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6)

Receive a packet

The procedure here is protocol-dependent but to illustrate, let’s consider the

packet format outlined above being sent to a UART. A UART interprets the start-

bit of a byte as a 1-0 transition. When the incoming data is 101010 or hash... it is

hard to actually find the start bit. This problem is solved by the UART Sync Byte.

The purpose of the Sync Byte is to create a high marking period of at least a byte

length so that the start bit of the following Start Byte can be correctly recognized.

The Start Byte following the Sync Byte is used by the receiving computer or

microcontroller to intelligently identify the beginning of a data packet. The Start

Byte value should be chosen so that it does not appear in the data stream.

Otherwise, a receiver may “wake up” in the middle of a packet and interpret data

in the packet as a valid Start Byte. There are many other, more complicated ways

to organize the protocol if this restriction cannot be met.

There is always a possibility of bursting errors from interference or changing

signal conditions causing corruption of the data packet, so some form of error

checking should be employed. A simple checksum or CRC could be used. Once

an error is detected the protocol designer may wish to simply discard the corrupt

data or to develop a scheme for correcting it or requesting its retransmission.

The preceding steps indicate the general events involved in using the transceiver

for a half-duplex communication application. To summarize, the system controller

will basically run through the steps in order and then return to step 1, flipping

back and forth between transmit and receive modes. As discussed, the designer

must take into account the timing considerations of both the transmitting and

receiving sides and carefully think through protocol issues to ensure product

reliability under field conditions.

USING THE TR-XXX-SC FOR AUDIO APPLICATIONS

The SC series is optimized for the transmission of serial data; however, it can

also be used very effectively to send a variety of simple and complex analog

signals including audio. The SC is an excellent choice for applications requiring

audio quality comparable to a radio or intercom. The ability of the SC to send

combinations of audio and data also opens new areas of opportunity for creative

design. In applications which require high-quality audio, the SC series may be

used with a compandor to further improve SNR. In the cases where truly high

fidelity audio is required, the SC will probably not be the best choice. For such

applications, consider devices which have been optimized for audio.

The audio source should provide a 3V P-P maximum waveform and should be

AC-coupled into the TXDATA pin. The size of the coupling capacitor should be

large enough to insure the passage of all desired frequencies. Since the

modulation voltage applied to the TXDATA pin determines the carrier deviation,

distortion can occur if the TXDATA pin is over-driven. The actual level of the input

waveform should be adjusted to achieve the optimum results for your application

in your circuit.

The RXAUDIO output of the transceiver should be filtered and buffered to obtain

maximum audio quality. For voice, a 3-4KHz low-pass filter is often employed. For

broader-range sources such as music, a 12-17KHz cutoff may be more appropriate.

The SNR of the audio will depend on the bandwidth you select. The higher the

SNR, the less “hiss” you will hear in the background. For the best SNR, choose

the lowest filter cutoff appropriate for the intended signal.