Raxel 在 2006-4-23 23:51:28 发表的内容
科研级旗舰 在 2006-4-23 23:47:49 发表的内容
其实纯粹的高频延伸量并不是很重要，但一些CD机不重视CD20K左右的锐截止滚降问题带来的高频相位偏差，和数码滤波器带来的某些非线形失真。。。。。。。。。。 |
我就是单纯谈LP的高频,因为我觉得有时候谣言传多了就变真理了.就是想讨论个究竟.LP上面有超高频一说在港台杂志上有时候也看到. |
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在七十年代.....日本JVC的CD4唱頭頻應就到45kHz啦！
當然要配合JVC的LP黑膠唱片.......鬼佬這方面比日本仔落后..哈哈！.
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The Victor Company of Japan (JVC) developed a different method for 4-2-4
transmission and distribution. The technique was called “CD-4” as well as “Quadradisc.”
CD-4 was not considered to be a matrix system, but a carrier system. It allowed for full
recovery of the initial four channels with absolute channel separation.
Just like an ordinary two-channel vinyl where the left channel was stored on the left side
of the record groove and the right channel was stored on the right side of the record
groove, the Quadradisc stored music in the same manner. The unique feature of CD-4
was that the signals stored on each side of the record’s grooves actually contained the
information for the left rear and right rear loudspeakers as well. The left side of the
groove contained the sum of left front and left rear signals, and the right side of the
groove contained the sum of the right front and right rear loudspeaker signals. The
signals had a frequency range of 30Hz-15Khz.
In addition each side of the record grooves contained a high-frequency carrier signal.
The 30Khz signal was a modulation made up of the difference between the front and rear
signals on each side of the groove. The signals on the left wall of the groove were
compared to the carrier signal of the left side and the channels were successfully
separated. The same process occurred with the signals on the right wall of the record’s
The Implementation Of Ambisonics For Restoring Quadraphonics A. Digenis
15
groove. This was the same technology used for the transmission of stereo signal over FM
radio. A single signal was transmitted, which contained the sum of both the left and right
channels. The FM receiver produced a carrier signal at a frequency of 38KHz that was
used to separate the left and right channels from the single transmitted signal.
When the record was played over a standard stereo set-up, the left front and left rear
signals were projected out of the left loudspeaker, and the right front and right rear
signals were played through the right loudspeaker. In mono, the signals from all the
loudspeakers were heard at the same level as the left and right groove that were already
the sums of the front and rear, as equally combined. Neither of the 30KHz carrier signals
will be heard, as anything above 20KHz is inaudible by the human ear.
In order to playback CD-4 discs with full surround capabilities, additional equipment
were required compared to the matrix systems. Most turntable cartridges and styli were
only capable of reproducing frequencies up to 20KHz since anything of higher magnitude
humans cannot hear. The use of a standard cartridge and styli would not allow for four
loudspeaker-playback as they would not pickup the 30KHz carrier signal. It was thus
necessary to use the Shibata stylus with
a cartridge that would allow for a frequency
response up to 45KHz. Secondly a small device, the demodulator, was required to
extract the four channels. A few manufacturers later developed amplifiers that had a
demodulator built-in (The institute of High Fidelity 1974).
Quadradiscs provided better channel separation than the matrix systems, as well as stereo
and mono compatibility. Apart from those advantages, the system also had its downfalls.
The frequency response was limited to 15KHz. Due to additional space required to cut
the high frequency carrier signal grooves, the entire record needed to be cut 3dB lower
than ordinary vinyl. Due to the high frequency response of CD-4, during the cutting
process of the master copy, the process needed to be carried out at half speed. This was
because the cutting heads at that time were not capable of cutting grooves of such high
frequency with efficiency. Unlike matrix systems that could be transmitted over the
radio, the FCC did not permit for the transmission of Quadradiscs (Runstein 1976).
