Narrow-bandwidth Television Association

logo

[ Home ] [ Contents ] [ Previous chapter ] [ Next chapter ]

Chapter 25

NBTV standards and recommendations

Members are encouraged to experiment with any standards they like but cameras and monitors should use the same standard when they have to work together. When you want to connect your equipment to that of other members, then more things should be the same. Members of the NBTVA work according to certain standards to enable co-operation.
Next to the club standards are also a few vintage standards for those people that want to do restorations on old original equipment.

Club standard
Number of frames per second: 12
Number of lines per frame: 32
Number of lines per second: 400
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

Baird standard
Number of frames per second: 12
Number of lines per frame: 30
Number of lines per second: 375
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: 7 : 3
Note: In the Baird Televisor the first three lines and the last three lines were made wider and placed more apart. This resulted in an aspect ratio of the viewed total picture of almost 2 : 1
German (TeKaDe) standard
Number of frames per second: 12
Number of lines per frame: 30
Number of lines per second: 375
Line scanning direction: horizontal, left to right
Frame scanning direction: vertical, top to bottom
Viewing position on disc: top
Rotation direction of disc: clockwise
Picture orientation: landscape
Picture aspect ratio: 3 : 4

Right-hand optics

Pictures viewed on a Nipkow disc monitor suffer from curvature distortion. Instead of rectangular the picture is a sector of a circle. When looking on the right-hand side of a disc to a picture that was recorded from a left-hand sided camera, this curvature distortion becomes very unpleasant.
To minimise this geometric distortion right-handed optics is strongly recommended. The sketches show what is meant by the term.
Right-handed camera Right-handed monitor

Video signal interconnection

For interconnecting video camera's and other picture sources with monitors we recommend to have a well defined type of video signals. These video signals can have sync pulses added or they can be without sync. In the second case there should be a black bar on the transition of lines (blanking).
Video signals
Video amplitude:
Video polarity:
Video bandwidth:
Note:

Sync pulses:

Note:



Line pulse width:
Frame sync:
Frame blanking:
Note:
1 volt peak to peak.
white is more positive signal, black is more negative.
2 Hz to 10 kHz, at -3 dB.
for the Baird standard the frequency bandwidth is theoretically 13 kHz.
33% to 50% more negative than video black, some­times called "blacker than black" or "ultra black".
AC-coupled video will have a black level voltage that floats up and down, depending on the mean picture brightness. DC-restoration or clamping may be done to stabilise the black level voltage.
0.1 to 0.25 msec.
line sync pulse missing between line 32 and line 1.
during the missing sync pulse the video is black.
Baird never used "ultra-black" sync pulses, but a normal black bar between all lines. This gave him problems in synchronisation when a black picture was transmitted. Therefore black pictures were never permitted.
Inputs and outputs
To make easy interconnections possible we generally use the same type of cables and connectors. When you use a different type, see that you have adapters available or cables with different types of connectors on both ends.
Cable:
Connector:
at equipment:
at cables:
colour for video:
colour for audio
Output impedance:
Input impedance:
screened, phono cable or coax.
Phono or RCA type,
female,
male,
yellow, white or black,
red, all in line with the audio equipment standard.
low, 500 Ω or less.
high, 5 kΩ or more.

Media for recording NBTV

NBTV signals can be stored and distributed using different types of media. The character of NBTV makes some media better suitable than others.
CD, CD-R, DVD
Direct recording on CD-R and CD-RW and also on DVD-Audio is almost free of waveform distortion and has a wide frequency band. Also the play-back speed is very precisely defined and is crystal stable. This makes CD and DVD ideal for distribution of NBTV signals.
Sample rate:
Mode:
Rec. level video:

Rec. level audio:
Channels:
44,1 kHz.
stereo, 16 bits per sample (CD-standard).
at play-back the line output should give a signal according to the video standard.
-10 dB, peaks to -2 dB.
video on left, sound or separate sync on right.
Bit rate reduced digital audio, MP3, WMA, I-pod
The bit reduction algorithms give large distortion of the waveform, because they are optimised for sound and not at all for video. Maybe you will not hear the distortion, but you will certainly see it as a noisy picture. These systems are not suitable for NBTV video.
Tape recorders and cassette recorders
With the advent of digital recording methods, the drawbacks of classical tape make it an uncommon choice for NBTV. However excellent results have been demonstrated in the past. Techniques used by club members to overcome some of the limitations include pre-distortion, frequency correction, FM modulation and 'negative time' distortion cancellation. At present there are no common standards for these systems.

Electro Magnetic Waves

Radio amateurs may want to transmit NBTV signals on the air. When common modulation standards are used they can receive each others signals.
AM pos
Polarity:
Mod. depth:
Bandwidth:

positive, white is the largest HF amplitude.
95%, sync HF amplitude is 15% of max. HF amplitude.
20 kHz.
AM neg
Polarity:
Mod. depth:
Bandwidth:

negative, sync is the largest HF amplitude.
90%, white HF amplitude is 10% of max. HF amplitude.
20 kHz.
FM
Polarity:
Deviation:
Bandwidth:

positive, white is the highest frequency, sync the lowest.
15 kHz sync to peak white.
35 kHz, occupies 3 channels in the 12 kHz grid.
Vestigial Side Band (VSB)
VSB techniques have been successfully applied to NBTV transmission. It has the advantage of enabling the necessary bandwidth for both AM and FM NBTV to be almost halved, enabling readily available receivers to be used.

Picture files

Nowadays lots of work are done in the PC. If we use the same type of files we might exchange files and are able to use them without any conversion.
CD
Sample rate:
Bits per sample:
Mode:
Type of file:
44,1 kHz
16
stereo, video on the left channel
.WAV
See also the chapter on Compatible Colour NBTV in this book.
DVD
On DVD several other sample rates and bit numbers are available. Wave-files are supporting this. For 60 line video a sample rate of 96 kHz on DVD is recommended.
EPROM's
For the preparation of EPROM's with the help of a computer some specific types of binary files are in use.

Number of bytes per line: 64
Order of bytes:
according to the NBTV scanning,
first byte (number 00) is botttom right, byte number 63 (3F) is top right,
last byte (number 7FF)is top left.
Bit allocation:
File extension:





Examples:



Sync position:
Sync polarity:
Frame sync:

Blanking:
MSB shifted
- first letter the use of msb bits:
P for still pictures,
V for moving video,
- number of msb bits used for video,
- second letter indicating the use of the remaining lsb bits:
S for sync, C for PCM audio, D for delta modulation audio.
.P8 : still pictures, 8 bits/pixel, bits 7...0,
.P7S : still pictures, 7 bits/pixel, bits 7...1, sync on bit 0,
.V6D : video, 6 bits/pixel, bits 7...2, bits 1 & 0 delta mod.,
.V5C : video, 5 bits/pixel, bits 7...3, bits 20 PCM audio.
on the first 3 bytes of a line, bytes xx0, xx1 and xx2.
bit = 1 indicates sync, bit = 0 indicates active line.
the sync of the first line (addresses 0, 1 and 2) should be zero (missing sync pulse).
during sync all video bits should be zero,
preferably the last byte of each line should be zero, this provides a front porch,
during the missing sync pulse the video is preferably blanked.

[ Home ] [ Contents ] [ Previous chapter ] [ Next chapter ]