Iíve found a few references to work done capturing a raw RF signal of a magnetic tape then using software to reconstruct sync, scan lines and frames, chroma and luma info etc. but whatís out there seems proto / conceptual for the most part.
A company claims to have a technology set with Betacam as their donor platform, but their site is unclear as to maturity. I havenít contacted them, myself. https://www.cube-tec.com/en/solutions/video/quadriga-video/quadriga-video-rf/analog
I also found a short clip on YouTube thatís tantalizing but no details were provided. https://youtu.be/t0eLxQY6-RI
Iím starting this thread mostly with the intention of collating information about the current state of the art on this topic. The above two leads are the only ones Iíve been able to find myself.... Iím hoping thereís more out there! Also hoping for some discussion about the long term efficacy, limitations of this set of techniques for signal acquisition and image reconstruction.
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I don't understand what you intend to recover that cannot be recovered by simply playing back the tape(s) on appropriate & maintained decks and capturing via the most straightforward, highest quality format given the deck's output options.
RF capturing requires much higher bandwidth (aka higher samplerate) than standard baseband video. Supporting this capability is certainly not economical.
Or are you just trying to see if it can be done from a scientific, theoretical standpoint?
This reminds me of the efforts to play phonograph records by trying to reconstruct the audio from reading a waveform created by a laser beam reflected off the grooves. It can be done, but even the modern efforts, using the CPU/GPU powers of modern computing, fail to fully reconstruct the audio, and the resulting audio is inferior to what can be achieved with a good cartridge and stylus.
You didn't make it clear if you are trying to use a signal that comes from a standard helical scan tape mechanism, but I assume that you are, since I don't think you can get the signal off the tape any other way (i.e., there isn't a "laser beam pickup" alternative).
Perhaps you could provide a simple block diagram of a typical VHS circuit (they are easily available on the Internet) and show where in that signal flow you are trying to read the signal.
Side to this novel approach with 2d high resolution photography allow to read whole plate at once.
both methods are superior to mechanical stylus as they are not damaging surface of plate.
For clean groove mechanical approach is less accurate.
There is many possibilities to read tape without head - but usually it is not necessary and head signal is easily available in VCR.
RF FM signal from head amplifier is routed to further processing - seem this is best place to start DSP.
Crucial can be combining FM head signal with servo/head sync pulses thus being able to compensate unavoidable mechanic issues.
Thinking on this from very long time ADC USB3 based acquisition board, perhaps RF (or rather 5 - 8MHz bandwidth 8 bit acquisition i.e. 16MHz sampling rate plus additional digital channel for head servo plus 2 audio 20kHz 16 bit channels for classic linear audio) all signal simultaneously sampled thus perfect synchronization provided. Core signal processing based around GNU Radio
Last edited by pandy; 25th Dec 2018 at 10:50.
Most of the this goes over my head, but the company in the OP's first link seems to be focused on selling their proprietary hardware (internal and external) with the only internal modification being to the video output stage with an internal board. Interestingly, at least that I could fine, there's no videos or even screencaps that compare their product's output to a conventional one.
From my layman's minimal understanding, what's being discussed is practical theory of what could be done. Could anyone address what they think is being done in the link given in the OP's first post https://www.cube-tec.com/en/solutions/video/quadriga-video/quadriga-video-rf/analog, which consists of replacement of an internal board and output to a proprietary digitizer:
"Simple but efficient
In order to modify simply open your machine and remove the DEC-42 board, if you are modifying a recorder. This is not needed for playback. In pure player devices the slot next to the DM-56 board will be already empty.
The DM-56 board has to be taken out and modified.
You only have to solder the supplied cables onto the Y/C demodulator board in order to directly tap the off-tape RF signals.
The next step is to open the back of the machine and solder the second set of supplied cables to two existing BNC connectors. For example, two of the composite outputs may be used to serve as outputs for the direct RF read out after modification.
Place the modified Y/C demodulator in its allocated space.
Than connect the QUADRIGAēVideo RF Amplifier board and place it in the free slot next to the DM-56 board.
Now your device is ready to be connected to the external QUADRIGAēVideo RF Digitizer. You are now able to digitize with 14 bit for a maximum of signal quality."
As has been stated, "Many professional players already have RF outputs for dubbing", so how is is different or superior?
Well for one thing, I don't think the Betacam BVW series has a stock RF output. Their custom board must provide some signal conditioning and then gets jury-rigged to a couple of BNC outputs.
Yes, for helical scan my thought was signal acquisition right off the drum head leads. I imagine that in addition to the software challenge of reconstructing image information (which my instinct tells me is solvable) the other challenge is having a shielded path to the aquisition system. Does anyone know what voltage/current levels are found coming right off the head?
A few mV into a high impedance, that's why the head amplifier is screened and fitted close to the head itself. It's to minimize noise pick-up and raise the video 'RF' level before it gets lost in the background. Using DSP techniques to process the head signal isn't new but substituting digital for analog circuitry doesn't fix the underlying problems of timing and frequency response because the same head mechanism, which accounts for almost all the signal degradation, still has to be used. There is some merit in performing timebase correction on the signal before it has gone through the normal VCR electronics instead of on the processed output but to be honest I doubt most people would notice the difference.
BA7252S datasheet describe head signal level at around 0.15 to 0.6mV i.e. 150 to 600uV - not so bad (usually MC pickup or tape audio magnetic head signal is bellow single uV around few hundred nV), gain is around 55dB i.e. few hundred (up to 1000) times - no problem for modern OPAMP's. But there is no need to build amplifier if VCR's amp can be used nicely so ADC must be able to deal with signals around 200mV and bandwidth maximum 10MHz - easy.
Checking datasheet seem in modern VCR's signal was sampled anyway - they began to use CCD as analog memory thus moving signal processing toward digital domain as another level of evolution. And looking on some problems with SECAM and uncommon video standards quite justified (not economically).
Last edited by pandy; 26th Dec 2018 at 05:57.