【正文】 form left to right on a linebyline basis.電視用的掃描是一種稱為柵格掃描的空間取樣形式，它把二維的圖像亮度轉(zhuǎn)換為一維波形，類似于我們閱讀印刷品時的逐行從左到右的掃描。 then the parity bit will be a 0 when the number of 1s in the data bit is even and a 1 when it is odd. Hence, if a single bit in a byte is received in error and thereby violates the even parity rule, it can be detected and then corrected through retransmission.假設，例如，通信同意使用奇偶校驗位；然后將在數(shù)據(jù)位的1的數(shù)目甚至和1時，奇怪的是0。Basically, there are two forms of data pression:?Lossless pression operates by removing the redundant information contained in the data of interest. The pression is said to be lossless because it is pletely reversible in that the original data can be reconstructed exactly. Lossless pression is also referred to as data paction.?Lossy pression involves the loss of information in a controlled manner。壓縮是無損的，因為它是完全可逆的原始數(shù)據(jù)可精確重構(gòu)。有損壓縮包括以受控的方式的信息損失；壓縮可能因此不完全可逆。In munication systems, the received waveform is usually categorized into the desired part containing the information and the extraneous or undesired part. The desired part is called the signal, and the undesired part is called noise.通信系統(tǒng)接收到的波形通常包括兩部分，所希望的部分包含信息，還有不希望的額外部分。Electrical noise may be defined as any undesired voltages or currents that ultimately end up appearing in the receiver output. To the listener this electrical noise often manifests itself as static. It may only be annoying, such as an occasional burst of static, or continuous and of such amplitude that the desired information is obliterated. 電氣噪聲可以定義為最終出現(xiàn)在接收機輸出端的任何不希望的電壓或電流。它可能只是令人討厭的偶發(fā)靜電噪聲；也可能是連續(xù)的，幅度足夠大的，以致淹沒了所希望接受的信息。由接收機自身引入的噪聲稱為內(nèi)部噪聲。Practical inductors (coils) used at RF frequencies and above have an inductance rating in henrys and a maximum current rating .Similarly capacitors have a capacitance rating in farads and a maximum voltage rating. When selecting coils and capacitors for use at radio frequencies and above, an additional characteristic must be consideredthe quality(Q) of the Q is a ratio of the energy stored to that which is lost in the ponent .用于射頻火更高頻率的實用電感（線圈）有一個標稱電感值，以亨利（H）為單位；還有一個額定的最大電流。當在射頻或更高頻率選用線圈和電容器時，還有另一個特性是必須考慮的，這就是元件的品質(zhì)因數(shù)（Q）。The most basic building block in a munication system is an oscillator . An oscillator is a circuit capable of converting energy from a DC form to AC. In other words, an oscillator generates a waveform. The waveform can be of any type but occurs at some repetitive frequency. A number of different forms of sinewave oscillators are available for use in electronic circuits . The choice of an oscillator type is based on the following criteria: