Page 83 - DCAP303_MULTIMEDIA_SYSTEMS

Unit 5: Video

notes
Frequency 4.433618 4.433618 4.433618 3.582056 3.575611
MHz MHz MHz MHz MHz
Video Bandwidth 5.0 MHz 5.5 MHz 6.0 MHz 4.2 MHz 4.2 MHz
Sound Carrier 5.5 MHz 6.0 MHz 6.5 MHz 4.5 MHz 4.5 MHz
paL versus ntsC
The NTSC receivers have a tint control to perform colour correction manually. If this is not
adjusted correctly, the colours may be faulty. The PAL standard automatically cancels hue errors
by phase reversal, so a tint control is unnecessary. Chrominance phase errors in the PAL system
are cancelled out using a 1H delay line resulting in lower saturation, which is much less noticeable
to the eye than NTSC hue errors.

However, the alternation of colour information—Hanover bars—can lead to picture grain on
pictures with extreme phase errors even in PAL systems, if decoder circuits are misaligned or use
the simplified decoders of early designs (typically to overcome royalty restrictions). In most cases
such extreme phase shifts do not occur. This effect will usually be observed when the transmission
path is poor, typically in built up areas or where the terrain is unfavourable. The effect is more
noticeable on UHF than VHF signals as VHF signals tend to be more robust.
In the early 1970s, some Japanese set manufacturers developed decoding systems to avoid paying
royalties to Telefunken. The Telefunken license covered any decoding method that relied on the
alternating subcarrier phase to reduce phase errors. This included very basic PAL decoders that
relied on the human eye to average out the odd/even line phase errors. One solution was to use
a 1H delay line to allow decoding of only the odd or even lines. For example, the chrominance
on odd lines would be switched directly through to the decoder and also be stored in the delay
line. Then, on even lines, the stored odd line would be decoded again. This method effectively
converted PAL to NTSC. Such systems suffered hue errors and other problems inherent in NTSC
and required the addition of a manual hue control.

The PAL and NTSC have slightly divergent colour spaces, but the colour decoder differences
here are ignored.
5.2.3 systeme electronic pour Couleur avec Memoire (seCaM)

The SECAM (Sequential Color Memory) is an analogue colour television system first used in
France. A team led by Henri de France working at Compagnie Française de television (later bought
by Thomson, now Technicolor) invented SECAM. It is, historically, the first European colour
television standard.
Just as with the other colour standards adopted for broadcast usage over the world, SECAM is a
standard which permits existing monochrome television receivers predating its introduction to
continue to be operated as monochrome televisions. Because of this compatibility requirement,
colour standards added a second signal to the basic monochrome signal, which carries the colour
information. The colour information is called chrominance or C for short, while the black and
white information is called the luminance or Y for short. Monochrome television receivers only
display the luminance, while colour receivers process both signals.
However, PAL and SECAM are just standards for the colour sub carrier, used in conjunction with
older standards for the base monochrome signals. The names for these monochrome standards
are letters, such as M, B/G, D/K and L.
These signals are much more important to compatibility than the colour sub carriers are. They
differ by AM or FM sound modulation, signal polarization, relative frequencies within the
channel, bandwidth, etc. For example, a PAL D/K TV set will be able to receive a SECAM D/K
signal (although in black and white), while it will not be able to decode the sound of a PAL B/G
signal. So even before SECAM came to Eastern European countries, most viewers (other than
those in East Germany and Yugoslavia) could not have received Western programs. This, along

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