Picture Quality Analysis of Digital TV Signals - Digital Video and Audio Broadcasting Technology - page 197

12 Picture Quality Analysis of Digital TV Signals

The picture quality of digital TV signals is subject to quite different effects
and influences than that of analog TV signals. Whereas noise effects in
analog TV signals manifest themselves directly as 'snow' in the picture,
they initially only produce an increase in the channel bit error rate in digi-
tal television. Due to the error protection included in the signal, however,
most of the bit errors can be repaired up to a certain limit and are thus not
noticeable in the picture or the sound. If the transmission path for digital
television is too noisy, the transmission breaks down abruptly ('brick wall'
effect, also called 'fall off the cliff'). Neither does linear or nonlinear
distortion have any direct effect on the picture and sound quality in digital
television but in the extreme case it, too, leads to a total transmission
breakdown. Digital TV does not require VITS (vertical insertion test sig-
nal) lines for detecting linear and nonlinear distortion or black-level lines
for measuring noise, neither are they provided there and would not produ-
ce any test results concerning the transmission link if they were. Neverthe-
less, the picture quality can still be good, bad or indifferent but it now
needs to be classified differently and detected by different means. There
are mainly two sources which can disturb the video transmission and
which can cause interference effects of quite a different type:

• the MPEG-2 encoder or sometimes also the multiplexer, and
• the transmission link from the modulator to the receiver.

The MPEG-2 encoder has a direct effect on the picture quality due to
the more or less severe compression imposed by it. The transmission link
introduces interference effects resulting in channel bit errors which mani-
fest themselves as large-area blocking effects, as frozen picture areas or
frames or as a total loss of transmission. If the compression of the
MPEG-2 encoder is too great, it causes blocks of unsharp picture areas. All
these effects are simply called blocking. This section explains how the ef-
fects caused by the MPEG-2 video coding are produced and analyzed.
All video compression algorithms work in blocks, i.e. the image is in
most cases initially divided into blocks of 8 x 8 pixels. Each of these