Narrow Band Television "NBTV"
(Oscilloscope converter)
NBTV is a form of low definition monochrome moving picture transmission & reception.
The low definition criteria is pre-determined because of the audio frequency passband
that is used to convey the "video" signals.
That also means that an NBTV video can be recorded as an audio sound file,
required upper frequency limit; about 10 KHz
Compotent NBTV practitioners can transmit video on a 'speech' bandwidth HF radio circuit!
The NBTV association
specification is:
32 lines raster scan with a frame repetition rate of 12 ½ frames per second.
This means each line lasts for 2.5 mS and the 32 line picture is completed in (32 x 2.5) 80 mS
Line scanning is vertical, from bottom to top, starting bottom right.
Field scanning is from right to left.
Video modulation is positive going with negative going sync.
Each picture line (except the last of 32) ends with a line sync pulse of around 100-250uS.
End of a field scan is defined by the absence of a line sync pulse, or 'missing sync' (line 32).
The audio bandwidth for a 32 line, 12 ½ fields/second (fps) NBTV signal can be calculated as below....
This circuit design will allow an NBTV picture to be displayed on an oscilloscope,
provided the oscilloscope has the following capability:
Two DC coupled inputs, so that the trace
can be deflected vertically (Y) and horizontally (X)
and also a (Z) input to modulate the brightness of the CRO trace.
The schematic:
Incoming NBTV (audio) is first buffered/isolated through a 1:18 ratio step-up audio transformer
I used a 5000:15 Ω speaker transformer (in reverse)
DC supply voltage is +5volt (but should work up to +15volt)
A LM324 single supply quad package op-amp was considered most suitable for the design.
The composite video signal is also fed to a times 20 gain (26dB) inverting op-amp for use as Z mod input to the CRO.
I found I needed an LM741 op-amp running on split plus and minus 15 volt supplies
in order to get sufficient voltage swing (up to 30V) needed to drive my oscilloscope (1960's valve type)
Other more modern CRO's may not need the same magnitude of voltage "drive"
I anticipated that an LM324 op-amp running @ 5volt would have been sufficient, but in my case - not so!
The input signal is also fed to an op-amp comparator whose output changes high > low when a sync pulse occurs.
This is traditionally referred to as a "sync-seperator" circuit.
The comparator input signal is "DC-restored" with a shottky diode & capacitor which conducts on the bottom tip
of each line sync pulse and 'clamps' it to a level of -0.2 V
An adjustable voltage divider pot' allows setting of the voltage trip point for the negative going "sync tips"
The detected sync event resets the line oscillator ready for the next line scan.
It acheives this by pulling LM555 pin 5 low, which is the 2/3 voltage divider sense inside the LM555 (see diag)
The 2 diodes at pin 5 of LM555 are essentially a diode "or" gate which enables resetting
to zero the sawtooth o/p voltage across the timing capacitor.
An LM555 configured as a missing pulse detector determines the absence of the last sync pulse
this signifying Vertical Sync and resets the field oscillator (nominal 85mS max timeout)
The missing pulse detector is a 555 re-triggerable monostable (one shot) with a pulse O/P width of approx 4 mS
Retriggering by the sync pulses every 2.5mS means O/P remains 'high' until the missing sync pulse event occurs,
the monostable then times out and the high > low O/P initiates a field retrace
by pulling low pin 5 of the LM555 field deflection oscillator.
Both vertical and horizontal deflection oscillators are LM555 timers configured as free running "sawtooth" oscillators.
However the waveshape isn't actually a sawtooth but more
the exponential capacitor charging curve shape.
This can be seen in the CRO screen photo, where the scanning lines are cramped-up on one side.
But as this is a "low definition" video system I don't feel that a slightly non-linear scan raster poses a problem?
For a more true linear spot deflection the constant-current variant type should be used as shown in diag' below.
For this "upgrade" 2 more resistor/transistor/zener's will be needed.
A Red LED can substitute for the zener diode (fwd voltage 1.7-2volt)
Test mode: first successfull vision of NBTV displaying on my 1960's Tektronix 502 oscilloscope. (Feb 2021)
NBTV audio of Spike Milligan came from NBTV website
showing linear field (X) sweep: no cramping of scan lines!
The broard diagonal band on the CRT screen is not seen in real time
but somehow appears in the video recording?
My circuit draws in part on a design presented by John Lawrence GW3JGA
which in fact I constructed but couldn't get it to work successfully!!
Reference's:
Oscilloscope Converter for Narrow Band Television by GW3JGA