CD-4 stands for Compatible Discrete 4 Channel. This was a true four channel surround sound system for vinyl records. In America, the system was known as the RCA Quadradisc system. The system was launched in the early 1970s in Japan. In September 1971, RCA committed to adopting the JVC system in the USA and made initial releases in May of 1972.
Stereo Lab now supports software decoding of CD-4 quadraphonic LPs.
CD-4 (Quadradisc) has the 4th largest discography of commercial quad systems after: Dolby Surround; QS and SQ with about 150 titles released on the RCA label and about another 100 titles on other labels such as: A&M, Arista, Project 3, Atlantic and Electra/Asylum.
RCA's discography includes classical and country repertoire as well as a disproportionate share of easy listening. The label did eventually release CD-4 discs by mainstream acts, for example; Lou Reed and Nilsson. Nevertheless, light-jazz/ easy-listening repertoire dominates many of the releases on CD-4; aided by Project 3 which was a label devoted to this genre. This bias reflects the somewhat middle-aged demographic of the early-adopters of what was an expensive quadraphonic system. Warner Brothers' Elektra Asylum label released some of the biggest acts encoded in CD-4, including: The Doors; Bread; Joni Mitchell; The Eagles; and Bob Dylan.
There exists an interesting parallel between the contemporaneous, rival, analogue television standards and the rival quadraphonic standards. Rather as with analogue television standards, a dimension exits which might be labelled quality vs. robustness when comparing the performance of the rival "quad" systems. Thus, a colour television standard like NTSC colour was capable of fine reproduction, yet it remained sensitive to channel distortions which made it a rather "fragile" system: whereas, the French SECAM system, whilst not capable of very fine reproduction, was so robust that colour TV signals could be passed (and even recorded) on monochrome equipment. Judged in this way, one can see that it's simply meaningless to declare one system as "best", since the question, "best for what?" follows inevitably.
Matrix quadraphonic systems like QS and SQ can be seen as the "SECAM of quad": clever, robust with the extra information hidden in the same bandwidth of the audio and undemanding of replay equipment: whereas JVC's CD-4, whilst certainly the only system capable of real, discrete 4-channel performance, was a relatively fragile quadraphonic system. A number of requirements both of the equipment and the medium need to be fulfilled to derive a stable and realistic quadraphonic effect; conditions which are even harder to satisfy half-a-century on from the system's heyday!
Because of this, and because we can't control the quality of the hardware needle-drops applied to the software decode, Stereo Lab is designed to "fail gracefully" when those conditions are not satisfied. To achieve this, the decoder in Stereo Sauce converts the CD-4 signal to an intermediate Ambisonics B-format signal before the final loudspeaker signals are calculated. To do this has a number of important advantages as explained below.
Baseband signals (20Hz - 15kHz) out of the groove are in fact the sum of the front and back signals (LF + LB) and (RF + RB). These signals are engraved on the 45° walls of the disc just as in a conventional stereo disc. The difference signals, used to separate back from front, are FM encoded on a pair of ultrasonic (30kHz) subcarriers above this baseband signal. (See diagram above.) The overall bandwidth of the CD-4 signal is thus from about 20Hz to 45kHz; a rather profligate three times greater than for conventional stereo LPs; a requirement which required CD-4 discs to be mastered at a third of normal speed.
A special Shibata stylus (alternatively named as Quadrahedral, Obata or Pramanik) is required to follow these high frequency subcarrier signals when the record is rotated at normal replay speed, and various precautions, such as: wide bandwidth preamps; correct cable types; low HF crosstalk in the cartridge and the subsequent electronics, are all required to recover subcarrier signals of sufficient and constant amplitude that successful decoding is possible. A micrograph of a recorded CD-4 groove illustrates the presence of the carriers as well as emphasising the concomitant requirement for scrupulously clean media and precision replay equipment.
Essentially, the CD-4 decoder in Stereo Lab has an Ambisonics "back-end".
This is included because Ambisonics is a formalised description of a sound-field, so it is possible to control the adaption of the decoding on a more elegant basis than simply collapsing the front-back separation in the event that the recovery of the FM signals is not entirely successful (which is all the CD-4 standard allows for).
Our approach has been to recover an enjoyable and satisfying musical effect from CD-4 records for final presentation using the standard ITU-R BS. 775 5.1 surround-sound loudspeaker layout.
It is hoped that, in this manner, the decoder will be generally useful to a wide range of users with different standards of replay equipment and that these adaptive techniques will help redress some of the hostility towards the CD-4 system where it has sometimes gained the epithets, sandpaper quad or more generally seedy four.
Importantly, ONLY needle-drops at 96kHz sampling are acceptable for CD-4 decode in Stereo Lab. All other sample rates simply produce an error (Unsupported frame rate) and fail to process.
Needle-drops must be be recorded without RIAA equalisation1. Attenuation of the upper sidebands of the CD-4 FM signals due to RIAA filtering is over 46dB relative to the baseband. Thus ninety-nine percent of the FM carrier level is lost in the RIAA filter and the FM tracks are reduced to 8-bit resolution. Our experiments have shown that reliable CD-4 FM carrier recovery is not possible with RIAA equalised needle-drops. (In any case, a non-equalised needle-drop is the usual input format for Stereo Lab phonograph processing and highly accurate RIAA correction to the baseband signals will be applied in software.)
JVC CD-4 decoding has its own preferences dialogue in the Stereo Lab GUI in which cartridge level matching and click-pop reduction may be selected. The initial CD-4 decoder is intended for CD-4 needle-drops of all CD-4 records, including those damaged in the fifty-odd years from their manufacture. A "best effort" approach is taken on the basis of the analysis of the incomming needle-drop data (as explained above).
Nota bene, by devoting a preferences dialogue to CD-4, we have provision to offer different decode "recipes" later. For example, we could offer a software decode which emulated exactly the old hardware systems with their maximum-separation decode to a four-square loudspeaker layout; something only desirable when a very high-quality CD-4 needle-drop is available.
Successful CD-4 decodes require some special requirements of the needle-drop hardware:
It is highly recommended that you use audio software such as Audacity* to record CD-4 needle-drops to the Mac. This software offers an instant view of the audio waveform as the needle-drop is made and the presence (or not!) of the 30kHz carrier is easily witnessed and judged; most obviously in silent sections where the presence of the subcarriers and their channel balance is easily inspected.
Using Audacity to record CD-4 discs also enables the correct recording levels to be set. CD-4 discs are recorded substantially quieter than conventional stereo discs, so be careful not to under-record CD-4 needle-drops. In fact, there is an excellent way to set CD-4 disc recording levels.6.
When looking at the waveform of an unequalised needle-drop (which is the only mode supported in Stereo Lab), the carrier should appear as a band of constant modulation.
The screen-grab below illustrates well the situation where, due to an incorrectly set skating-force adjustment, the carrier on the right channel (lower trace) is intermittent and inadequate for demodulation.
Once the turntable and tonearm setup is optimised, don't assume this will be remain the best set-up across the whole side of an CD-4 LP - the wavelength of the modulation in the inner grooves of the record is ⅓ of the wavelength of the modulation at the outer edge of the record, so just about every aspect of a CD-4 needle-drop becomes more critical as the record plays.
The state of cleanliness on CD-4 records is especially important. Cleaning CD-4 records is something of a specialist subject. We have found that standard methods should be considered a baseline for CD-4 types and that more Draconian methods are needed on discs which have been played many times.
If the quality of the ultrasonic subcarriers for either channels is judged inadequate, Stereo Lab will generate the warning "no RADAR" which appears in the process progress dialogue. (RADAR is the term JVC used for their carrier detection in the heyday of CD-4, so we use the term too; although what RADAR stands for in this context is not known.)
CD-4 difference signal processing is partially supended if this warning is given; although the full software decode is attempted and an output file(s) generated. Even in these circumstances, an attempt is made in the software to generate a partially valid surround-sound signal.
* Audacity is a free, easy-to-use, multi-track audio editor and recorder which runs on the Mac platform (and, in fact, on Windows and GNU/Linux too). Audacity is free software, developed by a group of volunteers and distributed under the GNU General Public License (GPL).
The first step is to separate the "quad" signal so as to
1) derive the baseband signals and
2) separate the HF carriers for the quad effect.
This separation is achieved in linear-phase, 8-pole filters; an improvement over the original analogue filtering (shown right). The low-passed signals are treated as standard stereo signals.
CD-4 discs have inconsistent carrier level, this is partially deliberate due to carrier level control during cutting, and partially due to "up-talk" in the physical interface of the groove, where out-of-phase amplitude modulation of the FM carriers appears to be due to stylus movements in the lateral axis as the the stylus is thrown against one or other of the groove walls as it weaves it's way through the baseband modulation.
Even using an FM discriminator with amplitude-rejection, this powerful amplitude modulation "gets into" the demodulated output adding distortion and noise. There is therefore the need to control the level of the carrier sent to the discriminator. In hardware decoders, this is accomplished by straightforward limiting (amplitude clamping), an option not available to us in a time discrete implementation without fabulous degrees of over-sampling. Instead a fast-acting carrier AGC system is employed. The image (left) illustrates the HF signal after the AGC stage.
The demodulators are the heart of the CD-4 decode and the demands on this element are severe in the CD-4 system as the message frequency range is half that of the carrier and the modulation index is over 30 at low frequencies. The graph left illustrates the modulation index (at 0VU) of the FM signal relative to frequency and how this is modified by the signal pre-emphasis prior to being applied to the FM modulator. (This is from a Service Manual of a CD-4 demodulator: the red-line, indicating a 1kHz deviation, is added for clarity2). Peak deviation at 1kHz modulation is ±7kHz on a 30kHz carrier5. Compare this with broadcast FM where a 10MHz IF is subject to ±75kHz deviation.
The FM discriminator design in Stereo Lab required considerable development. Not only does the FM signal require a very linear discriminator , but the algorithm must be relatively tolerant of carrier signal offset and drift due to the the speed variation and accuracy of the playback deck. It must also re-lock quickly in the event of carrier drop-out so that it does not produce noise like an FM radio when detuned from a station.
Below is a short video which illustrates the spectrum of a demodulated sweep at 0VU from 100Hz to 10kHz in the FM channels. The analysed signal is direct from the Stereo Lab demodulators so that you can also see the (unfiltered) sideband structure around the 30kHz carrier. Distortion due to demodulator non-linearity is a few percent (second and third harmonic).
The FM demodulator must mute very quickly if the carrier signal is lost, to avoid detecting noise and must recover very quickly when the carrier is re-established. The demodulator in Stereo Lab is very well behaved in this regard and performs much better than hardware demodulators. The oscillograms below illustrate the situation when a test signal (upper trace, 30kHz with with 3kHz FM modulation) is deliberately interrupted for 4 cycles of carrier. The resulting demodulation (lower trace) indicates how the demodulator deals with such an interruption.
With all these precautions, we have found that the quality of real-world demodulated signals from 50 year old records is considerably worse than the performance from the demodulation of our own test signals which leads us to the inevitable conclusion that considerable distortion is introduced in the encoding and replay of CD-4 discs. Why is this?
Theory suggest that the FM subcarrier system for CD-4 is capable of first-rate performance5. However the practical implementation of this system is bedevilled with issues which undermine the theoretical performance.
Any interference from the baseband channel (up-talk), not simply as a result of inadequate filtering, but due, for example, to harmonic distortion products from the baseband appearing in the FM band, are a source of particularly nasty, "splashy" distortion when demodulated. So is crosstalk between the two FM channels in any part of the system - including in the fragile cartridge. Most modern cartridges are unsuitable for first-rate performance because they offer inadequate channel separation at 30kHz. A few manage about 12dB separation, JVC stated that a minimum of -20dB was necessary for adequate performance of the CD-4 system. In our view, even -20dB is wide of the mark.
Listen to the effect of 20dB interchannel crosstalk on a track with a sinewave sweep on one-channel only (the right side is silent). Apart from the obvious effect of crosstalk, the live-channel audio is badly distorted in the encounter: it sounds like something from the BBC Radiophonic Workshop!
The frequency spectrum graphic, captured as the pure tone passes the 100Hz mark illustrates the distortion vividly with a procession of distortion components above the 100Hz fundamental.
Wow and flutter are further sources of "extra" (and troublesome) frequency modulation as well. If we select the period of silent subcarrier which exists in between tracks on a CD-4 record and beat it with a 31kHz signal, we generate an audio tone which warbles around 1kHz and thereby gives us a measure of the frequency stability of the carrier is as it read from the disc. Here is an audio clip of a few seconds derived in this way. The frequency deviation is about 100Hz pk-pk.
All these unwelcome frequency components in the FM band result in a host of spurious cross-modulation products which create distortion and raise the noise-floor. It is for these reasons that the practical CD-4 system included an aggressive noise-reduction system (ANRS). 3, 4 In fact, it's no exagerration to say that CD-4 would not work at all were it not for this multi-band expander arrangement. Interestingly, the Marantz CD-400 demodulator manual does say this regarding ANRS,
ANRS is used in the CD-4 system to reduce interference and noise caused by crosstalk between the left and right modulated carriers..
In other words, the "noise reducer" isn't there to reduce channel noise so much as to suppress co-channel interference.
We incorporate both modern correlation-based noise reduction techniques and frequency-based noise-gating within the software decoder. We base these techniques on software emulation of four hardware decoder references; the first is based on the Marantz CD-400 (JVC 4DD-5)2 and two are based on the QSI-5022 integrated circuit decoder designed by Louis Dorren3, 4. Finally, the late JVC CD4-50 has had a great influence on the software design. The CD4-50 is a rare hardware decoder but is widely reckoned to be a decoder which behaves where others fail. This is due to the rather different operation of the ANRS circuits in this late implementation. We have incorporated these changes in the Stereo Lab software decoder.
The matrix section in a standard CD-4 hardware decoder simply sums the FM signals and the baseband signals to generate the forward pair of signals and subtracts them to generate the rear pair of signals.
Our alternative approach in Stereo Lab is to derive the three (B-format) signals of first order, horizontal Ambisonics from the four CD-4 component signals. This enables us to exploit the power of Ambisonics to decode to different loudspeaker layouts. Using this technique, the CD-4 decoder in Stereo Lab decodes CD-4 to a standard, modern ITU-R BS. 775 5.1 surround-sound loudspeaker layout.
Hardware decoders require adjustment of the FM/baseband sensitivity; usually via a couple of potentiometers on the rear of the CD-4 decoder case. These are adjusted using signals from a special test record which was supplied with the unit. In Stereo Lab, no such special material is required and this adjustment is made automatically.
A short demo is available in the Phaedrus Audio listening room.
The input file to the CD-4 decoding process should be simple, two-channel, non-equalised, stereophonic track derived from the LP replay (or other source) at 96ksps ONLY. The output may be selected to be separate mono files (useful if you want to re-encode in DTS for example), or as a group of stereo files encoded:
CD-4 Preferences Tab
The CD-4 preferences tab groups together the controls for the various CD-4 decoding options in Stereo Lab.
Cartridge Channel Balance works by adjusting the measured RMS values of the left and right baseband signals. Note the points above concerning subcarrier quality and imbalance due to disc damage or due to a setup problem (like an incorrect anti-skate force) .
PHLUX-ll Crosstalk Compensation Only check this box is you are making needle-drops with the custom version of Phædrus Audio's PHLUX-II Libre active phono cartridge, fitted with a Shibata stylus. We could offer compensation for other cartridges here. See Output below for more information.
Pop-click removal for LPs Click-pop reduction systems are possible when decoding CD-4 The low recorded level of CD-4 baseband makes this noise-reduction especially useful.
Rumble filter Unless for test purposes, rumble filtering should always be selected as subsonic frequency components can upset the FM discriminator in CD-4 processing.
Hum Removal CD-4 needle drops should be substantially free of hum unless there is some major fault in the system. Only check this box if hum is not removed by other means.
Output For the moment, only two output decodes modes are available: Pspatial Audio CD-4, which is the decode described above; and Rears as Subcarrier.
If this latter decode is selected, the rear output audio signals are replaced by the filtered L/R subcarrier signals. This is intended for engineering purposes only. For example, this decode could be used to establish the quality of (or lack of damage to) the record or to check for "up-talk" from the baseband. It could also be used to measure cartridge crosstalk using the Shure Brothers square-pattern crosstalk test. The Shure Brothers test requires an audio goniometer (which is present in many DAWs). It is a simple and accurate way to measure crosstalk between the two subcarriers in a CD-4 system, including the cartridge.7
Pspatial Audio have received much help and advice in the development of this decoder. Thank you to all who have contributed ideas and samples of CD-4 material. We should especially like to thank Kirk Bayne and Jonathan Gatarz for their invaluable help.
1. Tucked away in the original reference relating to CD-4 is the information that all the "extra" subcarrier information is recorded constant-velocity. The diagram below indicates this detail that the modulator equaliser compensates with a downward slope for the upward slope of the RIAA equaliser in cutting. So, the unequalised signal directly from a MM or MC cartridge has the correct frequency balance to ensure the upper and lower sidebands are correctly balanced. This is why an unequalised needle-drop is preferred.
A DISCREET FOUR-CHANNEL DISC AND ITS REPRODUCING SYSTEM (CD-4 SYSTEM), Inoue, T. et al. Presented at the 39th Convention of the AES, October 1970. The manuscript for the presentation was expanded and revised in March 1971 and appeared in the JAES July/August 1971 (title unchanged).
2. Maranz Model CD-400 Service Manual.
3. Build a High-Performance CD-4 Demodulator. Dorren, L. Popular Electronics September 1975 (illustration above)
4. Surround Sound Decoders 1 & 2. Heller, D. Wireless World June, July 1976
5. Recording characteristic of the CD-4 FM channels (from Reference 1). If performance from these channels really delivered 74dB dynamic range in a practical decoder, noise reduction would not be required.
6. Comparing the recording levels of standard stereo and CD-4 discs is complicated by inconsistent use of RMS and peak velocity values in the various references. Standard level for a normal, stereo LP is 3.54cm/s. This is an RMS figure and relates to a single stereo channel. If both channels are recorded at standard level, the resulting velocty is the vector sum of the two individual channel velocities. Because the channels are inscibed on two, perpendicular groove walls is equivalent to the root of the sum of the squares of the individual velocities (Pythagoras' theorem). Thus,
√(3.542 + 3.542) = 5cm/s.
It's easy to get this mixed up with the peak velocity which relates to the RMS value of 3.54 cm/s which is, of course,
√2 × 3.54cm/s = 5cm/s.
But these two values are quite different from each other.
The CD-4 "standard" (the nearest thing we have appears to be Ref. 1) sets standard recording level at 2.23cm/s probably RMS and for one channel. So, CD-4 level is,
2.23/3.54 = 0.63 , or 4dB below standard level on a stereo disc.
Each, unmodulated carrier is recorded at a velocity of 3.54cm/s and thus appears (because it appears before the baseband moduation) as a signal at exactly reference level when recording a non-equalised needle-drop. In fact, this is an excellent way to set recording-level for a CD-4 disc (provided it is known that the cartridge has an extended frequency response).
7. Carrier Crosstalk Considerations in the CD-4 System. Anderson, C.R. JAES July/August 1977
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