weaver@pyrite.DEC (12/09/84)
The following is a re-type of the brochure on the Nakamichi OMS-7/OMS-5
Compact Disc Players. I thought it might be of universal interest.
Note that I left out the figures, see your local Nakamichi dealer
if they are of interest to you.
-Dave
Digital Sound From Nakamichi
________________________________________________________________________
If you've always wanted a compact disc player but never felt
comfortable with the harsh and unrealistic music of "digital sound"
that many CD players sound, the Nakamichi OMS-7 and OMS-5 Compact Disc
Players are what you've been waiting for. Nakamichi...the company
that created the OMS-1000 - the worlds first commercially available
optical memory system, a research tool capable of recording and
reporducing a variety of optical recording media - has designed compact
disc players that met Nakamichi's standards - Nakamichi Sound!
Perfect Filtering
Every CD player uses some form of low-pass filter to recontstruct a
continuous analog signal from the sampled data that emerges from the
D/A (Digital-to-Analog) converter. There are two possibilities: direct
D/A conversion at the disc data rate (44.1 kHz) followed by an
extremely steep ("brick wall") analog filter (Fig. 1), or,
"oversampling" the data to increase its frequency and permit digital
filtration, and then follow the digital filter with a gently sloping
analog filter (Fig. 2).
When data is converted from digital to analog form at the 44.1-kHz rate,
sidebands surrounding the sampling frequency extend as low as 24.1 kHz
(44.1kHz - 20kHz) as shown in Fig. 1 (c) requiring 9- to 11-pole
filters having an extremely steep slope. Such "brick-wall" filters
inevitably have severe phase distortion and non-constant group delay at
high frequencies which result in audibly inferior sound.
The approach taken in the OMS-7 and OMS-5 is completely different as
shown in Fig. 2. After quadruple oversampling digital filtering and
D/A conversion, the signal spectrum soncists only of the desired
baseband information (from 0 to 20kHz), and the sidebands surrounding
176.4kHz (4 times 44.1kHz) and its harmonics. As a result a gently
sloping [analog] filter is all that's needed as shown in Fig. 2(c)
producing constant group delay and linear phase-filtering and virtually
no phase distortion.
Perfect D/A Conversion
Many CD players share a single D/A converter between both channels,
converting left and right information sequentially - not
simultaneously - so there's a timing error between the two channels
that causes an interchannel phase difference.
Another problem is that left and right signals are separated after
conversion, that is, when they are in analog form. The switching
circuit used to separate the channels can introduce noise and
distortion.
In the OMS-7 and OMS-5, by using dual D/A converters, left and right
data samples are separated prior to conversion, that is, while they
are still in digital form. The deinterleaving switch cannot introduce
noise or distortion, and the two channels can be converted
simultaneously by separate D/A converters.
Direct-Coupled Linear-Phase Analog Signal Processor
In designing the OMS-7/OMS-5's analog circuitry, we've gone to
extraordinary lengths to preserve the pristine purity of the original
sound. To eliminate the possibility of digital noise from every
source, the OMS-7/OMS--5 analog circuitry is separately constructed and
enclosed in a special module called Direct-Coupled Linear-Phase Analog
Signal Processor.
The entire module is housed in a shielded enclosure to ensure
electrostatic and magnetic immunity. Not only is the analog circutry
powered separately from the digital circuitry, but left and right
channels are separately powered by regulated bipolar supplies. Even the
grounding systems are independent to ensure total shannel isolation at
all frequencies - not just in the midrange. The analog circuits are
directly coupled to eliminate distortion - causing capacitors.
High-purity oxygen-free-copper (OFC) wiring and gold-plated output
terminals are used to ensure the purity of the signal.
The results are flat response with minimum ripple right up to 20kHz,
THD that never exceeds 0.005%, and channel separation that approaches
100 dB over most of the band is a remarkable 90 dB at 20kHz!
Precision-Engineered Drive and Servo Systems
The Nakamichi OMS-7/OMS-5 disc-drive mechanism is mounted on an
exclusive zinc-alloy diecasting which is suspended on coil springs so
that it floats free of the disc-loading mechanism and the main chassis.
This makes it vitually impervious to external shock and vibration. The
disc is driven by a cog - and vibration-free linear torque motor to
ensure smooth rotation.
Focusing errors are detected through a critical-angle prism. Tracking
error is determined by a heterodyne detector. These two detectors feed
[the] precision servo mechanism to keep the laser on track and in perfect
focus even when the disc is slightly warped. The servos have
exceptionally fast response time to allow precise cueing and quick
access to any portion of the disc
OMS-7/OMS-5 Feature Comparison Chart
OMS-7 OMS-5
Quadruple Oversampled Digital Filters X X
Independent Left & Right D/A Converters X X
Direct-Coupled Linear-Phase Analog Signal Processor X X
Floating Disc-Drive Mechanism X X
Precision Centering Mechanism X X
Cog-Free Linear-Torque Drive Motor X X
Critical-Angle Focus-Error Detector X X
Heterodyne Tracking-Error Detector X X
Fast-Responding Precision Servo System X X
Direct Load-Play/Load-Pause X X
Direct Track Search X
Direct Index Search X
24-Program Memory X
Memory Clear X
Memory Recall X
Bidirectional Skip Search X X
2-Speed Cueing X X
Repeat Play X X
Remaining Track and Time Displays X X
OFC Wiring and Gold-Plated Output Jacks X X
Headphone Output with Volume Control X
Wireless Remote Control X
OMS-7/OMS-5 Specifications
System..............................Compact Disc Digital Audio
Signal Detection....................Contactless optical system
with solid-state laser element
Error Correction....................CIRC system
Number of Channels..................2 channel stereo
Sampling Frequency..................44.1 kHz
[176.4 oversampling]
Quantization........................16 bit linear
Disc Rotational Velocity............Approximately 200 to 500 rpm
(Constant Linear Velocity)
Wow and Flutter.....................Unmeasurable
Frequency Response..................5 Hz - 20,000 Hs +/- 0.5 dB
S/N Ratio...........................Better than 92 dB (A-WTD)
Total Harmonic Distortion...........0.003% (1 kHz)
Channel Separation..................Better than 92 dB
Output (Line).......................2 V (1kHz, 0 dB), 100 ohms
(Headphone)(OMS-7).................20 mW(1 kHz, 0 dB,
output control max.) 8 ohms
Power Source........................100, 120, 120/220, 220
or 240 V AC; 50/60 Hz
(according to country of sale)
Power Consumption...................30 W maximum
Dimensions..........................435(W) X 100(H) X
308(D) millimeters 17 1/8(W) X
3 15/16(H) X 12 1/8(D) inches
Approximate Weight..................7.5 kg; 16 lb 9 oz.
Remote Control Specifications
(Supplied with OMS-7)
System..............................Infrared pulse transmission
Power Supply........................3 V DC (1.5 V X 2)
Dimensions..........................57(W) X 175(H) X 19(D)
millimeters 2 1/4(W)
X 6 7/8(H) X 3/4(D) inches
Approximate Weight..................140 g; 5 oz
*Specifications and appearance subject to change for further
improvement without notice.karn@mouton.UUCP (12/10/84)
Hate to beat a dead horse, but I thought I'd throw out this fuel for the fire. I recently obtained the service manual for my Technics SL-P7 CD player. You know, one of the "el cheapo" models with (horrors!) analog reconstruction filters, and (double horrors!) a single D/A converter. All in all, a unit that "gives digital audio a bad name" (to quote one of the net experts). In scanning the schematic, I immediately noticed some assymetry between the two channels. It turns out that they put in an ANALOG all-pass filter in the left channel whose sole function it is to delay the signal by 1/2 sample to make it "match" the right channel. So there. They saved me the tedious effort of moving my left speaker back by a centimeter or so. Phil P.S. I recently attended a live performance of Saint-Saens Organ Symphony given by the Baltimore Symphony Orchestra. Compared to my reference CD player, the live performance lacked proper stereo imaging and had shrill, harsh highs. The noise level was also considerably higher (no audience should have been allowed) and there was obvious multi-miking in the acoustic design of the concert hall. (Seriously though. The live concert made me realize how far we've come in sound reproduction. Except for the audience noise, I could close my eyes and imagine myself sitting at home in front of the CD player. So why did I blow $16.50 per ticket?)