Direct sequence spread spectrum system

Direct sequence spread spectrum system (Direct Sequece Spread Spectrum CommunicaTIon Systems, DS-SS), usually simply referred to as direct sequence system or direct spread system, is to multiply the information signal to be transmitted with the high-rate pseudo-random code waveform to go directly Control a certain parameter of the radio frequency signal to expand the bandwidth of the transmission signal. The pseudo-random sequence used for spectrum spreading is called a spreading code sequence. See Figure 1-5 for a simplified block diagram of a direct sequence spread spectrum communication system.
In direct sequence spread spectrum communication systems, phase shift keying (PSK) modulation is usually performed on the carrier. In order to save the transmission power and improve the working efficiency of the transmitter, the spread spectrum communication system often uses a balanced modulator. Balanced modulation that suppresses the carrier is also beneficial to improve the anti-detection ability of the spread spectrum signal.
At the transmitter, the data signal to be transmitted is multiplied by the waveform of the pseudo-random code (spreading code) (or modulo 2 added to the pseudo-random code sequence), and the resulting composite code modulates the carrier wave, which is then transmitted by the antenna. At the receiver, a local reference pseudo-random code synchronized with the pseudo-random code in the transmitter is generated to perform correlation processing on the received signal. This correlation process is often called despreading. The despread signal is sent to the demodulator for demodulation to recover the transmitted information.

Figure 1-5 Simplified diagram of direct sequence spread spectrum communication system
(a) transmitting system; (b) receiving system

Aviation headphones are used in airplanes, cars, subways, etc., to facilitate passengers to listen to audio and watch videos to enjoy a pleasant journey.

The aviation earphones are mostly disposable, so the structure is extremely simple, the shape is relatively ordinary, the materials are small, and the annual usage is very large. The number of inquiry can reach several million or even tens of millions, so the cost is very cheap. earphones can cost as little as 10-15 cents. Therefore, they are disposed of as garbage after used, and passengers could take it away for personal use.

Styles can be divided into on ear headphones and in ear earphones.

Functional difference: power, impedance, and sensitivity etc. are also different. Airplane Headphone is mainly customized according to the equipment, place and user used. For example, 1 customer might  require 300 ohms impedances , and other customers may need 32 ohms. For adults, the sensitivity may reach 100DB or more.To children, the sensitivity may be around 85-90 DB.

In addition, aviation headphones are not all disposable ones. Some first-class cabins need to be equipped with headphones with good noise reduction, sound quality and functions. So the price will be much more expensive.

Airline Headphones

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