Digital Speech Transmission and Recovery

1. Digital Speech Transmission and Recovery Group 31 John Daugherty Chris Gass Tim Willenborg TA: Dave Crowe April 25, 2002

2. Overall System Transmitter Circuit Transmitter Circuit Input (microphone) Input (microphone) Channel (coax cable) Receiver Circuit Output (speaker)

3. What is Spread Spectrum? • This is the process of taking a signal that has been digitized and replacing the 1’s and 0’s with their own bit sequence (chip). • Example: 1 = (101), 0 = (001), then a coded sequence such as 1100 would become 101 101 001 001.

4. Spread Spectrum Communication Schemes • Direct Sequence Spread Spectrum • Code Division Multiple Access (CDMA)

5. Direct Sequence Spread Spectrum • 1 Transmitter  1 Receiver • The transmitter spreads the sequence and transmits it across a channel. • The receiver then decodes the spread signal and returns the original message.

6. Code Division Multiple Access • Multiple Transmitters  1 Receiver • Choice of unique sequences to spread the data bits • Ability of receiver to distinguish between data streams

7. Benefits of Spread Spectrum Communications • Security • Multiple users on a single channel

8. Project Goals • Implement a direct sequence spread spectrum communication system • Obtain a low level CDMA scheme • Transmit and receive speech signals through both systems • Learn about coding schemes in digital communications

9. Design Options • Modulate the signal out to high frequencies, or transmit across the baseband • What spreading sequence we choose and how long should it be • Choice for voice coding

10. Coding and Setup • The system was implemented using C language. • We wrote the filter code in TI assembly. • The transmission to the receiver was done over a coaxial cable.

11. Original Transmitter X(n) x(t) Channel X Spreading Sequence

12. LPF 1 Delta Modulation 8 Final Transmitter T Input 6 kHz Spreading Sequence Conversion D / A Channel

13. Transmitter • Sampling Rate: 44.1 kHz • After decimating by eight: 5.512 kHz. • After spreading sequence: 44.1 kHz.

14. Modulation Scheme • We transmit with a baseband BPSK scheme. • Baseband transmission was valid because we only implemented two users.

15. Delta Input  Spreading Sequence An Example Spreading Sequence: 1,-1,-1,1,-1,-1,1,1 Spreading Sequence Output 0 1

16. Voice Coding • We used a delta modulation scheme • We chose delta modulation because it allows the transmission of voice with a spreading sequence

17. Delta Modulation Input to Transmitter Delta Modulation Amplitude Time

18. Delta Modulation 64/8 Samples 64/8 Samples 16 Bits/Sample 1 Bit/Sample 1 Chip of 8 Bits 1 Chip of 8 Bits = 1024 Outputs = 64 Outputs

19. Delta Modulation Diagram To Spreading Sequence Conversion From Decimator Quantizer Delta Delay From DLL To Interpolator Delay Delta

20. Original Receiver Matched Filter Channel LPF Sampler D/A Y(t) Matched Filter Channel LPF

21. LPF 1 Delta Modulation Final Receiver T Delay-Lock Loop Matched Filter Channel 8 D / A Output 6 kHz

22. Matched Filter • It emphasizes the power of the correct spreading sequence and cancels out the power of the undesirable sequence. • The matched filter coefficients are equal to the desired spreading sequence.

23. Delay-Lock Loop Ensures that we choose the correct sample To Delta Modulation Offset Logic X On-time Sample From Matched Filter _ Early Sample Sampler + Late Sample

24. Delay-Lock Loop w/o Noise Encoded data Input to DLL 1 Magnitude 0 -1 Time

25. Delay-Lock Loop w/ Noise 1 Magnitude 0 -1 Time

26. Tests • Input: Sinusoidal Signal, Voice • 1 Transmitter  Oscilloscope • 1 Transmitter  1 Receiver • 2 Transmitters  1 Receiver

27. Performance • Delta Modulation: 0-3.0 dB attenuation • Overall System: 3.68 dB attenuation • Sound • Delta Modulation • 1 Transmitter  1 Receiver • 2 Transmitters  1 Receiver

28. Questions?