Eine akustische Digitalübertragung soll implementiert werden. Digitale Sendesignale werden generiert und über den Lautsprecher ausgegeben. Mit dem Mikrofon wird dieses Signal erfasst und anschließend demoduliert. Verschiedenste Verfahren der Übertragungstechnik können hier untersucht und implementiert werden.

Whats the goal:

To plot an audio data

     - sampling

     - conversion

This data should lead to the entrance of the transmitter

The transmitter should code the data signal

     The division in I- and Q- signal serves the transmission of more data

     (90 Grad phase delay) through

 

The multiplication of the sine oscillations (Q) and

the cosine oscillations (I)

You should add a sync impulse

     The sync impulse recognizes the starting point of the data signal

                                          

The transmitter modulates the data signal with the carrier frequency

      You should choose a los carrier frequency

      (audible acoustic such as 5 kHz).

     This means that the transmission is audible but the range is restricted

The expense uses a speaker (common computer supplies)

The receiver should receive the acoustic signal through a microphone

The input proceeds through a microphone (common computer supplies)

The signal is demodulated (known carrier frequency)

                                                                     

The phase shifting (I-Q) revoked (demodulation)

This data can be dispensed through a speaker.

                                                                                                                         

What ARE the problems?

Sync

The random signal to creat the Sync is a sensitv part of for the Tx an also the Rx.

QAM

The constallation diagram shows the available bitcombinations.

You can see the 90° differce between the both components.

I - in-phase-componment  - The carrierfrequency is a consine. (upper picture)

Q - quadrature-component - The carrierfrequency is a sine. (lower picture)

The next picture shows the added components - ready to transmit.

Bitrate

The bitrate is very slow, but needed for a secure transmisson between the speaker and the microphone (Low frequency = small bandwidth).

Transmission by accoustic wave

Thats a very difficult part of the assembly. Be aware we use very low frequencies as carrier.

The consequence is a short distance between the seaker and the microphone.

Its also a very sensetiv transmission channel.

Bit-Error-Ratio (BER)

The BER will fix detected errors.

But therefore is an imput (Digital Signal) from the Tx necessary.

Detection and Measurement

Usefull measuring tools for this experiment

Spectrum analyser

Scope

Constellation diagram

QUIZ

Test your knowledge - How to receive the incoming signal?

What is the right carrier frequency? - Can you detect the carrier in the given spectrum?

What is the right filter? - Can you image whitch is the right filter form in the given scope plot?

Whats the right constallation? - Are you able to determine the bitcombination in the given constallation diagram?

Answers

1: 5.5kHz ; 2: root raised cosine filter; 3: 0.7/0.7: 11, 0.7/-0.7: 10, -0.7/0.7: 01, -0.7/-0.7: 00 (Gray distribution)

Your own experience

Its up to you - try to transmitt your information and change the settings to train yourself. The transmission is done by wire and you can add noise, if you want. Launch the experiment at the end of this page to start your experience.

Buddy-Experiment

Ask your buddy to help you to transmit an digital information

You need to start the Tx (TRANSMITTER) and imput your data. You can use any digital sheet.

Your buddy uses the Rx (RECIEVER) to recieve the signal and output the data on his device.

You need just a simple speaker and microphone, as you own surely.

ENJOY AND GOOD LUCK!

Use the TRANSMITTER on your Computer (see below)

Your Buddy uses the RECEIVER (see below)

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