Narrow-bandwidth Television Association
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Chapter 13
NBTV STANDARDS
(and recommendations by the writers)
Members are
encouraged to experiment with any standards they like, but cameras and monitors
should use the same standard when they are required 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
cooperation.
Next to the club standards are also a few vintage standards for
those people that want to do restorations of old original
equipment.
Club standard Number of frames per second: 12½
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Baird standard Number of frames per second: 12½
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German (TeKaDe) standard Number of frames per second: 12½
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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.
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Right-handed camera |
Right-handed monitor |
Video signal interconnection
For interconnecting video camera's and other picture
sources with monitors it is recommended 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). A separate sync signal can be supplied.
Video signal
amplitude:0,7 V black to white, with the addition of a sync pulse of 0,3 V. (Total
amplitude is 1,0 V)
Video signal polarity: white is more positive, black is
more negative.
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Frequency bandwidth: 2 Hz to 10
kHz, ±3 dB. Inputs and outputs To make easy interconnections possible we generally use the
same type of cables and connectors. When you use another type, see that you have
adapters available or cables with different types of connectors on both
ends. NBTV signals can be
stored and distribited using different types of media. The character of NBTV
makes some media better suitable than others. We give a survey. Cassette tape CD, CD-R, DAT DCC, minidisc, MP3 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 FM 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. Compact Discs
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Note: for the Baird standard the frequency
bandwidth is theoretically 13 kHz.
Gamma of video: 2 (quadratic, parabola).
Sync pulses (when used): 0,3 volt more negative than video black.
Note: Video signals are not DC-coupled. The real
voltage for black can be any value, positive or negative, and can be changing
when the brightness of the picture changes.
Line
syncpulse width: 0,1 to 0,25 msec.
Frame synchronisation: line
sync pulse missing between line 32 and line 1.
Frame blanking: during the missing sync pulse the video is black.
Note: Baird never used
"ultra-black" sync pulses, but a normal black bar between all lines. This gave
problems in synchronisation when a black picture was transmitted. Therefore
black pictures were always avoided.
Cable:
Connector:
Output
impedance:
Input impedance:screened,
phono,
at equipment:
at
cables:
for video:
for audio:
low,
high,cable or coax.
RCA-type of
connector,
female,
male,
yellow, white or black,
red, as used
in CD players.
less than 500 ohms.
more than 5 kilohms.
Media for recording NBTV
For years cassette tape has been seen as
a suitable medium for distribution of NBTV signals: cheap, handy and made for
audio. However, direct recording on cassette tape gives huge distortions of the
wave form, because of the HF-biased recording process on magnetic tape. Also the
very low frequencies gave a problem during play-back. Several forms of
subcarrier recording have been tried, but they always needed a specially built
system for recording and play-back. No real standard for this method has been
established.
Direct recording on CD-R and
CD-RW and also on DAT (Digital Audio Tape) is almost free of wave form
distortion and has a wide frequency band. Also the play-back speed is very
precisely defined and is crystal stable. This makes CD 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.
The bit reduction algorithms
give large distortion of the wave form, because they are optimised for sound and
not at all for video. You will not hear the distortion, but you will see it very
well. These systems are not suitable for NBTV video.
Polarity: negative, sync is the largest HF
amplitude, white the smallest.
Mod. depth: 90%, white HF amplitude is 10% of
max. HF amplitude.
Bandwidth: 20 kHz.
Polarity: positive,
white is the highest frequency, sync the lowest.
Deviation: 15 kHz peak to
peak.
Bandwidth: 35 kHz, occupies 3 channels in the 12½ kHz grid.
For the preparation of CD's
with the help of a computer the use of Wave-files is recommended.
Sample rate:
Bits per sample:
Mode:
Type of
file:
44,1 kHz
16
stereo, video on the left channel
.WAV
EPROM's
For the preparation of EPROM's with the help
of a computer some specific types of files are in use.
Bit allocation: there are two standards in
use:
7 bits per pixel, bits 7...1, sync on bit 0, file
extension: .PS7
6 bits per pixel, bits 7...2, frame sync on bit 1, file
extension: .PS6
5 bits per pixel, bits 7...3, bit 2 always zero, file
extension: .PS5
bit 5
always zero, file extension: .DAT
preferably the last byte of each line should be zero
(front porch),
the sync of the first line (adresses 0, 1 and 2) should be
zero (missing sync pulse).
or a short sequence of moving
video.
8 192 for a 2764 with 4 pictures or 0,3 sec. video,
16
384 for a 27128 with 8 pictures or 0,7 sec. video,
32 768 for a 27256 with
16 pictures or 1,3 sec. video,
65 536 for a 27512 with 32 pictures or 2,6
sec. video, and so on.