The Memorex disc pack was typically driven at 1,800 rpm so that each revolution took 1/30th of a second. Since it took one-sixtieth of a second to generate one video field, exactly two fields of video were recorded on a disc corresponding to one revolution of the disc pack.
In skipfield recording only one field of video is recorded per frame. When replayed, each field is duplicated (and interlaced) to produce a fairly accurate reproduction of the original video sequence.
Gene Simon continues:
Because bandwidth was such a premium, only every other video field was recorded on the Memorex drives and they were only low bandwidth monochrome images. A separate single platter “skip field” disk drive was used to synthetically create the missing fields by creating a one-field delay.
Yves Faroudja recalls:
This method was the same as what Sony used with their helical scan tape machines at the time. There are ways of course to extrapolate better data and see the beauty of disk images with more heads but that was more complicated and more expensive than CMX wanted. The video was more or less preserved with the skip field method to achieve a way to edit in the most expedient way.”
Strobele recalls:
The sync coming from the original video feed then drove the servo and we could treat the whole system as a closed circuit. Then we could destroy the sync pulse, the back porch and remove all of that and insert our signal in there with only a narrow leading pulse of the leading edge.
The skip field method had solved the storage issue by enabling more video to be crammed onto the disk platters but it created another problem. Bargen, Strobele, Anderson, Adams, Youngstrom, Faroudja, Scaggs and Eppstein cited it in their subsequent patent:
… the visual quality of a reproduced skipfield recording at the editor's monitors is sufficiently high that editing functioning is possible. However, while it is satisfactory to eliminate one-out-of-two or more fields of video, the same is not true for audio. In fact, it has been found that for satisfactory audio reproduction, the audio samples associated with each field of video must be retained and ultimately reproduced.
Jerry Youngstrom recalls:
Every field of audio had to be retained and kept in sync with the skip field video.
Faroudja adds:
Tony Eppstein and Lee Scaggs designed a very smart scheme to keep the audio in sync and at high quality.
Gene Simon continues:
The missing fields of audio could not synthetically be created, so both fields of a single audio channel were recorded on the back porch of each video line (where color burst would have resided) with a pulse amplitude modulation scheme. The skip field disk was used in both record and playback to delay every other field of audio so it could be reconstructed.
Since the video line rate is 15,750 the line rate amplitude modulated audio pulses could be filtered to become reasonable sounding audio.
The system of placing audio signals, into video control pulses in a manner which did not disturb the control function to be carried out by these control pulses, had been devised and patented by 3M engineer Fred Hodge in 1965.
Strobele recalls:
The pulse amplitude modulation scheme had a great side benefit because when it was displayed in video, an editor could ‘see’ the audio. The sound was displayed for the editor to work with and while it was never intended to be a feature, once we showed CBS editors this, they loved it.
Gene Simon adds:
I’m not sure if this was by design or not, but having still frame audio turned out to be a great feature because edit points during silence (e.g., between words) could easily be found when frame by frame jogging.
On the other hand, if you were not the editor trying to do actual creative work, the buzz of still frame audio will drive you crazy. The hardware and media development was critical but a significant amount of software development was needed to drive the system.
Jim Adams adds:
As time went on, it became apparent to me that the hardware side of the 600 was far more difficult that had been originally surmised. Adapting the digital disk drives to handle analog video, sound and frame count signals, as well as converting the drive motors from standard AC motors to servo-controlled DC motors was very challenging to the electronic design engineers.
Then another audio issue arose. The previous method to reduce noise introduced along a transmission path, be that a coaxial cable, telemetry link or videotape, was to use pre-emphasis and de-emphasis circuitry. However such an approach caused audio distortion, image streaking and could not guarantee equal time delays for signals.
Strobele recalls:
Because it was an FM signal and we had to ensure that the bandwidth of the two signals didn’t impinge on each other, we had to come up with a technique of limiting the level. The answer was to create a hard limit with a slow decay but we didn’t limit the sound limit except for the first cycle.
Lee Scaggs created, and patented, a new method that logarithmically amplified low level signals and logarithmically de-emphasized the original input signals. Then further video issues as Strobele recalls:
There were some image issues with artefacts when using 24fps footage. You would see it particularly on footage with zooms clashing with key stoning from the lens, but because we knew this was going to be purely for offline editing, and it was a technical limitation of the field and frame conversion that it was impossible to fix, we let that go.
Faroudja recalls:
My job was to get decent pictures for the two black and white monitors that were planned for the system. The picture had to be vaguely recognizable but not use too much storage space. Through trial and error we discovered that with about one quarter of the original camera data being used you could edit well.”
Strobele’s work on defining the analog signal process and its circuitry was almost done.
We had made sure the electronics played RF to RF, and the RF reproduction of the disks was good enough. Once Tony (Eppstein) could reliably run the packs at 3600rpm, we could put the first field’s audio on and the second field’s audio on and then combine them and do all of that processing and have a data stream ready to put onto the disks.
The digital team was now in a position to improve and finesse the process. Better flying heights for the heads, make vertical recording possible for increased bandwidth.
Faroudja continues:
Remember this is 1970 and I was fascinated by how easily you could click and click, decide this shot or that and then you would see the shots on the screen as an edited version.
Across America, Harvey Dubner was out of work.
He had graduated as an Electrical Engineer in 1949 and was then employed by companies to work on military projects during the Cold War.
In 1958 I was involved with a missile system that required extensive optical calculations so that we rented a new thing called a digital computer. To make a long story short, that changed