Antialiasing Video Filters
Antialiasing Video Filters and Reconstruction
Filters
The main application of the
video filter is in the conversion from
analogue to digital (A-D) and digital to analogue (D-A) for full broadcast standard signals. The CCIR
recommendation 601 specifies the video filters to be used. These filters are usually referred to
as antialiasing filters and reconstruction filters.
In sampling theory the Nyquist rule that the
minimum sampling rate must be at least twice the maximum frequency of the signal to be
retained is used. As filters cannot have an infinite cut off rate (if they did
unacceptable time domain ringing would be introduced) a sampling frequency slightly
greater than twice is usually chosen. Before sampling, the bandwidth of the signals should
be limited to restrict the lower side-band frequency components of the samples signal.
Otherwise these may modulate with the base-band signal after conversion back to analogue.
This would lead to noticeable aliasing components appearing on the picture.
After the digital to analogue (D-A) conversion the sampling frequency and
its sidebands remains and the signals are seen as made up from discrete packets of
frequencies. A reconstruction filter removes the sampling frequency and its sidebands
which results in the reconstruction of a smooth analogue signal.
The diagram below illustrates the above for a system using a normal
sampling system.
Fig 1. Diagram of Antialiasing Video Filters
Oversampling
Filters
As can be seen the filter needs to have a fairly sharp cut-off rate
and hence be group delay and amplitude equalised. To reduce the complexity of the
video filter it is possible in many systems to employ oversampling techniques. In standard definition
video systems for the luminance channel the standard oversampling frequency is 27 MHz. A
digital filter is included in the decimator to attenuate the energy from the end of the
passband up to 19.25MHz. An analogue filter having a relatively slow cut-off rate and
little or no equalisation can then be employed. This removes the clock frequency and the
sidebands and reconstructs the signal.
Fig 2. Diagram of Oversampling Video Filters
Sinx/x Distortion
When a signal is sampled the sampling window is usually constant and
hence as the frequency being sampled increases less energy is ‘collected’. This
results in the signals having a loss against frequency. For a 5.5 MHz bandwidth signal
sampled at 13.5 MHz the loss at 5.5 MHz is 2.56dB. The shape of the loss frequency curve
is a function of frequency and is defined by an equation of sinx/x form. It is usual to
include a correction for this in the passband of the reconstruction filter.
Timing Considerations.
In many video systems, for instance with component video, the
luminance and colour difference signals pass through filters with widely different cut off
frequencies. This results in the video filters having different insertion delays. In order to
restore the correct timing it is necessary to delay the luminance signal by the delay
difference. For a typical full CCIR rec.601 system the delay may be of the order of 600ns.
As video delay lines are expensive and will add further distortion to the signal. A more
elegant solution is to delay the luminance signal by a number of clock cycles in the
digital domain and to either introduce a small analogue delay line or to purchase filters
which are matched in insertion delay to the nearest integer of the clock frequency. The
difference in the reconstruction delay between the luminance and chrominance channels must
also be taken into account.
Comprehensive
range of Anti Aliasing Video Filters
Faraday has specialised in the design and manufacture
of video filters for the Professional Broadcast Industry by combining a knowledge
of filter design and manufacture with a knowledge of television technology.
It has the most comprehensive range of
antialiasing video filters
available. |
