Download Modulation, Modulation Methods - Telecommunications - Lecture Notes and more Study notes Telecommunication electronics in PDF only on Docsity! Part 10: Modulation Introduction This section briefly discusses modulation, the process a modem uses to convert the digital data into analogue tones which are sent over the dial up connection. Modulation Is the process used to describe how the digital signal is changed so it can be transmitted across the analogue link. Modulation methods This refers to how the digital signal is altered so that it can be sent via the analogue PSTN. There are a number of different methods. The more complex methods allow much higher transmission rates (bits per second) than the simpler methods. Amplitude Modulation This uses a signal carrier frequency, on for high, off for low. Note that there is no carrier signal being sent for approximately half the time. This modulation method is suitable only for low speed transmission. The frequency of the carrier signal used depends on the protocol standard being used. Frequency Modulation This method uses two carrier signals, one for high one for low. The higher frequency is associated with binary 1, the lower frequency with binary 0. Also called frequency shift keying this method is used for 1200 bps modems or slower speeds. Higher speeds are not possible. If you increase the number of bits per second of the digital signal, the corresponding carrier tones are on for a shorter duration. Eventually the stage is reached where only a few cycles of the carrier tone is being sent for each digital bit. In order to reliably detect the carrier signal at the receiver, up to 5 complete cycles are required. Less than this and the carrier signal may be misinterpreted. Docsity.com To compensate for this, you might consider the option of increase the frequency of the carrier signals. But, you just cannot raise the carrier frequency higher as the voice channel will cut it off (remember of the bandwidth limits of the voice channel). Phase Modulation This method uses a single carrier frequency and alters the phase of the carrier Variations of this method allow coding of multiple bits (i.e. two or four bits) to represent a single carrier change. Normally, a change from binary 1 to binary 0 is represented as a phase shift of 180 degrees. By coding two bits per phase change, this doubles the number of bits per second. This is called two level coding. This method is suitable for 2400 bps modems (CCITT V.26) Bit Pattern Degrees Phase Shift 00 45o 01 135o 10 315o 11 225o Remember that baud rate is the number of changes per second. By moving to two bits per phase change, the number of changes per second is still the same (baud rate), but the number of bits per second is doubled. Baud rate is only equal to bit rate where single level encoding is used. Three bit encoding is used for CCITT V.27 modems at 4800 bps. Higher speeds are obtained by suing four bits, eight phase change, with two or four different amplitudes. Where changes in amplitude are combined with phase changes, this is called phase amplitude modulation. Docsity.com
