VIDEO SIGNALS
Restore correct time base
for non-standard TV signals
By Frank Kearney,
Staff Engineer,
Analog Devices, Inc.
Despite the rapid advancement
of digital TV, analogue TV will
remain dominant for both transmission and display for several
years to come. Analogue video
formats, like digital formats, have
precise specifications as to how
to format video properly. These
specifications include how the
luma, chroma, and synchronisation information is packaged to
provide the line, field, and frames
of video information that recreate the images observed on TV
screens. To display the video information accurately, each of these
components must be extracted
correctly from the video signal.
Similarly, the timing and phase
information must be maintained
or recreated as it was when the
video signal was first encoded at
the source.
This article outlines the challenges encountered when decoding and restoring a correct
time base to non-standard input
video sources. Examples of where
restoring a correct time base
is critical to maintaining good
image quality include analogue
tuners (NTSC, PAL, and SECAM)
used on most TVs today and
the venerable VCR. Since most
people today continue to rely on
analogue tuners and VCRs, it is
important that advanced digital
video decoders utilize the latest
clock reconstruction techniques
to produce outstanding image
quality in DTVs.
Noise-induced time-base
inaccuracies
NTSC, PAL, and SECAM video consist of lines of video information
packaged in fields and frames.
The lines, fields, and frames are
identifiable by embedded synchronisation. The correct extraction of this information enables
Figure 1: Line Synchronisation Stream without Noise at Source
Figure 2: Stable Synchronisation Extraction with No Vertical Jitter on Displayed Image
Figure 3: Noisy Synchronisation Information Resulting in Extracted Synchronisation Jitter
the receiving device--TV, VCR, or
projector--to reconstruct the images and provide a visual display.
Video signals consist of various components, each of which
can be altered or corrupted
within the transmission path,
resulting in distortion of some
video package aspects. For RF
transmitted signals, the synchronisation information is normally
present on the recovered signal,
but its detection and extraction
can be difficult or impossible
because of excessive noise. It
is important to note that even
when recovering the synchronisation is possible, its detection
can be offset due to noise, which
in turn introduces jitter on the
recovered synchronisation information.
Figure 1 shows a representation of a typical stream of line
synchronisation information. All
line lengths are the same and
conform to nominal specification requirements. Receiving and
extracting the correct synchronisation results in a proper and
stable display. Figure 2 shows
that slicing the synchronisation
information from this stream
results in a stable display.
Noisy sync
Noise is commonly introduced
in the RF transmission path. The
induced noise results in the acEE Times-India | eetindia.com
in a misinterpretation of the
synchronisation. This shows the
resulting stripped synchronisation that can be caused by the
noise. Note that the time base is
corrupted and jitter introduced.
The effect of the jitter on the displayed video is serrations at the
start and end of each line (see
Figure 4).
A typical noisy input from a
tuner source is shown in Figure
5. It demonstrates the difficulty
in determining the synchronisation information.
The very high noise level
within the signal makes it difficult, firstly, to determine where
the synchronisation information
is and, secondly, to determine its
exact start and stop positions. A
critical requirement for digital
video decoders is to meet this
challenge.
High-performance
video decoders can maintain
lock to RF video signals with
powers of less than 20 dBµV,
thus enabling the display device (TV) to show these images
without horizontal jitter or vertical rolling. Although noise is an
objectionable artefact on any
display, its presence becomes
much worse when the display
loses its time base, resulting in
vertical roll and horizontal jitter.
Figure 6 illustrates the
decoder output re-encoded
into an analogue format.
Figure 4: HSYNC Jitter Resulting in Line to Line Shifting of Displayed Image
Figure 5: Actual Video Output from Tuner Source Measured on Input to Decoder
Figure 6: Decoder Output with Correct Noise-Free Synchronisation Restored
tive video region that will be
displayed becoming ‘snowy’, soft,
and corrupted by noise artefacts.
However, it is not just the active
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video data that becomes corrupted. The noise can also corrupt the
embedded synchronisation necessary for the receiving device to
reconstruct the image.
Figure 3 is an example of how
noise can distort the synchronisation information, resulting
Amplifier distortions
The introduction of noise is a
common issue for video that has
come through a transmission
path. It is also common for components of the video signal to be
attenuated or amplified, resulting
in nonlinear characteristics in the
video package.
Many display devices use
the synchronisation depth as a
reference and apply gain until
it reaches its nominal value. In a
situation where the video package has become nonlinear, this
results in incorrect gain being
applied to the video image.
An example of a video signal
seen at the input to a decoder is
shown in Figure 7. The synchronisation level is reduced close
to the blanking level, while the
ing the other signal components
at their nominal level, as shown
in Figure 8.
VCR-induced time-base error
Unlike common transmission
path induced errors where synchronisation is normally present
but can be distorted, video from
VCR sources can have missing or
incorrect synchronisation information.
VCRs are essentially mechanical devices. Variance in
the mechanism, including motor speed, belt wear, and head
switching, can introduce timebase corruption. In the case of
head switching and VCR trick
modes, the time base is not only
corrupted, but can also be lost
(see Figure 9).
When head switching occurs,
all video and synchronisation is
lost. The result is a flat dc output
for this duration. When such
errors occur, the downstream
display devices do not receive
adequate synchronisation information to reconstruct the image
correctly.
Depending on the amount
of missing information and the
ability of the decoder to reconstruct the TV images with this
information, the effect can vary
from a minor top curl artefact to
total loss of horizontal and vertical synchronisation. Figure 10
shows a typical top curl artefact
caused by poor time-base signals from a VCR source.
The latest advanced digital
line-length tracking (ADLLT)
technology used in devices such
as Analog Devices’ ADV7180,
ADV7184, and ADV7188 decoders ensures the correct regeneration of the missing or inaccurate
synchronisation
information
ensuring that the top curl like
artefacts are eliminated (see
Figure 11).
Figure 7: Input Video Signal with Attenuated Synchronisation
Figure 8: Output Video Signal with Correct Synchronisation Level Restored
Figure 9: VCR Missing Signal During Head Switch
other components of the video
signal remain at the proper am-
plitudes.
Advanced digital video pro-
cessing will restore synchronisation information while maintain-
Processing non-standard
input video with advanced
digital video decoders
Advanced digital video decoders
filter the window in which they
look to detect the synchronisation. In addition, the decoders
use HSYNC and VSYNC processor
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Figure 10: Output Image with Top Curl
Figure 11: Output Image with Top Curl Eliminated
Figure 12: LLC Jitter Performance
blocks to ensure that the synchronisation information is correctly
extracted. The filters ensure that
the decoder gates the time period in which it looks for synchronisation information. Previously,
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excessive noise outside of this
region would have dipped below
the slice level and been seen as
synchronisation. The synchronisation PLL and processor blocks
ensure that synchronisation de-
tected within the gated period
is correctly aligned. Because line
locked decoders use the HSYNC
as a timing reference for colour
burst detection and subsequent
video decoding, its proper detection is essential.
A critical requirement for any
decoder is to correctly separate
luma and chroma information.
This is dependent on the ability of the decoder to extract the
colour subcarrier and correctly
generate the proper number of
samples in each horizontal synchronisation period. Advanced
video decoders use fixed frequency 4x oversampling to
digitize the input video. A resampler block within the decoder
ensures that a fixed number of
samples per line are consistently
output. The resampler PLL varies
in frequency to obtain this fixed
number of output samples. This
resampling method that delivers
a fixed number of samples in
each horizontal synchronisation
period is referred to as linelocked time-base correction.
With this type of architecture,
a simulated line-lock clock (LLC)
is generated. Note that although
the line-locked time-base correction results in a fixed number
of samples per line; samples are
not at a fixed 27 MHz rate, but
vary with the resampler PLL, i.e.,
27 MHz ± 5%.
Pixel information is fed to the
output FIFO before it is output
from the decoder. Special FIFO
control techniques are used to
allow a smooth flow of data from
the FIFO to the output pixel drivers. Although the long-term clock
jitter is still present on the output
clock, the short-term jitter variations are smoothed out. Figure
12 provides a representation of
output jitter across two fields of
video. The peak jitter measurement is the same on both plots,
but the short-term jitter in the left
plot has been removed. It is the
ability to remove this short-term
jitter that allows the decoder to
now operate in a direct backto-back configuration with the
digital video encoder.
Digital video decoders, such
as those from Analog Devices,
incorporate the functionality of
synchronisation detection and
extraction blocks with a resampler and advanced back end
FIFO management.
Summary
Analogue TV is still more widely
used than digital TV for transmission and display purposes.
In order for video information to
be displayed accurately on an
analogue TV, it is important that
luma, chroma, and synchronisation information is extracted correctly from the video signal, and
that timing and phase information is maintained or recreated as
it was when the video signal was
encoded at the source.
This article described several of the processing problems
that can be encountered when
decoding and restoring a correct time base to non-standard
input video sources. The ability
to restore such a time base has
direct consequences for the
quality and stabilisation of the
displayed image. Failing to correctly process the video causes
vertical and horizontal synchronisation issues that can, in turn,
cause rolling and jitter on the
displayed image.
The latest techniques for
the correct regeneration of the
missing or inaccurate synchronisation information, such as
advanced digital line-length
tracking (ADLLT) technology,
used in the ADV7188, ADV7184,
and ADV7180 decoders, ensures
optimised processing of a wide
range of non-standard signal
types. Even for the weakest
signal strengths or the poorest
quality VCR sources, it is now
possible to display an image on
a user’s TV that is both stable
and viewable.