...we might as well just deliver a 4-channel digital audio file instead of bothering to create a CD-4 record...
And then what? Twin-track the disc with two grooves one-groove-width apart from one another and have a four-pole twin-stylus cartridge? Have each stereo pair be offset by 180-degrees and play it with two tonearms and two cartridges facing each other? Have one stereo pair go from the edge of the disc to halfway-through and the other two channels start halfway-through and go to the center? Use both sides of the disc at the same time in ``opposite'' directions and be only able to play the disc on its' edge?
... but it can be also be done for fun!!
Oh it was fun alright dealing with all those crazy formats. (Name them all and get the Last Two Wedgies and the Game, Match and the Million Dollars).
I believe it is the only step missing to demodulate CD-4 recordings in the PC, starting from a two-channel high bandwidth recording.
Except the ``high bandwidth'' would have to be ridiculous in terms of a sample rate.
As we all know, the MINIMUM sample rate required to capture an analog signal into digital is twice the highest frequency, one sample for the negative swing of the highest-frequency sine wave and one sample for the positive swing.
In reality though, trying to get an even CLOSE to accurate sampling of a sine wave with only two samples is absolutely ridiculous. Ten-times or a hundred still wouldn't give you THAT good of a sampling. A thousand times BEGINS to approach acceptability.
So, say you have your 20 Hz to 15 KHz bandwith-limited CD-4 baseband audio signal, processed in this manner in order to avoid interfering with the modulated carrier wave above it. This carrier wave, while centered at 30 KHz can swing down to as low as 20 KHz or as high as 45 KHz, again, bandwidth-limited in order not to interfere.
So, already we have not one but two stereophonic bandwidth-limited signals to deal with which we'll get to in a minute, and all those nice musical overtones which give recordings their depth of presence and height of clarity all stripped away by the limiting necessary for CD-4 in both the baseband as well as the carrier wave.
Which means CD-4 has no greater bandwidth on either of it's bands than FM-Stereo which is similarly limited to 15 KHz in order to avoid interfering with the 19 KHz unmodulated stereo beacon subcarrier, nevermind the 38 KHz Left-minus-Right subcarrier needed to recreate stereo radio.
But in order for the computer to EFFECTIVELY capture the CD-4 tone ALONG with all of it's modulations, the computer would have to sample at a minimum of 900 KHz, 9,000 KHz or 90,000 KHz (of ten times, a hundred times and a thousand times respectively of twice-the-highest-frequency needed for sampling).
The only computer that can do that on a regular basis is the Cray Supercomputer Arrays at Government installations such as NASA, Jet Propulsion Laboratories or the Lawrence Berkeley Research Center. And then, not only would the CRAYS have to be ganged together in order to do it with any degree of speed, but all their hard drives and other storages would have to be assembled in a huge RAID as well JUST to be able to save it to disc fast enough.
Forget real-time recording on THAT one. You'll be there forever, like the first digital recording engineers were back in the mid `50's trying to record a three-minute selection of the First Movement of Beethoven's
Fifth Symphony onto IBM punch cards.
The resulting three-minute selection was in fact transcribed, onto millions and millions of punch cards occupying floor after floor after floor of card-catalog style storage. It took three years to punch all the cards, and another year to load them all back into the computer for playback.
At eight-bit word lengths and a 10 KHz sample rate.
Playback was done for three minutes of music, and it sounded like extremely clear AM radio.
Nothing more. Baby simple. But out of that has grown..... .
Oh, sorry, what were we talking about?
Dropped into Bernstein's Omnibus show from the 50's there for a little bit.
http://www.youtube.com/watch?v=xK1BkymfaM4 Slide over to 3:25 and begin.
And then back to our other topic, the main problem with the ORIGINAL CD-4 system was NOT the mis-placement of the RIAA curve or the NAB curve from the tape being wrong, it was the fact that when you divide frequencies in a cutterhead of the period down, the result is what's known as a bass contour effect.
The bass frequencies become lost in the dirt of the bottom end of the cutterhead's frequency response (which now must engrave 10 Hz, 12 Hz, or 15 Hz respectively to accurately reproduce 20 Hz later on).
Then, due to various ``bumps'' and ``valleys'' in the not-perfectly-flat response of the cutterhead in question, rather than adding a nice pleasing effect to a vocal or lead instrument, this will now mess with the frequencies in the high-bass or low-midrange areas, making the recording sound terrible in comparison to standard stereophonic releases cut on the same lathes in the same period.
Which, especially in the early days of CD-4, they often did, but people forgave a lot in order to have double the channels.
So that's why you really couldn't master the ``two-channel modulated program'' into the computer the conventional way and retrieve the signals digitally, even if you HAD a program to do so.
Trying to load master tapes into a computer with their original supersonic bias frequency tracks intact in order to create an inverse to cancel out the miniscule analog speed variations on the original recording date creates a similar problem.
But Jamie Howarth at
www.plangentprocesses.com does similar work with their proprietary-at-the-moment technology for retrieving bias frequencies off master tapes and using that to create an inverse sync and run a computer off that newly-created inverse sync without actually having to capture the original supersonic bias to do so.
So, while excellent CD-4 retrieval will still be for the foreseeable future accomplished by Lou's new demodulators, master tapes can now be restabilized using similar technology as the sync on tape carries no music modulated within it as does CD-4.
And then, you never know. New developments could be just around the corner to take care of the bass-contour problem and midrange problems incurred from mastering at half-speed, 2/3rds speed or 3/4 speed. Add to that, mastering onto DMM and mastering for playback at 45, and any way you slice it, you'll have a considerably better product than you had in 1974.