h a l f b a k e r yBaker Street Irregulars
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Current VCR technology uses a tuner (like in a TV) that selects the channel signal desired, then records that channel. But the entire spectrum of available channels is already there in the cable. Why not have the VCR record the cable signal first, then when you play it back, that signal is run through
a tuner. That way, you can record an entire cable signal time slot, and watch any of the channels from that time slot.
In addition to the home use for this, imagine places like casinos with multiple security cameras. They currently use one tape for one to four cameras. With this system, they could record as many cameras as they can create channels for, all on one tape.
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Hmmm. Interesting idea. The
only problem I see is that the
cartridge would be about the
size and cost of a VCR, and the
VCR itself the size and cost of
a computer. |
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Croissant for Videodrome-ness. |
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Couldnt it be stored in another format? DVD maybe... |
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The problem is with information density. Ordinary videotape, run at normal speeds, has a bandwidth of probably 5MHz tops. Fortunately, that fits within the realm of what composite video requires(*). To record even just the VHF band would require over 50MHz bandwidth which would thus require running the tape and head at over 10x normal speed. |
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(*) A VHS VCR doesn't actually record a composite video signal directly to tape, but instead decodes the color information therein and recodes it differently. The bandwidth requirement is similar, but the VHS encoded format is much less less sensitive to timing variations than NTSC composite video. Even a 0.1% timing jitter in the playback head speed would render the colors in NTSC almost totally useless. A red object, for example, would shimmer between purple and orange. By contrast, after the signal is recoded for VHS a 0.1% timing jitter merely causes a slight jitter in the position of objects near the right side of the screen. Jitter would probably have to exceed 0.25% in most cases not be noticeable, and even a 0.5% jitter would probably not render a picture unwatchable. |
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This recoding does cause some loss of picture quality (which is why professional formats don't use it) but it's much easier to design equipment with a 0.25% jitter tolerance than 0.025% (which might still be pushing it). |
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Would you get jittering if you used a DVD? |
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I like the idea. I don't know diddly about whether it's practical or not, so I defer to [starcat]'s obviously greater knowledge. But... what [RK] said... Couldn't we use a format that has greater data-storage capacity? |
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Liie a 100GB IDE UMDA100 Hard drive or something. These Start from about £200+VAT at the moment though... |
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Hmmm. IDE UDMA100 has a max theoretical transfer rate of, I believe,100 MB/s, or 800 Mb/s. I don't know what the uncompressed information rate of an NTSC signal is, maybe 80-100 Mb/s? So you could only squeeze 10 channels worth of TV-quality video through a UDMA100 disk interface. You'd have a hard time finding a drive that could actually write that data to the disk that fast, though. A 100GB disk would only store about 3 hours of video anyway (or 20 minutes of 10-channel video). |
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To make a useful digital recorder, you really have to compress the video (TiVos, replays, etc. all incude MPEG-encoders). But each channel you want to record will need its own costly, complex, power-hungry MPEG-encoder chip. Darn. |
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I think the most practical solution is the one supercat suggested --- record in analog, and use a higher tape speed. Maybe wider tape with multiple parallel tracks. |
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<trivia>Full-bandwidth component video (YUV) at 8-bits per channel flows at 270Mb/sec. 10-bit at 300Mb/sec. Composite is considerably less. |
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Fantastic idea. [+] If you're utilizing this system for security you probably would need very low-fi sound if any at all.. |
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..and for home use - to get a similar result to VHS quality sound and video, I think a system that separated then compressed the video and sound of each channel individually, could combine them to a signal similar to that coming from cable, but small enough to be recorded on a larger version of a DV tape(which can store 3 hours of digital footage+sound on something fairly small)? |
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I'm sure that every VCR/DVR maker has explored this possibility and found the technology wanting, but it's not out of the realm of possibility. If I can have 60 VCRs, I ought to be able to have 1 VCR that's 60 times as big. Scientifically, it's impractical...but so was cross-country flight once. |
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