Suggestions for voiceband television wanted

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Suggestions for voiceband television wanted

Postby tubefriend » Sat Mar 16, 2013 6:57 pm

Hello and thank you for letting me participate in your forum. I am an young service technican and a few of my hobbys are to collect junk, keep old tubes glowing and design new circuits.

Recently i got an large soviet osciloscope tube and this brought me to the idea to build some kind of narrowband TV receiver with it.

My major question to you is if you can suggest me very narrowband modes that fit into voiceband channels. I have espically the idea to build the equipment such that i could not only use it with local signal sources like converters and audio players but also connect it to an CB radio and transmit and receive video signal over the air.

What would you think about:

32 Pixels wide * 48 Lines * 4 Frames per second or alternatively 24 * 32 * 8 , both result in an pixel clock of 6144Hz that is an common 18,432Mhz crystal divided by 3000?

Line sync by making the first pixel blacker than black as common practice, but frame sync by transmitting an half line at peak white that could also be used as an reference for AGC circuits?
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Postby gary » Sat Mar 16, 2013 8:52 pm

The Chris Long Shortwave format available in Video2NBTV V2 - alpha is meant to be voice bandwidth, at least it is meant to be suitable for transmission over shortwave AM:

Number of frames per second: 6¼
Number of lines per frame: 32
Number of lines per second: 200
Line scanning direction: vertical, bottom to top
Frame scanning direction: horizontal, right to left
Viewing position on disc: right
Rotation direction of disc: anti-clockwise
Picture orientation: portrait
Picture aspect ratio: 3 : 2
White level: 1V
Black level: 0V

This format is sans sync because as long as the signal source has an accurate clock it is unnecessary.
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Postby tubefriend » Thu May 30, 2013 10:54 pm

I have now finally settled to the following values that are optimized for CRT display but should be well compatible to Nipkov discs and other mechanics as well. This is now declared to be my official VBTV (Voiceband Televison) standard.

Image format is in Landscape 4:3 with line scan from left to right and frame scan from top to bottom. This is similar to the usual television systems.

Pixelclock is 4800Hz

Every line consists of 32 pixels which is equal to an line frequency of 150Hz.

Line sync is done by transmitting pixel number 0 with sync level while pixels number 1 to 31 carry the active video line.

Every frame consists of 24 lines which is equal to an frame rate of 6.25Hz.

Frame sync is done by transmitting line number 0 with sync level while lines number 1 to 23 carry the active video lines.

Some circuits tend to tearing the upper lines so i recommand to break up this sync line and use the following sequence instead:
Pixel 0 with sync level
Pixels 1 and 2 with black level
Pixels 3 to 31 with sync level again

Electrical interconnections should use -250mV with respect to black for sync level and +750mV for peak white to give an total signal level of 1Vss. Inputs should be AC coupled and have at least 10dB spare amplification for weaker signals.

HF transmission should be AM modulated from the negative of the above signal (maximum amplitude for sync and minimum for white) and as much as the transmitter allows without distorting.
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