by David Ranada
CBS' NEW "CX" ENCODED DISC
IN mid-May, and with surprisingly little fanfare, CBS demonstrated for the press its "CX" (Compatible eXpansion) noise-reduction system for discs (as announced in STEREO REVIEW'S "Bulletin" in January).
CX is the only encode-decode disc noise-reduction system claimed to be audibly acceptable in non-decoded playback--in other words, compatible with ordinary playback systems. At first hearing it seems that CBS has done all the necessary psychoacoustic homework needed to perfect such a system, for CX lives up to everything claimed for it, at least with the musical selections and pressings used at the demonstration, a sampling of broad-dynamic-range jazz and classics.
The claims for CX are impressive. When played through an inexpensive CX decoder (expander), a CX-encoded record will offer a dynamic range 20 dB greater than that of a normal pressing. This gives a total disc dynamic range of up to 85 dB, depending on the pressing quality, comparing favorably with the dynamic range offered by a stereo mixdown from a multitrack sixteen-bit dig ital-audio master tape. Noise from a CX encoded disc played without a decoder is said to be no worse than that from a conventional pressing. In addition, there is no alteration of frequency response in non-de coded playback and a minimum of "pumping" of music or noise. (It is this last point that will undoubtedly be the crux of the audiophile debate that is sure to be started over the CX system.) Eventually all CBS discs will be released in CX-encoded format, and licensing agreements with other major labels are expected.
CBS is not asking for royalties on CX-en coded records and expects the prices of en coded records to be no higher than those for normal discs. Their stated goal is to make CX ultimately "the industry standard for disc manufacturing" since the compatible nature of the encoding eliminates the need for double inventories. The first CX-encoded pop record has already been released (New Music's "Sanctuary," Epic NFE 37314).
On the hardware side, the circuit looks quite simple. It requires only common, off-the-shelf parts with no special selection or matching required. By the time you read this there may be at least one custom-made integrated circuit (IC) available that will contain almost all the necessary circuitry for a CX decoder. Add-on CX boxes are promised from Phase Linear, MXR, Audionics, Sound Concepts, and others, and they are expected to cost between $50 and $100. That cost is dominated by the cost of the cabinet, chassis, and power supply, not by the CX circuitry, which means that CX decoders could be built into preamplifiers, amplifiers, and receivers at very little additional cost.
It should be noted that CX is incompatible with other tape and disc noise-reduction systems (Dolby B and C, dbx) and in its present form is optimized for disc noise reduction only, not for tape or the audio channels of videodiscs. Like the Dolby tape systems, however, the CX decoder is sensitive to absolute signal level; unlike the Dolby circuits, a level mismatch of up to 6 dB in the CX system is said to be "almost unnoticeable." (Level matching at home need be done only once using a reference-level calibration disc and a decoder control.) The CX system is also similar to the dbx tape and disc systems in that it uses a 1:2 expansion ratio in playback. The CBS sys tem, however, does not use this expansion over the entire dynamic range as does dbx.
Signals more than 40 dB below the reference level are not processed, and this is one of the three main reasons that a CX-encoded record does not sound unnatural in non-decoded playback.
ANOTHER reason for CX's acceptable non-decoded performance is that, unlike Dolby and dbx, the system does not use pre-emphasis in encoding. Non-decoded CX records thus have the same overall frequency balance as normal pressings. The last major factor in CX compatibility is that the encode-decode process has been carefully designed to take advantage of the ear's limited ability to follow rapidly changing loudness levels. The internal circuit timings and levels have been chosen so that perceived noise pumping is minimized in both decoded and non-decoded playback. Amazingly, you should be able to check this out for yourself by early fall.
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Source: Stereo Review (USA magazine)