General Principles
The use of frequency modulation to convey the
picture information makes analogue SSTV very immune to impulse interference.
Although co-channel amplitude interference will degrade an SSTV picture the FM
capture effect means that a strong SSTV signal will tend to override weaker
signals.
The image on the right shows a typical spectrum display for an SSTV transmission.
The left side of the display represents an audio
frequency of 1,000 Hz and the right hand side represents 2,400 Hz. The three
vertical lines are at 1,200 Hz, 1,500 Hz and 2,300 Hz.
Picture Content
The image information is transmitted between
1,500 Hz and 2,300 Hz. Black corresponds to a frequency of 1,500 Hz and
white corresponds to 2,300 Hz. The 800 Hz band in between represents a
continuous gray scale between pure black and pure white, so that 1,900 Hz
represents mid gray.
The number of gray levels which can be
transmitted is limited by the ability of the receiving software to discriminate
between one frequency and another, but in practice it is possible to display a continuous
spectrum between black and white without visible steps.
It should be noted that since only a single audio
sub-carrier is used the transmission will only contain a single
frequency at any instant. The above display represents the information
gathered over a short period, which includes a segment of image information and
at least one sync pulse. This particular image segment contains some pure
black picture content ranging through to mid gray, but there is no pure white
content.
Line Sync Pulses
Line synchronization pulses (sync pulses) are transmitted at a lower
frequency than the image information, normally at the start of each line so that
the start of the line can be clearly identified. They are visible in the
spectral display as a peak around 1,200 Hz. (The small peak around 1,350 Hz is
probably noise.)
The
main function of the sync pulses is to allow an image to be properly synchronized even if
the start of the transmission was missed. Once horizontal synchronization
has been established the sync pulses can generally be ignored provided the timing of the
receiving equipment is accurately aligned to the transmitting equipment.
This has the advantage that noise and interference will not affect picture synchronization,
but if there is any timing discrepancy the picture will be received with a
slant. For this reason, some programs provide an option to make use of the
sync pulses to re-synchronize the lines as the picture comes in. Although
this may correct any slant, results can be unreliable under less than perfect
conditions.
If the image is sent by SSB and is not tuned in
correctly this will cause the spectrum to shift to the right or the left. The signal is correctly tuned
when the sync pulses fall at 1,200 Hz.
Decoding Considerations
At any one time during picture transmission the
frequency of the SSTV sub-carrier determines the gray level of the image. Thus,
picture quality depends on the ability of the receiving equipment to accurately
measure the instantaneous frequency of the sub-carrier. For most practical
purposes one half cycle is the theoretical smallest quantum of information which
can be sent using such a system.
At black level: f = 1,500 Hz, so cycle time t =
666 uS and t/2 = 333 uS
At white level: f = 2,300 Hz, so cycle time t =
434 uS and t/2 = 217 uS
There
is therefore little point in attempting to transmit picture information with a
pixel time significantly less than 217 uS.
Although digital signal processing techniques
have been employed to try and accurately measure the frequency of the received
waveform within less than half a cycle with some degree of success, the tried
and tested method is to measure the time intervals between the points at which
the waveform passes through the zero axis. This has proved to be a simple,
accurate and robust technique, although it is subject to one major pitfall.
Consider a constant audio frequency as produced
by a uniform gray image. If the signal input circuits are less than perfect they
can easily produce an asymmetrical waveform, shifting the waveform above or
below the zero axis. This will cause adjacent half cycles to be alternately
longer and shorter, which is interpreted as two different shades of gray.
Depending on the number of half cycles which go to make up each pixel, this can
result in noticeable patterning on the image.
The accepted solution is to average the last two
half cycles so that variations due to waveform asymmetry are smoothed out. This
effectively eliminates such patterning problems, albeit at the expense of image
resolution. As a consequence, the practical limits of resolution are generally
closer to the full cycle period rather than the theoretical half cycle
limit. Therefore, pixel times of less than about 400 uS cannot be achieved
without degrading picture sharpness, and significantly greater times are
required to obtain true pixel-for-pixel reproduction.
20 January 2008
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