Timeline Analog 2. John Buck
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Название: Timeline Analog 2
Автор: John Buck
Издательство: Ingram
Жанр: Изобразительное искусство, фотография
Серия: Timeline Analog
isbn: 9781925108583
isbn:
The complexity came with the desire to display the selected frames at other than 30 fps; faster, slower and frame-by-frame. A further complexity was that only the first field of each frame was on the disk, the second field for interlaced video came from an auxiliary disk which recorded the first field and delayed in a line-time to provide the second field.
Dave Bargen wrote:
King Anderson did much of the digital hardware design, for control and interface. Later, he headed the manufacturing of the products.
Jim Adams continues:
Dave had selected the PDP-11 as the computer to use in this system and I believe that turned out to be the best decision made in the entire project.This computer had many attributes of larger systems and the machine assembly code was extremely versatile.
Add-on (interface) boards for the PDP-11 were designed to link the necessary control features of the disk players, video switching circulatory and light pen position to the processor.
I was provided with a high-speed paper-tape reader and punch and an ASR-33 teletype for my software development.The printing speed of the ASR-33 is 10 characters per second.After a couple of months, I was able to persuade the company to get a Memorex printer which was about ten times as fast, but I still edited my programs on the ASR-33.
The analog team were still dealing with issues as Cal Strobele recalls:
I spent many days, in a fog, trying to figure out how to apply what we already knew about compression and recording to this project. In the end I came up with an idea to skip every other video field, in black and white and to record the audio on the back porch of the video for the missing field.
The ‘backporch’ is a term referring to the signal time between the horizontal sync pulse and the next active portion of the video signal. This is the area where the audio signal was stored.
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 СКАЧАТЬ