Digital Speech Digital Speech Transmission Transmission and Recoveryand Recovery

Overall SystemOverall System

Output (speaker)

Channel (coax cable)

Receiver Circuit

Input (microphone)

Transmitter Circuit Input

(microphone)

Transmitter Circuit

What is Spread Spectrum?What is Spread Spectrum?

This is the process of taking a This is the process of taking a signal that has been digitized and signal that has been digitized and replacing the 1’s and 0’s with their replacing the 1’s and 0’s with their own bit sequence (chip).own bit sequence (chip).

Example: 1 = (101), 0 = (001), Example: 1 = (101), 0 = (001), then a coded sequence such as then a coded sequence such as 1100 would become 101 101 001 1100 would become 101 101 001 001.001.

Spread Spectrum Spread Spectrum Communication SchemesCommunication Schemes

Direct Sequence Spread SpectrumDirect Sequence Spread Spectrum

Code Division Multiple Access Code Division Multiple Access (CDMA)(CDMA)

Direct Sequence Spread Direct Sequence Spread SpectrumSpectrum

1 Transmitter 1 Transmitter 1 Receiver 1 Receiver

The transmitter spreads the sequence The transmitter spreads the sequence

and transmits it across a channel.and transmits it across a channel.

The receiver then decodes the spread The receiver then decodes the spread signal and returns the original signal and returns the original message.message.

Code Division Multiple Code Division Multiple AccessAccess

Multiple Transmitters Multiple Transmitters 1 Receiver 1 Receiver

Choice of unique sequences to Choice of unique sequences to spread the data bitsspread the data bits

Ability of receiver to distinguish Ability of receiver to distinguish between data streams between data streams

Benefits of Spread Benefits of Spread Spectrum Spectrum

CommunicationsCommunications

SecuritySecurity

Multiple users on a single channelMultiple users on a single channel

Project GoalsProject Goals

Implement a direct sequence Implement a direct sequence spread spectrum communication spread spectrum communication systemsystem

Obtain a low level CDMA schemeObtain a low level CDMA scheme Transmit and receive speech Transmit and receive speech

signals through both systemssignals through both systems Learn about coding schemes in Learn about coding schemes in

digital communicationsdigital communications

Design OptionsDesign Options

Modulate the signal out to high Modulate the signal out to high frequencies, or transmit across the frequencies, or transmit across the basebandbaseband

What spreading sequence we What spreading sequence we choose and how long should it bechoose and how long should it be

Choice for voice coding Choice for voice coding

Coding and SetupCoding and Setup

The system was implemented using The system was implemented using C language. C language.

We wrote the filter code in TI We wrote the filter code in TI assembly.assembly.

The transmission to the receiver The transmission to the receiver was done over a coaxial cable.was done over a coaxial cable.

Original TransmitterOriginal Transmitter

Xx(t)X(n)

Spreading Sequence

Channel

Final TransmitterFinal Transmitter

T

Delta Modulation

Spreading Sequence

Conversion

8

Channel

Input

D / A

6 kHz

LPF1

TransmitterTransmitter

Sampling Rate: 44.1 kHzSampling Rate: 44.1 kHz

After decimating by eight: 5.512 After decimating by eight: 5.512 kHz.kHz.

After spreading sequence: 44.1 kHz.After spreading sequence: 44.1 kHz.

Modulation SchemeModulation Scheme

We transmit with a baseband BPSK We transmit with a baseband BPSK scheme.scheme.

Baseband transmission was valid Baseband transmission was valid because we only implemented two because we only implemented two users. users.

Delta Input Delta Input Spreading Spreading SequenceSequence

Spreading Sequence Output

An Example Spreading Sequence: 1,-1,-1,1,-1,-1,1,1

1 0

Voice CodingVoice Coding

We used a delta modulation We used a delta modulation schemescheme

We chose delta modulation We chose delta modulation because it allows the transmission because it allows the transmission of voice with a spreading sequenceof voice with a spreading sequence

Delta ModulationDelta Modulation

Input to Transmitter

Delta Modulation

Time

Am

plit

ude

Delta ModulationDelta Modulation

64/8 Samples

16 Bits/Sample

1 Chip of 8 Bits

= 1024 Outputs

64/8 Samples

1 Bit/Sample

1 Chip of 8 Bits

= 64 Outputs

Delta Modulation DiagramDelta Modulation Diagram

Quantizer

Delay

DelayDelta

Delta

To Interpolator

From Decimator

From DLL

To Spreading Sequence

Conversion

Original ReceiverOriginal Receiver

LPF Matched Filter

Channel

LPF Matched Filter

Channel

Sampler

D/A

Y(t)

Final Receiver Final Receiver

T Matched Filter

Delta Modulation

Delay-Lock Loop

8 Output

Channel

D / A6 kHz

LPF1

Matched FilterMatched Filter

It emphasizes the power of the It emphasizes the power of the correct spreading sequence and correct spreading sequence and cancels out the power of the cancels out the power of the undesirable sequence. undesirable sequence.

The matched filter coefficients are The matched filter coefficients are equal to the desired spreading equal to the desired spreading sequence. sequence.

Delay-Lock LoopDelay-Lock Loop

SamplerEarly Sample Late Sample

On-time Sample

+

X

Offset Logic

_

Ensures that we choose the correct sample

From Matched

Filter

To Delta Modulation

Delay-Lock Loop w/o NoiseDelay-Lock Loop w/o Noise

Mag

nitu

de

1

-1

Time

0

Encoded data Input to DLL

Delay-Lock Loop w/ NoiseDelay-Lock Loop w/ Noise

Mag

nitu

de

1

-1

Time

0

TestsTests

Input: Sinusoidal Signal, VoiceInput: Sinusoidal Signal, Voice

1 Transmitter 1 Transmitter Oscilloscope Oscilloscope

1 Transmitter 1 Transmitter 1 Receiver 1 Receiver

2 Transmitters 2 Transmitters 1 Receiver 1 Receiver

PerformancePerformance

Delta Modulation: 0-3.0 dB Delta Modulation: 0-3.0 dB attenuationattenuation

Overall System: 3.68 dB attenuationOverall System: 3.68 dB attenuation

Sound Sound Delta ModulationDelta Modulation 1 Transmitter 1 Transmitter 1 Receiver 1 Receiver 2 Transmitters 2 Transmitters 1 Receiver 1 Receiver