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G8MNY  > TECH     30.04.22 09:15z 106 Lines 4832 Bytes #15 (0) @ WW
BID : 1077_GB7CIP
Subj: Recording whole LF + MF bands
Sent: 220430/0910Z @:GB7CIP.#32.GBR.EURO #:1077 [Caterham Surrey GBR] $:1077_GB

By G8MNY                                  (Updated Feb 10)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

Here is a system for recording 100kHz - 3MHz on an old VCR. A VCR can record
40dB Signal to Noise over that whole DC - 3MHz bandwidth, or 70dB S/N in each
3kHz, & that equates to about 60dB S/N in each 9kHz AM channel!

Old VCRs (not HiFi) are best for this, provided the tape speed & head drum are
still good, as they do less cleaver video processing than later ones, so there
is less to corrupt the RF signal.

To get the right RF level into the 75 VCR input, an MF amplifier will be
needed. Here a simple NPN follower to do that using any MF transistor. If you
look at the VCR signal on a monitor you should see noise & patterning as bright
as a normal picture, the VCR will have a few dB of AGC as well to keep the
levels approximately right.

Wire  \/    68k                  
Aerial         /            u1 ===
        100pF    \e                
                  pot                ____________
             1mH         1mH       |  Old VCR   | 10nF 
TV Camera >(((())(((())| In     Out |Ĵ> LF/MW
 With Lens          ===             ~~~~~~~~~~~~         Rx
  Cap on  >          u22                      >

The VCR is designed to lock up to 50Hz     0dB.        _.--,
video syncs & the best way to get these   -3dB  Video   \      /   HF   VCR
is from a camera. Here the camera's HF          Syncs        /  Filter Video
is filtered off to not cause QRM. The RF        Filter      /           Wall
signals need to peak to 700mV p-p to add      
to the 300mV of frame syncs.                    30    3k    30k   300k  3M  Hz 

Black Cam signal            Filtered Cam         RF       
¿ - 300mV   ĿĿ        Added   1V P-P
312lines     -  P-P                                        _
  \ 50Hz /                                        VCR sees this as video

If your MF aerial supplies enough signals, without a too strong predominant
one, it will be possible to tune into recorded LW & MW AM/DRM broadcast
stations as well as 160M SSB/CW stations, but with some 25Hz FM due to the
normal VCR head drum-tape instability! 80M will not be possible due to the 3MHz
video wall filter on record & playback.

The VCR converts the video baseband DC-3MHz to higher frequencies by modulating
a FM carrier between 3.5-5MHz.
The AF is edge recorded or on hifi machines uses 2 lower FM carriers (around
100kHz) added into the head drum FM signal, these Hifi carriers could cause QRM
in this application.
The complex 4.4MHz PAL colour signal is down converted to 600KHz & also added
into the FM signal. All these added signals use the FM carrier as a record bias
(like an AF tape recorder), again these converted PAL frequencies could cause
QRM in a machine that can't drop to plain black & white mode.

     VIDEO BASEBAND                          TAPE SIGNALS
   Luminance      Colour             HiFi  Colour       FM Video
----------------  _______           _  _  _______  ----------------
                ||       |         | || ||       ||Sync Black White|
DC               3   4.4 MHz         0.1      0.6   3.5             5  MHz

Have fun trying this one out.

N.B. It will NOT work with any digital recorder as they only save a few % of
the picture's information per frame is actually encoded & stored, they rely on
similar picture storage of up to 15 frames to complete the task!!

If you want more bands try a down converter with 5MHz xtal & aerial BPF for
5-8MHz, that would be quite interesting!

> Hans PA0CX has done some experimenting, & he has published his findings in
> the Dutch magazine Electron Feb 1998 p59-63 De Luistermachine by pa0cx/f2zi

73 - Andre, PE1HWO @ PI8HGL
From         : G6DTW
Date/Time    : 15-Feb 2010
Title        : mf video receive converter
wonderful circuit; so easy: a good aerial helps tremendously.
much better than i thought it wd be not hifi, but then it IS MF.

Why Don't U send an interesting bul?

73 De John G8MNY @ GB7CIP

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