Differential gain


Differential gain

Differential gain is a kind of linearity distortion which affects the color saturation in TV broadcasting.

Contents

Composite color video signal

Composite color video signal (CCVS) consists of three terms:

  • Luminance (monochrome) signal
  • Auxiliary signals (sync pulse and blanking level signals )
  • Chrominance, which is actually a subcarrier modulated by chroma information

The first two terms are usually called composite video signal (CVS)

The modulation technique of the color subcarrier is quadrature amplitude modulation (QUAM ) both in PAL and NTSC systems. The amplitude of the color signal represents the saturation (purity) in both systems. On the other hand, the level of the CVS represents the brightness. So in order to reproduce the original vision in the receiver the ratio between these two pieces of information should be kept constant in the receiver.

Non linearity in the broadcast system

The main steps of visual signal from the scene to receiver screen (for terrestrial broadcasting) are as follows:

  • Cameras and associated buffer stages
  • Recording circuits and storage medium (CDs, tapes etc.)
  • Playback units and studio equipment
  • Cable, microwave or TVRO equipment
  • Transmitters (and sometimes also transposers)
  • Receivers (and sometimes also antenna amplifiers and distribution units)

In cable broadcasting and satellite broadcasting some of the above maybe replaced by other equipment.

All of the above circuitry include active circuit devices. These devices are only approximatelly linear devices. In particular, amplification factor is not constant for all levels. Usually the amplification factor decreases as the input level increases. This is known as gain nonlinearity. In system specifications, the nonlinearity in percentage is almost always specified. It must be under a tolerable level depending on the required sensitivity of the system.

Differential gain

Differential gain is a special case of gain nonlinearity. Let CCVS is given by

CCVS = CVS + color

The output of an ideal amplifier with the amplification factor of A is;

A\cdot (CVS+color) =A\cdot (CVS) + A\cdot (color)

However, in practice this goal is almost never attained. The amplification factor of color signal is always slightly different than that of CVS. (The color signal is superimposed on CVS.)

Problems caused by differential gain

Any color can be totally described by luminance, saturation and hue. When the gain of the reproduced color signal is lower than that of luminance, the perceived colors are paler than their originals. Conversely, when the gain of the reproduced color signal is higher than the luminance, the perceived colors are too loud.

Measuring

Top:Input, Middle:Output, Bottom:Output after HPF

To measure the percentage of differential gain, the standart sawtooth signal with superimposed color carrier is used. (4.43 MHz. in PAL and 3.58 MHz. in NTSC) The sawtooth represents the CVS with all possible levels between 0.3 v to 1 V. The waveforms are shown in the accompanying image frame.The top figure shows the input. Note that the amplitude of the superimposed color signal is constant. The middle figure shows the distorted output. In this example, color signal superimposed on a high level luminance signal has been attenuated.The bottom figure shows the same waveform after passing through a high pass filter to facilitate measuring. (CVS is filtered out leaving only color signal) .

\mbox{DG} =\frac{a-b}{a}

According to European standarts DG should be less than % 10.

A special case of PAL

DG is a problem both in NTSC and PAL. But PAL is more sensetive to this kind of distortion. System PAL averages phase differences of color signal in two consecutive lines[1]. Let α be the phase lag.

\mbox{f(t)} =\frac{1}{2}(\sin(\omega t+\alpha) +\sin(\omega t-\alpha))= \sin(\omega t)\cdot\cos(\alpha)

So the phase differences between two consecutive lines (so called differential phase) are converted to differential gain distortions. ( Hue is preserved at the risk of decreased saturation)

For example, a differential phase of 100 introduces an additional gain loss of % 2.

Reference

  1. ^ V.F.Samoylov-B.P.Khromoy: Television(trans:Boris Kuznetsov) Mir Publishers, Moscow,1977, p375

Further reading


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • differential gain — skirtuminis stiprinimas statusas T sritis automatika atitikmenys: angl. differential gain vok. differentielle Verstärkung, f; differentielleVerstärkung, f rus. дифференциальное усиление, n pranc. amplification différentielle, f ryšiai: sinonimas… …   Automatikos terminų žodynas

  • Differential gain — A&V A change in subcarrier amplitude of a video signal caused by a change in luminance level of the signal. The resulting TV picture will show a change in color saturation caused by a simultaneous change in picture brightness …   Audio and video glossary

  • Differential amplifier — symbol The inverting and non inverting inputs are distinguished by − and + symbols (respectively) placed in the amplifier triangle. Vs+ and Vs− are the power supply voltages; they are often omitted from the diagram for simplicity, but of course… …   Wikipedia

  • Gain compression — in an electronic amplifier circuit is a reduction in differential or slope gain caused by nonlinearity of the transfer function of the amplifying device. This nonlinearity may be caused by heat due to power dissipation, or by overdriving the… …   Wikipedia

  • Differential phase — is a kind of linearity distortion which affects the color hue in TV broadcasting. Contents 1 Composite color video signal 2 The colorburst 3 Differential phase distortion …   Wikipedia

  • Differential cryptanalysis — is a general form of cryptanalysis applicable primarily to block ciphers, but also to stream ciphers and cryptographic hash functions. In the broadest sense, it is the study of how differences in an input can affect the resultant difference at… …   Wikipedia

  • Differential interference contrast microscopy — Micrasterias furcata imaged in transmitted DIC microscopy. Differential interference contrast microscopy (DIC), also known as Nomarski Interference Contrast (NIC) or Nomarski microscopy, is an optical microscopy illumination technique used to… …   Wikipedia

  • Differential association — Criminology and penology Theories Causes and correlates of crime Anomie Differential association theory Deviance …   Wikipedia

  • Fully differential amplifier — A fully differential amplifier, usually referred to as an FDA for brevity, is a DC coupled high gain electronic voltage amplifier with differential inputs and differential outputs. In its ordinary usage, the output of the FDA is controlled by two …   Wikipedia

  • Limited slip differential — A limited slip differential (LSD) is a modified or derived type of differential gear arrangement that allows for some difference in rotational velocity of the output shafts, but does not allow the difference in speed to increase beyond a preset… …   Wikipedia


We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.