Questions tagged [demodulation]

Demodulation is the act of extracting the original information-bearing signal from a modulated carrier wave.

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23
votes
2answers
7k views

Extracting Binary Magnetic-Strip Card Data from raw WAV

I am faced with a tricky challenge: To extract binary data from an iPhone magnetic strip card reader. This is what the magnetisation on the card looks like: Source Here is the .WAV the iPhone ...
14
votes
3answers
13k views

How to demodulate an AFSK signal in software

I am trying to transmit binary data from one device to another over an audio channel (speaker/mic). I use AFSK (Audio Frequency Shift Keying) as in Packet Radio, with $1200 \text{ Baud}$ and two ...
14
votes
2answers
2k views

Improving SNR using DSP techniques

I am building an optical OOK (On off Key Ring) system without a carrier frequency. [However I have a guard time in between symbols, so a consecutive "1" messages will result in a pulse train as ...
13
votes
1answer
10k views

How to perform carrier phase recovery in software?

What are the options for recovering the phase of a BPSK signal in software? The only resources I can find online contain circuit diagrams – there seems to be no one willing to explain this to people ...
11
votes
2answers
10k views

How to demodulate an OFDM signal

I'm looking at decoding an OFDM signal which consists of 6 carriers (or tones) which are BPSK modulated and a pilot tone which aids tuning. This is the first time I have worked with OFDM so I need to ...
10
votes
1answer
4k views

FSK Demodulation, Bit Timing Recovery

I am currently implementing acoustic FSK modulation and demodulation. I am not a signal processing guy so any help about the bit timing recovery would be very appreciated. Currently I implemented the ...
8
votes
2answers
4k views

How to soft decode DQPSK?

I'm successfully soft-decoding D-BPSK by taking the dot-product of the constellation-position of the symbol and of the previous symbol. If the result is >= 1, then the symbol phase hasn't changed and ...
8
votes
1answer
466 views

Which digital modulation scheme would be suitable for transmitting digital data through the output of a PC soundcard

I need to output a data stream at $40\textrm{ kbit/s}$ or faster out of a computer system whose only accessible output peripheral is the audio interface. This interface has reasonable specs, $96\...
8
votes
1answer
4k views

How to determine where to sample for demodulation of BPSK signals?

I have a simple BPSK demodulator. Very simply, the signal comes in and is split into two branches, one for I and one for Q. The I branch is mixed with a sin wave of the carrier, and the Q branch ...
7
votes
2answers
557 views

QAM constellation slicer with unknown carrier phase

As shown in the image, I have a 16QAM constellation that is misaligned due to a phase rotation. In this case you can see that rotation amount is approximately $\theta = \pi/4$, but this won't be the ...
7
votes
1answer
1k views

What is the connection between analog signal to noise ratio and signal to noise ratio in the IQ plane in a quadrature demodulation system?

We would like to compute the quantitative relation between analog noise near the LO frequency and the statistics of points found in the IQ plane after IQ demodulation. In order to completely ...
7
votes
5answers
502 views

Angle Averaging

I am working on an 802.11a demod that works, for the most part, but has a bug that causes intermittent errors. I haven't finished characterizing it, but it appears that the problem is in my ...
6
votes
3answers
2k views

Common methods of transmitting digital data through an extremely noisy audio signal?

I am developing software for transmitting data from a computer to a mobile device using audio. Specifically, from the computer's speaker to the device's microphone. (I am developing both the sender ...
6
votes
1answer
2k views

Why can Quadrature Demodulation demod a Frequency Modulated Signal?

I use a Quadrature Demodulator in my SDR application, which is defined as: $\angle (S_n, S_{n+1})=arctan(S_{n+1}*\overline{S_{n}})$ So practically its amplitude is the angle between two Samples $S_n$...
5
votes
1answer
3k views

Phase synchronization in BPSK

In BPSK for demodulation at the receiver end we do phase recovery between received wave and carrier wave. We can find the phase difference between the carrier and received waves using : PLL And ...
5
votes
1answer
1k views

FFT window phase syncronising

I'm currently devising a solution for demodulating a DPSK signal in C#. So far I've got a working FFT with phase detection, BUT: I now have an issue of aligning the window of the FFT to synchronise ...
5
votes
1answer
1k views

How exactly to implement the fractional part of a 'fractional equalizer'?

I have written my own LSE equalizer (that works on the bit level), and literature I have read says that 'fractional' based ones are much better. So how exactly does one get 'fractional' bits? Is it as ...
5
votes
1answer
10k views

QPSK Modulation/Demodulation in C++

I'm trying to implement a QPSK Mod/DeMod system in C++. So far i have a bit generator that generates N bits and stores it into an array: ...
4
votes
2answers
1k views

Doppler effect on Baud Rate

I'm having a hard time wrapping my head around doppler and how it affects a digital communications signal's baud rate. The examples that I've seen explain that the center frequency of a signal is ...
4
votes
1answer
4k views

Demodulating FSK audio in Python

I am trying to write a Python script that can demodulate an FSK modulated audio file and return the data encoded in the audio. The data being transmitted is GPS NMEA strings which are embedded as the ...
4
votes
0answers
2k views

Demodulation of noisy AFSK signal

I'm trying to implement a software audio modem with FSK to transmit and receive short digital data between two devices (e.g. a PC and an iPhone) so I'm simulating simple cases study using Scilab/...
3
votes
2answers
5k views

FSK and IQ modulation

I am having trouble understanding the FSK modulation process. There are many ways of demodulating the FSK signal. What is the advantage of performing the FSK using IQ modulation? How to demodulate ...
3
votes
2answers
141 views

Why would we simulate baseband rather than passband

My question is, Why do we usually use baseband in simulation rather than passband? however, if we want to test our simulated idea in real environment, it must be sometimes in passband !! So, should ...
3
votes
2answers
700 views

Really basic question about OFDM

I am new to signal processing and am currently doing research into the subject. I am learning about Orthogonal Frequency Division Multiplexing and have a question. Most definitions of OFDM mention its ...
3
votes
3answers
2k views

Demodulation of a BFSK frequency hopping signal using FFT

I am not very familiar with DSP techniques, and I am currently working on my research project to develop the firmware of a DSP processor, using C/Linux. It involves the modulation and demodulation of ...
3
votes
2answers
96 views

Getting the bitstream of an unknown RF signal

I am trying to get the bitstream of a signal sent over 433MHz. I have installed SDR# (I'm on Windows), connected my NESDR SMART, and I identified the signal here: I can see there is only one peak ...
3
votes
1answer
697 views

Matlab: What should be the BER performance for BPSK using Constant Modulus Algorithm equalizer

I need help in plotting the Bit error curve or the symbol error curve for BPSK modulation scheme for varying Signal to Noise ratios or Eb/N0. The plot should show the simulated versus the theoretical ...
3
votes
1answer
955 views

Estimating optimum sampling point for 16-QAM in MATLAB

I need to write a MATLAB function to estimate the best initial sampling point for demodulation of a 16-QAM. I can do it by looking at the eye diagram, the point which at which the eye is "open", but ...
3
votes
1answer
467 views

Implementing a basic demodulator

I'm trying to implement a simple demodulator as an evening project (to understand how this works - my background is in physics not engineering) and was wondering if anyone has any advice. To make a ...
3
votes
2answers
2k views

How to transmit the real+imaginary output of an IFFT over a transmission channel?

I'm working on a modem system using a 16-QAM scheme. The transmitter runs an IFFT to modulate the signal, and has a 12-bit DAC. The receiver runs an FFT to demodulate the signal, and has a 12-bit ADC. ...
3
votes
1answer
284 views

Improving the communication speed on a 10km cable (with high capacitance)

I'm working on improving the modem communication speed that can be achieved over a 10km cable. The current modem achieves a speed of 9600 bps via hardware (phase locked loop) and I'm looking for ways ...
3
votes
1answer
195 views

What are the options for turning a bit stream into a byte stream?

I am creating a system (modulator and demodulator) for broadcasting binary data through an analog signal. I'm using BPSK with great success – at least, I'm generating great graphs. Green is input ...
3
votes
2answers
494 views

Demodulating AM signal from carrier signal on Matlab

I'm trying to demodulate a voice signal (bandwidth approximately 5 kHz) from a carrier signal (100kHz). [y,Fs]=audioread('Signal1.wav'); h=10*sinc(10*(-10:10)); y1=filter(h,1,y); But ...
3
votes
1answer
2k views

How to improve an FFT AM demodulator for NOAA APT satellite imagery?

My question involves proper FFT Technique for AM demodulation, in the special case where the modulation rate exceeds the carrier frequency. This question has a specific hobbyist application, the ...
3
votes
0answers
99 views

Synchronization of BPSK with variable-length symbols (ISO 14443)

Does anyone have an idea how one would perform BPSK symbol synchronization if symbol length is allowed to vary? As dumb as that sounds, this is not a theretical question.The "Proximity" Type B NFC ...
2
votes
1answer
1k views

How bandwidth is used in modulation

In a typical Digital cable environment in the US, the spectrum is divided into 6MHz bands. When a tuner locks to a signal at say 800MHz, I would assume that the band for that signal is 797 - 803 MHz. ...
2
votes
1answer
3k views

why LTE doesn't use DC subcarrier in the downlink?

I know LTE doesn't use the DC subcarrier for OFDM/OFDMA modulation for the downlink, but no author gives a detailed explanation to this stuff. Can you tell me the reason behind it? Thank you!
2
votes
1answer
979 views

Integrator and the multiplier in the Optimal Binary Detector

The optimal binary detector for asymmetric, equiprobable PAM pulse normally have structure as shown below, But I cannot understand what is the multiplier and the integrator using for. Why the ...
2
votes
2answers
8k views

How to normalize the power of a QAM signal?

If I have, for example a 16-QAM signal, and the constellation points are (3,3),(3,1),(1,3),(1,1),...(-3,-3). Now I have a simulated in coming signal with arbitrary ...
2
votes
1answer
174 views

Quadrature demodulation, homodyne detection, lock-in detection - what's the difference?

Quadrature demodulation Homodyne detection Lock-in detection What is the difference between these terms? Are they all synonomous? In my mind they kind of are, so I'm never sure which to use. I ...
2
votes
2answers
336 views

Mystery signal - FSK, BPSK?

I have a mystery audio signal that I need to decode, it's from a discontinued product. Inspecting the object code shows a module called decodeFSK so that suggests ...
2
votes
3answers
427 views

OFDM violates sampling theorem

The "textbook" OFDM block diagram is as the below figure. But I wonder if it satisfies the sampling theorem. Here, we have N points $X[k]$ before IFFT, and $$x[n]=\frac{1}{\sqrt{N}}\sum_{k=0}^{N-1} X[...
2
votes
1answer
146 views

How is a signal with SNR less than unity demodulated?

The title pretty much says it all. What technique can be used to demodulate a signal with SNR less than 1?
2
votes
3answers
1k views

Real-time BPSK demodulation

I am currently studying BPSK demodulation. Most probably my question very basic but i am having difficulties to understand the case of real-time bpsk demodulation. Basically, all the examples (matlab,...
2
votes
2answers
3k views

How to recover complex values (downconvert) from FM modulation (composite signal)?

How does one recover the complex IQ data from the output of a FM modulator. I have combined my signals correctly and performed FM modulation on them, now I'm looking for a way to find the IQ values ...
2
votes
1answer
1k views

Find beginning of signal for BPSK Demodulation

I want to store a small amount of data in a sound file recorded by a microphone but I know nearly nothing about signal processing. I managed to demodulate my data encoded using binary phase shift ...
2
votes
1answer
760 views

How to Shift Carrier Frequency in QAM Signal?

If I wanted to apply a 100Hz frequency shift to a QAM signal with a 500MHz carrier frequency, I would Demodulate it at the carrier frequency Apply a frequency shift to the original signal Modulate it ...
2
votes
1answer
638 views

Understanding BFSK Modulation (MATLAB/Simulink)

Could someone explain to me BFSK Modulation on the Simulink model below? The simulation above simulates a binary FSK system with a correlation (or matched filter) receiver. I have to understand the ...
2
votes
1answer
539 views

FSK analysis: only ever find one frequency

I've trying to write this software as part of my amateur radio and SDR hobby for analyzing various two-level FSK signals. I'm very new to DSP but have been having a lot of fun so far learning more ...
2
votes
1answer
299 views

Denoising effect in GnuRadio OFDM Serializer block

Why does the OFDM serializer has such a strong denoising effect in the flow graph below ? Is that normal ? The upper constellation is AFTER the OFDM serializer block and the lower is BEFORE the OFDM ...