On Sun, Jan 18, 2004 at 06:41:29PM -0600, Kent West wrote: > Jeff McAdams wrote: > > >Nano Nano wrote: > > > >>Could a device in theory record every channel simultaneously? > > > > > >Yes. > > > >>Could it in practice? > > > > > >Depends on what you consider "practical", I guess. > > > >I would say "no." > > > >Your typical TV or VCR has one or two "receivers" or "tuners" in it. > >A receiver or tuner is capable of receiving or tuning a single channel > >at a time, so a dual-receiver or dual-tuner TV or VCR is capable of > >extracting two channels at once. > > > >Software-defined "radio" (the same principles apply to receiving > >channels on a wire that they do to receiving channels over radio waves > >in the air) could, in theory, receiving the whole spectrum at once and > >extract individual channels out of the stored data at a later time. > >Again, it becomes a question of what you consider "practical." > > Or with a fast enough processor (does it exist?) controlling the tuner, > sample each channel in real time, like a couch potato surfing through > the channels and getting a fair idea of everything that's on, only much > much faster. It'd have to sample all 155 channels (or whatever) each > 30th of a second (for analog TV signals) in order to get a full > 30-frames per second for 155 channels. > > But that's probably closer to the theoretical rather than the practical. It'd be pretty hard to do it quite like that, with one analogue tuner continually being retuned. You'd have to tune it through all 155 channels every 0.125us to get a 4MHz video bandwidth on each channel. As a side effect the selectivity would be reduced and adjacent channels would interfere with each other. You could ease the task by only trying to recover the audio, perhaps at less than the full audio bandwidth. Better to do the whole thing digitally - A-to-D the incoming signal directly and have a fast DSP implement 155 tuners and demodulators in parallel, in software. That's less impractical than it sounds. GPS receivers work vaguely like that, to receive several satellites at the same time. DAB receivers are also vaguely like that, to receive the multiple carriers of the DAB signal simultaneously. Both cases are less demanding, of course, but it's certainly possible in theory. In practice you'd probably have to use a combination of software and hardware parallelism - eg. 15 DSPs doing 10 channels each - with current processor technology. (For an off-the-shelf solution, you could have a room full of 155 VCRs...) -- Pigeon Be kind to pigeons Get my GPG key here: http://pgp.mit.edu:11371/pks/lookup?op=get&search=0x21C61F7F
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