make a level five output, the first and third bit would be on, and the second
one off, so that 4 plus 1 equals 5. Different ways to achieve the same thing.
Both designs have advantages and disadvantages. Single-bit designs inherently guarantee
amplitude linearity, while multi-bit designs are
harder to keep linear in very high resolution applications because the bit values have to
be so small, and are therefore hard to manufacture within necessary tolerances without the
use of laser trimming. For instance, if your second bit accidentally were set to put out a
level 2.2, then it's no longer in the correct ratio to the other bits. A single-bit only
has to be proportionally correct to itself and, regardless of the value, it is. On
the other hand, single-bit designs also generate a lot more noise in the conversion
process, which must be then filtered out and isolated lest it
interfere with other components in the player, such as the clock, which could introduce
jitter and subsequently distortion. Is either one better?
No, just different, and the performance depends entirely upon the parts and design of the
machine as a whole. The ultimate resolution is determined by the linearity, distortion,
and noise floor of the system, including the analog components. For instance, many
"24 bit" players actually only have 18 bits of resolution, and some 24 bit
converters use single-bit technology and achieve 20 - 21 bits of resolution. Is it even
important when we listen to 16 bit recordings (at least for now)? Yes, because many of the
16 bit recordings use noise shaping techniques to increase their resolution in the more
audible band, so that the playback system must surpass 16 bit resolution to take advantage
of that.
As for DVD players, 9 or 10 bit refers to the video converters which do essentially the
same thing described above, but at higher frequencies.
Q
I want to purchase a 35" to 40" direct
view TV in the near future. I've narrowed the choices to three makes with prices ranging
from $1,500 to $3,600. If I were to spend say $3,600 on a 40", will it be backward
compatible with HDTV and a wise use of my money? My viewing habits are 90% from cable and
10% renting movies. My concern is the tradeoff in spending $3,600 now on a 40" that
may not be compatible with HDTV or spend say $1,500 on a 35" figuring I don't lose as
much if it's not adaptable to HDTV. I'd really like to get the 40", but I'm torn
between the larger screen and the premium price it commands.
Second, in your report from the 1998 CES show, you mention the advantages of the new audio
format coming out as Digital Audio Discs (DADs). You mention current DVD players can play
these discs but will only resolve 20 of the 24 bits. The next generation DVD players will
be able to resolve all 24 bits. Do you know what the time frame is for these DADs to hit
the shelves? I'm also thinking about purchasing a Marantz 5 disc CD changer and a Sony DVD
player, but I'm wondering if these are wise purchases versus waiting for a player that
would play all formats such as CDs, DVDs and DADs.
A All TVs will be able to view
HDTV broadcasts, but our current NTSC sets will require an adapter that converts the HDTV
signal to NTSC. The 40" TV you are considering will be able to use this adapter just
as any other TV will. However, the image you see on that TV will not be an HDTV image, but
rather, a standard NTSC image (like we have right now). So, if you are really thinking
ahead to viewing HDTV images, you will need to wait until the HDTV sets arrive in the
stores. Instead of $3,600, they will probably be in the range of $5,000 - $7,000, but we
won't really know the price until they actually are here. HDTVs will be able to view all
DTV formats as well as NTSC. I have seen some 35" NTSC sets for around $1,000, and
32" models for $600. My suggestion is to get one of the 35" or 32" sets at
as good a price as you can find, for now, and save the money for HDTV. After you get the
HDTV, you can move the other TV to a different room. If it's going to be the kitchen, I
would suggest the 32" or 29" set. Spouses don't usually like large TVs in the
room when dinner is by candlelight! I was absolutely blown away with HDTV images at the
CES, and you know how seldom I use that description.
All DVD players, including the first generation, will play CDs,
DVDs, and DADs. It's the resolution capability that has changed with the latest
generation. All you need to do is look at the spec sheet on your potential DVD purchase to
see if it says it will decode 24 bit / 96 kHz digital bitstreams. Some of the earlier ones
said 96 kHz, but not 24 bit (these are the ones that are 20 bit). It is hard to say when
DADs will become available for the mass consumer. In the meantime, however, you should not
be worried about going ahead with buying the DVD player, since the DADs will play on them
when the discs finally come to market. There is always the possibility that some other
form of DVD audio will also rear its head, but technology is never at a standstill these
days. There is no assurance of total forward compatibility with any product.
Q
I understand that the " .1"
LFE channel in Dolby Digital carries frequency information from 3 Hz to 120 Hz. Does this
mean that the other 5 channels are devoid of frequencies of 120Hz and below? Or is the LFE
channel included to compliment the other channels by accentuating some of their info.?
Example: Let's say a setup includes "full range" speakers for all five full
frequency channels. Would a separate sub hooked up to the LFE channel be necessary?
A The LFE channel is designed
for separate low frequency effects, but it is not generally used that way right now.
Instead, the LFE channel carries a duplication of bass from the other channels. This makes
it easy for a subwoofer hookup, but, for the time being, is a waste of the technology. At
this point, the only advantage to using the LFE output from a DD or DTS decoder is that
you will bypass any crossover that exists in the signal path of the regular subwoofer
output of a receiver.
Q
Is it possible to connect the "color
stream" jacks to a RGB scart? The jacks have a red green blue color, but there is no
jack for the synchronization.
A No, not without a
Component-to-RGB converter. Component is a matrixed version of RGB. There are a couple of
DVD players on the market that offer RGB with or instead of component.
Q
My question is concerning S-Video coming from an
outboard AC-3 decoder. If I were to buy an AC-3 ready A/V receiver that did not have an
S-Video jack on it, could I later buy a decoder with S-Video output in order to make a
full S-Video connection? (I plan on buying a TV that has one, and a DVD player in the
future.) I know you can help me!
A Your A/V receiver does not
need an S-Video jack to enjoy the benefits of DVD with S-Video. Just run the S-Video cable
straight from your DVD player to your TV. This is not the most convenient way to go but
gives the best image, even when the receiver has an S-Video connection. But, if you buy a
decoder that has an S-Video switcher in it, then you can use it, and it will be especially
helpful if you have several S-Video sources.
Q
If laserdiscs are capable of working with DTS
decoders by just hooking up a digital cable between the PCM output and a DTS decoder,
why is it such an issue for current first generation DVD players not being able to send
the same bitstream out of their own PCM output (assuming the discs have that data there)?
Am I correct, or am I not, that all first generation DVDs are incompatible with DTS
decoders? I had read in a magazine that both the DVPS3000 and DVPS7000 Sony units were DTS
capable. Do you think this was a misprint or were they referring to CDs played back on the
machines using the PCM output?
A From what I have heard, DTS
had originally planned on using the PCM track for DVD. The DVD consortium said NO, they
were not allowed to use the PCM track. This is why both first and second generation DVD
players will NOT play DTS DVDs. This includes Sony. Panasonic, Denon, Pioneer, Faroudja,
Meridian, and Runco all have players on the way that will play DTS discs, but none are
here yet. I asked one of the Sony technicians at CES about this subject, and he said that
even their 3 new players scheduled to come out later this year will not support DTS,
although this could change, and I hope it does. I hate to say this, but DTS has really
dropped the ball, and actually they have been fumbling since the very beginning with
laserdisc. This is unfortunate, because DTS is a great format.
Q
(1) What advantage does digital convergence
have, and what is the other type (analog convergence)?
(2) What allows certain projectors to support various aspect ratios, while others
can't?
A (1) Analog convergence
uses potentiometers on the projector. These tend to vary over time at a greater degree
than digital ones. It also requires you to do the adjustments from the projector. One slip
while adjusting these pots and you will have to start over. With digital convergence, you
can make adjustments from your remote control. The values are usually represented by
numbers on the remote, so you can write all the initial values down and then start playing
with it. You should always be able to get back to where you started because of the known
values you started with.
(2) It is the projector's innards that allow multiple aspect ratios, and this
feature is an added cost that the budget projectors leave out to keep costs down. For
every aspect ratio, there is a requirement for additional memory to store the convergence
and gray scale info. A lot of the new projectors coming on the scene now include multiple
ratios as standard.
� Copyright 1998 Secrets of Home Theater & High
Fidelity
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